# Garn water temp - difference between front and rear?



## foxt (Jan 28, 2009)

I've had a GARN 2000 operating for about a month now, and am curious to know if others see the same thing that I see with temps.  The Dectra-supplied temp gauge on the front of the unit consistently reads 5-6 degrees hotter than my own temp gauge I have mounted in-line on the black iron pipe hanging off the top supply fitting in the rear.  

My gauge is installed about three feet away from the unit, and that black iron pipe is insulated, so I can't believe I am losing 5-6 degrees in the pipe.  My circ runs 24/7 at the moment, so I would have assumed I don't have much stratification in the tank.   I've considered that my gauge could be at fault (Letro/Pentair dial gauges with well fittings), but I've swapped out between 4 different gauges and they all show about the same results.  Could be an effect unique to the gauge design (the probe is immersed in the well which is mounted in a T fitting, but the dial is hanging out there in unheated space and I wonder if that affects the accuracy).    The Dectra-supplied gauge could be reading high, but I won't be able to swap that out unless I frain down the tank.

I wonder, do other folks see the same difference?  I'm wondering if the location of that front gauge (being right above the combustion chamber) yields a difference in temp between front and rear?  With the circulator running I would think there wouldn't be much difference between the front and back, especially hours after the fire is out, but there is a bit more thermal mass at the front of the unit, and heck, I'm just guessing here.  I have been planning on swapping out the dial gauge for an electronic model with a probe that will fit in the existing well, but before I do that I figured I ask here ...

Tom


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## Jim K in PA (Jan 28, 2009)

Tom - I also have seen a difference in temp between the front gauge and my thermowell sensor gauge at the supply pipe.  I don't see quite that high a difference, and mine balance out to within a degree or two after the burn is done.  The bungs on the front wall are very close to the air collar, and I see a cool down on the front gauge when starting a new fire with the fan pulling cold air in.  This reverses during the burn when the upper portion of the air collar gets mighty hot.  

My pumps are also running 24/7, but hopefully they won't by the end of the week.


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## TCaldwell (Jan 29, 2009)

tom, there might be a calibration screw on the back of the thermometer, most bimetal types have one. there is one on the flue gas thermometer, i found that the flue temps were about 25 deg off ,low untill it reached 350deg+. i switched to a type k thermocouple. i would think jim's theory might affect it also to some degree.


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## foxt (Jan 29, 2009)

Thanks for the replies guys.   No adjustment screw on my gauge at the back of the unit.  I do notice a bit higher temp differential (maybe as high as an extra 3 degrees) when the fire is at it's peak, that could be due to the effect that Jim suggests.  But when the fire's been out and I'm just drawing heat, I see a consistent 5 or 6 degree difference.  It's got to be the design of my gauge.  Gonna be cold tonight, I'll wrap the whole thing in fiberglass and see what I have in the morning.

Tom


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## foxt (Jan 29, 2009)

Sure enough, wrapping the face of my gauge at the rear of the unit with some fiberglass seemed to work.  Both gauges are reading within 1-2 degreess of each other at this point, so mystery solved ...

Tom


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## Rick Stanley (Nov 5, 2009)

While the fire was burning and after it had burned down to only coals, I had 200* on gauge on front of Garn and 199* at the hx 170 ft away in the house. About 16 hours later with the fire completely out, I have 176* on the garn gauge and 148 getting to the house. This has been a pattern since I started. Seems like the turbulence of the hot firebox while burning and shortly after, is causing some nice mixing. After the water and the firebox are the same or closer temps, there is some short cycling in the back of the garn. Make sense??


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## foxt (Nov 5, 2009)

I'd love to compare notes on this.  I am still seeing something similar, although my temp drop is not as significant - more like 20*.  I didn't have time last heating season to test the effect or dropping a submersible pump into the tank and letting that mix the tank, but I intend to do that in the next few days. 

I'm still guessing that this is due to short-circuiting between the in/out at the back of the unit, perhaps related to the fact that I only have a single loop plumbed to the GARN, and it consists of a total length of about 300' (most of that doubled-up 1" pex with 60' under ground) and a heat exchanger.  I could be wrong but, for some reason, I don't think I've read of this phenomenon when people have a primary/secondary system set up with a relatively short primary right at the GARN.  How is yours plumbed?

Tom


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## Rick Stanley (Nov 5, 2009)

Mine is plumbed the same way Tom. The 1-1/4'' underground dual-pipe Insulpex is 170' long for a round-trip of 340'. When you add the piping in each building it's a round-trip of about 380' to a flat-plate hx and back. Just a single loop like yours.

The pump is a 15-58 grundfos and I've tried different speeds and it doesn't change anything as far as temps. When I first noticed it, it scared the crap out of me. I was thinking, "great, I'm losing heat into the ground". Sinking feeling. But, as long as the firebox is hot I only see a 1-2 degree difference.

I've been burning late afternoon because this is a two family set-up and the biggest demand is in the evenings. Four showers, sometimes two at once, laundry, dishwashers, heat on everywhere, there are seven zones, you name it. So while that's going on, I have approx 200* water reaching my hx. In the morning, the fire has been out completely for hours, the heat has run all night and is still running and I have 145-150 water reaching the house, which is fine, I can run on low temps with the radiation I have, but when I go check the garn it's 160-170 or higher most mornings.
   So, it keeps pumping as needed with those temps and slowly draws down the garn temp throughout the day. Then I'll build a fire later in the afternoon and within an hour or two the gauge on my hx will reflect the garn temp again within a couple degrees.
    My needs are being met as it is, right now. I feel it may be different when it gets cold, so I'd be real interested in hearing how the submersible  works. Good idea.


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## foxt (Nov 5, 2009)

I even have the same pump, and have also not noticed any affect on the temp behavior based upon pump speed.  Where do you have your supply plumbed - top or bottom of GARN?   Mine is on the top per the older install manual, and I have wondered if swapping the connections would make a difference.

Tom


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## foxt (Nov 5, 2009)

I should have also referred to  this other thread where there was some exchange on this starting on page 2.


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## Rick Stanley (Nov 5, 2009)

foxt said:
			
		

> I even have the same pump, and have also not noticed any affect on the temp behavior based upon pump speed.  Where do you have your supply plumbed - top or bottom of GARN?   Mine is on the top per the older install manual, and I have wondered if swapping the connections would make a difference.
> 
> Tom



Mine is the same. Supply from the top.


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## Gooserider (Nov 6, 2009)

Have you tried looking at the PIPE temps leaving the Garn, as opposed to the heat that's showing on the tank gage?  That should help narrow things down to being either a problem in the Garn or something strange happening in your piping to the HX...

Not having been in a Garn, I'm wondering do the pipes on the supply and return have any sort of diffusers in the tank to spread the returning water / suction around?  Or do the pipes just go through the end wall and dump into the tank?  I'm wondering if adding something of the sort might help if it doesn't exist already, I know that it appears to make a significant difference in maintaining stratification for the storage tank folks...

Gooserider


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## foxt (Nov 6, 2009)

Goose,
   Yes, I can measure/observe a 10-15 degree delta between the front and back of the tank via thermometers installed in thermowells at the front bushing and at the top supply port.  
   There are no additional fittings or diffusers on my GARN at the supply and return ports - the water just mixes via whatever turbulence is created by the geometry of the tank fittings themselves ...

Tom


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## Gooserider (Nov 6, 2009)

foxt said:
			
		

> Goose,
> Yes, I can measure/observe a 10-15 degree delta between the front and back of the tank via thermometers installed in thermowells at the front bushing and at the top supply port.
> There are no additional fittings or diffusers on my GARN at the supply and return ports - the water just mixes via whatever turbulence is created by the geometry of the tank fittings themselves ...
> 
> Tom



OK so it does sound like a Garn problem...  I am kind of surprised that they are just dumping the water into the tank w/o providing some way of spreading it out...  Probably no way to do it w/o draining, especially on the bottom port, but I'd think it would really help to put some sort of diffuser on those pipes, or possibly even just an extension on the top pipe so that it is pulling from the front  end of the tank, diagonall opposite from the return...  One of the approaches I've heard being used that struck me as good is to put a long pipe inside the tank with a cap on the end, and a series of relatively small holes drilled in one side - put the holes facing up on the top and down on the bottom pipes, this is supposed to minimize mixing and help maintain the stratification...

Gooserider


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## Rick Stanley (Nov 6, 2009)

I just checked my temps. I have only a 16* difference this morning, which is more in line with what you are seeing Tom. It has occurred to me that the large delta-t's I have seen might be because I have no way of 1- seeing both temps at once without walking up and down stairs and across the yard and in and out of two buildings (guess I should run) and 2- knowing how long the pump has been running when I do check the temps. Maybe when I see the large difference, the pumps had just started and the temp in the house was on the rise and I didn't notice it.

Kind of a long story, but I can't read the temps on the piping on the back of the garn very well either. I have two different brands of gauges and wells that aren't interchangeable AND seem to read 10* different from each other. Also, the supply water leaving the back of the garn goes through about 15' of uninsulated copper/iron piping, in an unheated uninsulated space before reaching the gauge. I don't know how much the water can cool between the garn and the gauge but maybe more than I think, especially when  there's only a 1'' board between it and 26* outside overnight temps. 
    So after I get things tightened up and fully insulated and maybe buy an infared gun, I'll better see what's happening.

Jim K,
Your garn is a sister-ship to mine. Am I imagining it or was there a pipe on the lower return port extending into the tank a couple of feet??


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## foxt (Nov 6, 2009)

I had about a 20* difference this morning, and decided it was time to drop a submersible pump into the tank and see what happens.  I don't recall offhand what size the submersible is.  I dropped it in the tank and plugged it in, and also had the circ that serves my supply/return loop running.

Within about a minute or two, the temp at the top supply port of the GARN had jumped 6-7*, and had dropped about 4* at the front gauge.  After about 5 mins, the front temp had dropped a total of 7-8*, and the rear temp had no further change.  The front/rear were now about 4-5* apart.  I walked away at this point, and came back about 30 mins later.  At that point, the front/rear had equalized within a degree or two (I'm calling that even, what with different gauges and all). 

Interestingly enough, overall the front temp eventually dropped by more  (11-12*) than what the rear rose (7-8*), indicating that the front was more of a hot-spot than the rear was a cold spot, if that makes any sense (eventual steady state temp was closer to the colder vs. warmer starting points).

So, there is clearly not enough mixing going on within the tank in this configuration.  I can only think of 4 options (maybe others will have more), in order of simplicity:

1. leave the submersible in the tank and have it cycle on/off (5 mins on was enough to mix, I don't know what the off cycle would have to be), or let it run when the GARN circ is running.  I'm sure this could be set up with an even more complex control at some point to trigger off delta-T at front/rear.
2. Swap the supply/return ports and see if that makes a difference.   
3. Switch to a primary/secondary config, I don't think people see this effect in that config.
4. Drain the tank and extend the supply or return further into the tank (how long of an extension, and which port to extend are all great questions that I have no answers for)

I know #1 works and is easy enough to do.  I don't know if #2 would have any impact, and would love to know if anyone has a GARN plumbed with only one loop  (not p/s) with the supply coming off the bottom port and *doesn't* see this effect.  #3 seems to be the "right" solution given that folks don't see the front/back delta-T in this config, but I didn't go that way in the first place because I didn't want the extra complexity and the install manual indicated that what I was doing was ok.  I really don't want to tackle #4 at the moment, if ever.

So, what do you all think?  Is it worth a question in to Dectra at this point as well?

Tom


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## heaterman (Nov 6, 2009)

Question for Fox

Is this situation causing you problems as far as system operation and heat delivery?


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## heaterman (Nov 6, 2009)

A couple other comments..........

The Garn does have an extension on the return line in the bottom of the tank. It terminates approximately 3-4' inside the unit as I recall. The purpose is to help eliminate "short circuiting", (for lack of a better term) whereby the return water gets drawn directly back to the supply. Can't happen to a great extent the way it's made. The pipe is also angled slightly downward to help keep sediment off the bottom of the tank.

I have noticed this occuring on every Garn I have checked whether it is run constant circ or not. Circulation would have to be really high to get blending in a 1500 or 2000 gallon tank because if you think about it, most residential applications are running about 12-15gpm through the Garn. at that rate it would take about 100 minutes plus just to turn over the tank one time. Add another 20 minutes or so for a 2000.

What I have seen is good stratification in the unit when the fire is down. I checked on this past week with an infrared and it showed 125-130 at the bottom (return) and 165 at the supply port. The temp gauge on the front was reading about 172 IIRC with a freshly started fire in the combustion chamber. When I checked it after about an hour and a half of work in another part of the building, it was pretty clear to see that the temp was equalizing in the tank as there was less than 15* difference return to supply. The temp gauge at that point was reading about 185* and the return port was close to 170*.  So, with no fire in the Garn and the heating load drawing off what it needs, the hottest water obviously is at the top of the tank. As heat is used up, cooler water begins to fill the tank. You'll find that with an infrared thermometer you can see a marked difference from top to bottom with a distinct band about a foot deep where the temps begin to blend. When the Garn has been sitting for a long time with heat being used out of it that band of blended water eventually reached the supply port. At that time you can see quite a bit of variation from the front mounted temp gauge to the actual temp at the supply port.


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## foxt (Nov 7, 2009)

heaterman, yes, it is causing me a problem, or at least a perceived one.  The control for the GARN circ uses a sensor mounted in a thermowell next to the front gauge.  When there is a 20* difference between the front and back temps, the system is delivering water that is 20* cooler than it thinks it is.  To correct for this, I need to artificially raise the setpoint 20* above the low end of my operating range.  For instance, I can take water temps as low as 120* for my distribution system, but I have to set the control for 140* (not considering loss in the pipes, fphx, etc).  Isn't the net that I am effectively reducing my operating range by 20* (burn to 200*, and then run down to 140* instead of 120*)?

Thanks for your other thoughts, a couple of questions:

1. I took pictures of the inside of my GARN while I was in there cleaning it out prior to first fill.  I need to dig them up, but I don't recall the extension that you mentioned.  I could be wrong, but I do recall thinking it was odd that there wasn't one.  An extension makes sense to me.  I have been thinking  that I don't have one, hence my thinking that I am seeing short circuiting. 

2. 





> You’ll find that with an infrared thermometer you can see a merked difference from top to bottom with a distinct band about a foot deep where the temps begin to blend. When the Garn has been sitting for a long time with heat being used out of it that band of blended water eventually reached the supply port.



Makes sense to me, I expect vertical stratification in the tank.



> At that time you can see quite a bit of variation from the front mounted temp gauge to the actual temp at the supply port.



I got confused there.  Is the front-mounted gauge at a different elevation than the top supply port?  I've been thinking that they are roughly at about the same height.  Plus, the front is always warmer than the back, never the other way around.  Given that there is vertical stratification, does this difference between front/back mean that there is also horizontal stratification?  I suspect that the geometry and location of the combustion chamber might be contributing to what I am observing, maybe preventing the water that sits above the chamber from mixing with the rest of the tank?

3. A final question: IIRC the newer (or was it older?) GARN manual calls for the supply to come off the bottom of the tank, right? Have you installed any that way, and is there enough experience with the different config to know whether it makes any difference in this front/back delta-T?  Could one potential difference be that there might be less resistance to mix with the water over the top of the combustion chamber if the return is pushing water to the top front of the tank vs. the bottom?

My net on all of this is that I'm potentially having to fire the GARN more frequently than necessary, because I am not getting all of the stored BTUs out of the tank when I choose to run it to the low end of my operating range.  If I am wrong about that, I'll just accept that this is the way it works ...

Tom


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## TCaldwell (Nov 7, 2009)

tom, gary switzer builds a simular boiler to the garn, he has 2 extra bungs, one high and one low that he uses a for tank mixing when the boiler is firing, i think they are 2 inch and 40gpm mix rate to homoginize tank temps


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## heaterman (Nov 7, 2009)

*"supply to come off the bottom of the tank",*

You know..........I think I have one of the first ones we did hooked up that way......I have not been back to that install lately but the next time I'm there I will do a little checking and see. .......Interesting.  I will drop an e-mail to Martin regarding this subject tomorrow.......to tired right now.

TC. Have you noticed anything like this with yours?  BTW I got your message yesterday evening but didn't know how late you were up. Left for a job early this AM and finally sitting in my chair undisturbed now. So how late are you up and when can I return your call? I'll get in touch.


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## brad068 (Nov 7, 2009)

I guess I will have to check my tank temps too. I welded 4 extra heavy 2" couplings in the front plate of my unit. Two high and two low. Being that they are full length and installed all the way through, I put 5' extension on the return and draw right short from the front. My solar uses the other two via a plate heater and two pumps.

Its to early too tell, but I use to pull off the bottom and return on top, I also seen older garn manuals. I switched it this year and will see if it is a improvement.


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## TCaldwell (Nov 7, 2009)

my wood circ used to run 24/7, i noticed a moderate difference from the front of tank thermometer and the temp gauge on the supply line that pulls off the bottom bung, it would equalize a few hrs after the fire was out, circ delivers 14gpm. now the wood circ is only energized when a zone calls, so it probably never runs more than 10 minutes at a time, annoyingly after a burn the temp gauge will read 200, and the supply[stagnant] will read 155, and increase when a zone calls. this i can live with, but i believe it would  be beneficial to run the wood circ when the inducer is running, to mix the tank?


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## leaddog (Nov 8, 2009)

I would think that with the big tank that it would be benificial to have statification. Why don't you take the hot off the top and return to the bottem like we all do with storage. You would get more btu's at the higher temp that way. The garn shouldn't work any different than our propane tanks!! we might have 180* on the top and 120* on the bottem and can still feed the baseboards with hot water.
leaddog


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## heaterman (Nov 8, 2009)

leaddog said:
			
		

> I would think that with the big tank that it would be benificial to have statification. Why don't you take the hot off the top and return to the bottem like we all do with storage. You would get more btu's at the higher temp that way. The garn shouldn't work any different than our propane tanks!! we might have 180* on the top and 120* on the bottem and can still feed the baseboards with hot water.
> leaddog



You are absolutely correct there Leaddog. At least for the actual heating part of the equation.  You want stratification when you are in heating mode with any storage system. I think the problem that this guy is encountering is the result of insufficient flow during the firing mode. You want to encourage blending while firing so you can bring the entire storage volume up to temp. I was just talking with TCaldwell on the phone about that very issue. We found that a plugged up HX was severely limiting flow on the Garn side of the system in this case. What was happening was that the tank thermometer would show 180-190* but then drop off very rapidly a short time after the burn was done. No turnover of the storage was happening during firing so the temp gauge would pop right up there quick and show that everything was up to temp. The reality of the situation was that with very limited flow (probably less than 5 gpm) the bottom of the tank never heated.


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## Gooserider (Nov 8, 2009)

Not a Garn expert, never seen the inside of one, but I don't understand why the Garn would work any different from other storage setups, where mixing is generally considered undesirable...    I would also think it best to get the hottest supply water from the top, and return the cooler water to the bottom, hopefully in a way that would discourage mixing...

It would seem that unless there is hardware inside the tank that would be in the way, that it should be possible to add an extension to the top intake pipe with minimal draining.  Just pull down low enough to get that connection above the water line, and pull the fitting.  Make a setup with an extension that you can screw in, then mount it in the hole.  I would probably go in the top manhole and try to rig up some support for the pipe as it goes by.

I'd probably try to make the extension long enough to almost reach the front of the tank, cap it, and drill a series of holes in it, possibly making the holes larger near the far end, such that the area of the holes was far greater than the area of the pipe.  Position the extension so that the holes are on the top side.   Alternatively, not put a cap on the end, and then you'd be pulling from the same general area as the front thermometer dial...

Either way you would be preventing any potential short circuiting, and hopefully pulling from the hottest portion of the tank, without inducing any added turbulence.

Gooserider


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## foxt (Nov 8, 2009)

leaddog said:
			
		

> The garn shouldn't work any different than our propane tanks!! we might have 180* on the top and 120* on the bottem and can still feed the baseboards with hot water.
> leaddog



That's what I thought would happen.  I get the top/bottom stratification; what is puzzling me is the variation that apparently sets up in at least a couple of GARNs front-back after the burn is done and the tank has been supplying heat for a few hours.  I bet that you don't see the same front-back variation in your propane tank after it is charged to temp and as it draws down on the stored BTUs.  When my GARN is behaving like any other storage tank and is drawing down the stored BTUs, not only does it set up top-bottom stratification, but there can be as much as a 20* delta between top front and top back of tank.  

That difference has to be due to something unique about the way that the GARN behaves when it is just a storage tank - most likely related to the presence/geometry of the burn chamber at the front of the tank?  Otherwise, like you said, it's just a big storage tank after the burn cycle is completed, and it should behave like one (no front-back variation).  What am I missing?

Tom


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## TCaldwell (Nov 8, 2009)

leaddog, i agree on the tank stratification, after the tank is fully charged. i think it takes alot of gpm for a short period of time[ length of burn] to fully charge the non pressureized tank .most time i wait till 135deg to fire, 2000 gal up to 200 deg. not sure that ceramic thermocouple cover will last, it looks like the probe tip is not protected and the lead wire not having a protective sheath, unless it is already outside of the boiler at that point. the other part that fails on thermocouples is the potting material  that covers the connection of the probe to the leedwires, needs to be temp rated. sometimes it is worth having a thermocouple built to your needs, not expensive, espically if the install is difficult


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## Rick Stanley (Nov 17, 2009)

Just "for what it's worth" info, thought it was interesting.....................................

 The fire had been out for 12 hrs, the pumps had been running as needed for 12hrs (continuous for last 2 hrs).

I calibrated my 2 temp gauges on supply and return of garn and the 2 on the hx (one in from garn one out to oil boiler). I adjusted all four so they read the same as a digital kitchen thermometer w/meat probe.

The gauge on front of garn said 168 2 hrs before any other readings were taken. After the pumps ran continuous for the 2 hours, it read 162,  so I was moving heat into the house. Warm and cozy.

Anyway, what's interesting is that with the garn/front gauge reading 162 (12" below water surface), i reached through the manway and stuck the meat probe in there and it read 171 (2" below surface 48" back from front gauge), the gauges in the supply AND return on the back of garn read 120, the two gauges in the house read 125 in from garn and 130 out to oilboiler. So I had reached the tipping point I guess and was backfeeding btu's the other way it seems. 

I have the oil burner disabled so I'm gonna just let the pumps run and see if I can EVER get he heat that appears to still be in the garn, the front of the garn.

So, Foxt, I'm more interested than ever in finding out what kind of submersible pump you dropped into your garn. I have a couple of those black plastic submersible sump pumps kicking around here. Is that what you used? wondering about the high temps???

Anyway, like I said, for what it's worth


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## foxt (Nov 17, 2009)

Rick,

The pump I dropped in there for that experiment was indeed one of those black submersible sump pumps.  IIRC, it was a Flotec 1300x, I think it's something like 1400gph max.  As  I reported, it equalized front/back temps within minutes.  Having said that, I am sure it also disrupted whatever vertical stratification existed, and I don't know if that is a good or bad thing, or if in the long run it hurts more than it helps.  

You're seeing  a much bigger delta between front/back (almost 40*) than I think I have ever seen on my WHS2000.  I think my max has been about 25*.  Can you confirm that when you are nearing the end of the burn cycle all of your temps are close (mine are)?  

As you said, and for me as well, this is all about getting the most out of that tank between burns.  I'm surprised that the more experienced GARN crowd hasn't been as bothered about this as me (and maybe you).  I keep wondering if it's more of an issue with the way that our systems are configured, or if we're operating under some flawed assumption that such a big difference between front and rear temps means we're not getting the most out of the GARN when it is in storage mode?

Tom


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## Jim K in PA (Nov 17, 2009)

Tom,

I have been following this thread with interest.  I have not noticed much of an issue with my system.  I am running a P/S piping arrangement right off the GARN.  The primary loop is all 1.5" black pipe, with ~ 7 elbows, and only about ~20 feet of pipe.  Have not calculated the head loss, but it ain't much.  My primary pump on that loop is a nice fat Grundfos UPS 43-44 3 speed unit running on low.  Even if I have 5-10 feet of headloss, she is still circulating 20-30 gpm.  Even if the GARN had no extension pipe into the return line (which it does), I doubt it would be short circuiting.  

Even without a continuously running pump, convection will eventually equalizes temps horizontally, while stratification takes place vertically.  I cannot fathom how you are getting 170 degree readings at the manway, and 120 degrees drawing off the top of the tank 4 feet away.  I am at a loss there.

I have not checked temps in the piping much this year since things have been running so well for me.  I re-piped my in-house distribution and included a temp/pressure guage just downstream of my circ pump in the house.  I rarely see much more than a 5-7 degree difference between the front temp gauge on the GARN and the temp gauge in the piping.  And there is a FP HX in between them, too.  

With outside temps at night still at or just above freezing, I am running my BB temps down to 130 degrees F quite comfortably.

Doubt this helps, but not sure what to offer.


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## foxt (Nov 17, 2009)

Jim,
   Thanks for that.  Just to clarify - the max front/back delta I see is about 25*, I think Rick reported that he saw over 40* this morning.  He and I both have a single loop off of the GARN feeding our distribution system via fphx.  You, and I think most other GARN owners, are p/s.  I keep thinking that's the difference that is contributing to this somehow.  Maybe we dump a higher volume of cooler return water into the tank at a slower rate than you do, and perhaps that's driving it.  The configuration of the tank internals at the front might be contributing as well, somehow trapping the warmer water up near the front gauge.  At one point, I thought that the actual "hot spot" at the front might be extremely localized  (as in just right there at the gauge), but Rick's analysis this morning tends to indicate that this is not the case.  
   I don't know what to do about it, other than resign myself to running the front temp down no lower than 140-150 (which supplies 120-130 at the rear), which just means that I might fire more frequently than I have to.  I don't think I want to run a circ in the tank because it will disrupt whatever vertical stratification I do have.  I could repipe to p/s, but I am not really sure it's worth a try (lots of work at the start of the heating season for a fix that might not really correct the situation).  
   So, my last hope was that there were others out there besides Rick and I that have seen this and knew why it happens, and could provide enough info upon which I could decide a course of action.  The folks who have responded have been great, don't get me wrong, I'm just surprised that there isn't anyone else besides Rick that has seen this ....

Tom


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## heaterman (Nov 18, 2009)

I don't have an answer other than to say you should measure the temps at your different locations with the same thermometer to ensure an accurate and meaningful reading. There's too much variation in different types of thermometers to give you a reliable indication of what is really going on. 

Now....... I spoke with Martin regarding tank turnover during firing as well as stratification during the "off" cycle. He told me their testing has shown the entire tank will turn over in a matter of minutes once even a moderate fire is established. Their testing regimen uses 8 type T thermocouples (accurate to within 1/2*)  spaced from top to bottom They originally built the 3200 with an integral circ thinking that the big tank would need help equalizing temp during the burn cycle. That proved to be false. The tanks turn over far more quickly than you can pump even at 40-50GPM.

As to vertical stratification during the off cycle he said a lot depends on flow rate and temp drop the heat emitters are able to generate. They have seen units with very low flow rates (<6gpm) develop thermoclines of nearly 60* within just a couple inches. Water above the hot/cold interface could be 160* and below it only 100-120*. I have seen this phenomena in radiant floor systems with a mixing valve in place and now that I think about, on one that we did that had a section of snowmelt incorporated in the design. Water coming back to the Garn was less than 90* IIRC. Interestingly, when the owner would fire that particular unit you could see the tank "blend" as the fire became established. The temp reading would be at 120-130 at the start of the burn and within 5-10 minute it would drop 10-15* then start to come back up. This is because the colder water on the bottom was being mixed with the hot stratified at the top.  Same thing happens when you start one up with cold (<70*) water in it. They seem like they sit there forever and all of a sudden the temp starts to climb.

He also said that they used to hear occasionally of some units/systems with front to back variation in models prior to when they added the return "injection" pipe but never to the extent you are describing. Maybe 4-5* at most. He suspected that the variation was coming from different types of thermometers and suggested that you use the same one everywhere in order to get a valid set of data.


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## Rick Stanley (Nov 18, 2009)

I let my pumps run all day. When I got home I had 120 coming into the house and the front garn gauge was just below 140. I built a fire and just checked it after 20 minutes of strong burning (stack temps over 400*) and I show almost EXACTLY 144 on both gauges.


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## nt3041 (Nov 18, 2009)

Hi;

I have a Tekmar 150 installed in a well next to the Garn supplied gauge. The 150 is read in my kitchen area about 60 feet from the boiler. Both read within one degree of each other. My main circulator operates on demand,ie on a call for space heat or DHW.

Regards


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## foxt (Nov 18, 2009)

Thanks for the replies.

I am pretty sure that the variation isn't due to the different temp gauges/devices - as Rick reported, I too see front/back within 1-2* of each other at some stage of a burn and for a while thereafter.  The differential only develops after  btu's  stored in the tank have been drawn down for a while.

 For grins, I measured the elevation of the front tap for the gauge, and the rear top supply.  The front is maybe 5 inches higher up the tank (as near as I can tell, I'm not able to get to the actual tap at the rear without some work, so I am measuring the black iron pipe that runs horizontally off of it for a while).

Martin (and Steve, I think), reports that there can be huge temp differences at, or even within a couple of inches of, the cold/hot vertical barrier that develops over time.  So can Rick and I just be seeing this thermocline in action as it gets to the top levels of the tank (front gauge is in the hot layer, rear supply is in the cold)? And might that dramatic vertical differential be due to presumably lower flow rates in our configuration vs. the flow in the p/s plumbed installs?  Since the supply is slightly below the front gauge, and there's what, another foot of water above that? is it possible that I'm not effectively pulling heat from that uppermost layer of water?

If so, that would beg several questions:

1. do the p/s plumbed units also see such a strikingly thin hold/cold interface layer at some point?  From the feedback we've seen here, it doesn't sound like you can observe it at the top level of the tank, but does it develp down lower?  Or is it less of a dramatic discontinuity?
2. presumably that layer rises through the tank as the btu's are drawn down?  does it rise faster or slower in a p/s system (assuming it develops there at all) vs. a single loop system?
3. all other things being equal, which  config yields more usable heat between burns?

These may be up for guessing vs. observable, but if the answers pointed to p/s providing a longer storage cycle as compared to single loop, I'd bite the bullet and replumb.  Or do I increase the flow rate of my circ to try and pull water from that top hot layer?

Tom


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## allan (Nov 18, 2009)

I piped my boiler so that I pull the supply water off of the back of the boiler near the top of the tank and I piped my return water to the front of the boiler about mid way up the tank. My tank is 10.5 feet long by 7 feet in diameter. I notice something different then what you are experiancing, my gauge will read say 195 after a burn, but my supply water is actually reading something over 200 degrees. My supply water is always hotter then my gauge on the front of the boiler. My supply line is maybe 10-inches higher then my temp gauge in the tank. My unit stores about 3000 gallons of water and so far I'm lossing about 25-30 degrees in 24 hours while heating my 2500 SF house and heating my domestic hotwater for 5 people. I think that if you would take your supply from the rear of the boiler near the top and return the water to the front of the boiler closer to the bottom, you should get good mixing  of your tank. I'm using a B&G Pl-36 pump that runs 24/7. When there is no load on the heat exchanger, I have about 1 degree loss between the supply and return water at the boiler which is about 85 feet from the house. When I have a load on the heat exchanger, I have about a 10-15 degree delta between the supply and return line.


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## brad068 (Nov 18, 2009)

You know guys, the garn heat exchange tubes are set in the tank at different levels and two passes are right close to the bottom. With that in mind that setup is the same as if you were charging a tank with an intank exchanger which most that I've seen are set close to the bottom to get maximum stratification. This is the same setup only using flue gas instead of water so why wouldn't the tank turn itself over naturally during firing?

One thing that I noticed when I switched my supply return around is that I get hot water sooner during the burn, but now that I have a bigger circ running parallel with my inducer timer the hot water isn't as "soon" as it was.


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## heaterman (Nov 18, 2009)

Garnification said:
			
		

> You know guys, the garn heat exchange tubes are set in the tank at different levels and two passes are right close to the bottom. With that in mind that setup is the same as if you were charging a tank with an intank exchanger which most that I've seen are set close to the bottom to get maximum stratification. This is the same setup only using flue gas instead of water so why wouldn't the tank turn itself over naturally during firing?
> 
> One thing that I noticed when I switched my supply return around is that I get hot water sooner during the burn, but now that I have a bigger circ running parallel with my inducer timer the hot water isn't as "soon" as it was.



I was wondering how that was working out for you. So at present I assume you are supplying from the bottom tapping, returning on the top?


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## Rick Stanley (Nov 18, 2009)

would a rig like this help with taking temps at different places on a heating system?
http://www.amazon.com/gp/product/images/B0017L9Q9C/ref=dp_otherviews_z_5?ie=UTF8&s=hi&img=5


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## brad068 (Nov 18, 2009)

heaterman said:
			
		

> Garnification said:
> 
> 
> 
> ...



 Presently, supply from top.  Last year I supplied from the bottom.


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## heaterman (Nov 21, 2009)

Hey Fox!!

I was at my favorite farmers place all day Thursday and Friday and I think I observed what you are talking about. This guy has two very abused 2000's which are piped together via a common manifold. They were an absolute mess and we were cleaning them inside and out. We drained, power washed and flushed one, filled it and fired it up to 200 in the morning and were doing the same with the other one while the single boiler kept everything heated in the barn. I would say that we had the first unit up to 200 by 1PM and it was sitting there with a 300,000 btu load on it for about 3 hours after that when I looked at the temp gauge. It still read 188-190* and I'm thinking...........we just took about 8-900,000 btu's of heat from this thing and it still shows almost 190*. So I got out the Fluke temp probe and started checking. I went to the barn and checked supply water temp going to the indirect tank and sure enough, it read 146.4*. Went back to the Garn Barn about 100' from the mechanical room and checked the supply on the manifold (2" steel) 146.8*. Then I took the infra red and shot the back of the Garn (we had the insulation off) and starting at the bottom took measurements about every 6-8". They were as follows from the return port up.  135, 135, 137, 138, 140, 140, 143, a couple inches below the supply port I hit 146, a couple inches above it the temp read 163 and 6" above it read 187.  Sounds like what you are seeing.
 I would call that pretty good stratification. I was interesting to see that the temp gouge on the Garn hardly budged during that time period with that size load on it and that's what got me thinking about your scenario. Now here's the wild thing about the stratification and it matches what Martin told me about tank blending when there's a fire in the Garn. I stoked it back up again with a good size load of wood and kept an eye on the tank thermometer. About 5 minutes into the burn the temp gauge still read 178-179 but while I didn't actually see it happen about 15 minutes later the gauge read 150. Either the return water hit the thermometer or else the tank turned over. The temp gauge stayed at 150 for about a half hour with a raging load of wood in it before it started to climb again. I would guess that it took that long for the entire tank to blend plus that fact that the beast was still carrying a good size heating load. 
I don't know if that matches what you're seeing but it was interesting to see that stratification in action.

Now, you should know that we have a 1/2hp Taco 1400-70 circ in the main loop, which according to Taco's spec's and my head calculations should be cranking  close to 45gpm through the Garn. Definitely not a low flow situation.


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## foxt (Nov 22, 2009)

Hey heaterman,

   As always, so glad you are here!  I think you have now seen what I have seen, and what Rick may see.  What is curious about this is that others have not seen it, or is that just because they haven't been looking for it?   Or maybe they fire before the thermocline reaches the supply port?   If we are right and can attribute the front/rear differential to the existence of a very thin boundary between hot and cold water that moves up the tank as btu's are drawn down long after a burn, a couple of things come to mind:

1) I assume this very sharply defined stratification is a good thing, right up until the moment that layer passes the supply port?  In other words, I shouldn't keep the tank mixed during storage mode?

2) Is there any way to get to the btu's stored in the upper reaches of the tank once the thermocline gets above the supply port?  Or is that warmer water just going to sit there and taunt me?

3) Since the tank mixes very well during a burn, does it matter where that tank gauge is mounted?  Can I drop it below the supply port, maybe off of one of the bungs for the electrical elements?  I want to be able to measure the temp in the tank so that I don't overfire, but I also want to reliably know when the tank needs to be refired, ideally when that thermocline hits the supply port?

4) my root problem is that my GARN circ is controlled by a temp sensor at the front bung next to the temp gauge - since we are pretty sure that, at least in my case and now maybe in your farmer friend's case,  this location will continue to report temps above my set-point even though the temps at the supply port can be 25* lower or even more, any advice as to how to work around this?

5) and finally, if this condition is directly related to configs that are not p/s (not quite sure how your farmer's setup if plumbed), do I replumb to a p/s or not bother?

Tom


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## heaterman (Nov 22, 2009)

foxt said:
			
		

> Hey heaterman,
> 
> As always, so glad you are here!  I think you have now seen what I have seen, and what Rick may see.  What is curious about this is that others have not seen it, or is that just because they haven't been looking for it?   Or maybe they fire before the thermocline reaches the supply port?   If we are right and can attribute the front/rear differential to the existence of a very thin boundary between hot and cold water that moves up the tank as btu's are drawn down long after a burn, a couple of things come to mind:
> 
> ...



1) I assume this very sharply defined stratification is a good thing, right up until the moment that layer passes the supply port?  In other words, I shouldn't keep the tank mixed during storage mode?

I don't think a person could keep the tank blended with less than 80-100gpm of flow. Good stratification will keep the temp supplied to your system consistent until the cooler water reaches the supply port. Then it's time to fire

2) Is there any way to get to the btu's stored in the upper reaches of the tank once the thermocline gets above the supply port?  Or is that warmer water just going to sit there and taunt me?

There is no way  to get at that last 100-150 gallons of water that I can see.  Other than welding in a new supply port at a higher location. I think that one would want a little cushion water level wise over the top of the port. If it was too close to the top of the water you might run into a situation where you would create a little whirlpool and start sucking air into the line.

3) Since the tank mixes very well during a burn, does it matter where that tank gauge is mounted?  Can I drop it below the supply port, maybe off of one of the bungs for the electrical elements?  I want to be able to measure the temp in the tank so that I don't overfire, but I also want to reliably know when the tank needs to be refired, ideally when that thermocline hits the supply port?

I think I'd leave that one where it is and put an additional one at a lower point if you wanted to track the thermocline. If your only gauge is at a low point you won't be able to tell how much hot water you have left.

4) my root problem is that my GARN circ is controlled by a temp sensor at the front bung next to the temp gauge - since we are pretty sure that, at least in my case and now maybe in your farmer friend's case,  this location will continue to report temps above my set-point even though the temps at the supply port can be 25* lower or even more, any advice as to how to work around this?

If you could enlighten me as to why you are controlling the Garn circ in this manner maybe I can offer a suggestion.  Shooting from the hip, I'd say get a strap on aquastat and install it on the supply pipe.

5) and finally, if this condition is directly related to configs that are not p/s (not quite sure how your farmer's setup if plumbed), do I replumb to a p/s or not bother?

The condition is related to the fact that the Garn stratifies very well. Piping configuration would have virtually no effect on it that I can envision.

Now, assuming that your Garn circ is in a main loop feeding a heat exchanger, does the aquastat near the thermometer location kill the pump at low temps and if so why?


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## foxt (Nov 22, 2009)

heaterman said:
			
		

> If you could enlighten me as to why you are controlling the Garn circ in this manner maybe I can offer a suggestion.  Shooting from the hip, I'd say get a strap on aquastat and install it on the supply pipe.





			
				heaterman said:
			
		

> Now, assuming that your Garn circ is in a main loop feeding a heat exchanger, does the aquastat near the thermometer location kill the pump at low temps and if so why?


Not sure I understand this last one, but I'll explain what I have and maybe that will answer both or your questions ...

GARN circ supplies fphx that's plumbed in to my original manifold distribution system in series with backup fossil boiler.   When the tekmar controller in the distribution system decides that my manifold has a boiler demand, I use the sensor at the front of the GARN to determine if the GARN circ should come on, or if the oil burner should fire.  When I first set this all up, I ran with the GARN circ setpoint at about 125* above/at which the GARN circ is enabled (I can use water down to 120* in the house, it's all radiant floor).  With a setpoint at 125*, and with the thermocline, the supply temps from the GARN would actually fall below 100*.   Over the course of maybe a week, I kept boosting the setpoint until I discovered that the lowest I could go was 150* to maintain supply temps above 125*.

Since it does indeed work this way,  and since there doesn't appear to be a way to get the remaining btus out of the tank, maybe I should just leave it alone.  On the other hand, I would like to control the system a little bit more directly (i.e. base the control on actual supply temps from the rear vs. this empirically derived value from the front).  I thought about just strapping the sensor to the supply pipe, but when the circ has not been running for a while, won't that give me a false low reading - or does all of that steel/iron conduct heat well enough that if I can attach the sensor to within an inch of the back of the tank on the supply pipe I'm good to go?




			
				heaterman said:
			
		

> If your only gauge is at a low point you won’t be able to tell how much hot water you have left.


Actually, I guess the only temp that will really matter is the temp at the supply port, right?  When that's below my setpoint, I fire, and when it's above 190* I don't.  Makes the front gauge seem kind of useless, no?  I wonder why that bung is not at the exact same elevation as the supply port?



			
				heaterman said:
			
		

> The condition is related to the fact that the Garn stratifies very well. Piping configuration would have virtually no effect on it that I can envision.



This is the part that still confuses me - so there's a pretty good chance that other GARNs are doing this, but folks haven't noticed?

Tom


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## jebatty (Nov 22, 2009)

Jumping on to monitor. Working with an organization with the 3200, and actual supply temps are a real issue, so this is interesting. Also, fphx installed to handle 950,000 btuh with a 30F temperature difference. This concerns me, thinking that it should have been sized at a 20F temperature difference. F/B temperature in the Garn may impact this for getting usable heat into the system.


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## Gooserider (Nov 22, 2009)

Speculating a bit, but the reason they might put the front gage lower than the supply port could be to give you some "inertia" by getting you to fire while the tank is still hot enough to be supplying acceptably hot water at the supply port...  The sloping thermocline seems to screw that up, but it seems like a reasonable idea in theory.

As to temperature monitoring in general, my understanding is that a strap-on aquastat or other sensor will pretty much track the temperature of the water going through the pipe it's strapped to, as long as it's reasonably well insulated...

If I was wanting to monitor the temperature of the tank itself, I would try to get a sensor attached to the wall of the tank itself...  If I was feeling really ambitious, what I'd think would really be neat would be a few strings of DOW (Dallas One Wire) temp sensors attached to the tank in different places with thermal epoxy.  Depending on how many strings one ran, and how many sensors were used, it would be possible to do a very precise map of the tank showing just where the thermocline was, and the various temperature distributions in the tank...

Gooserider


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## foxt (Nov 22, 2009)

Gooserider said:
			
		

> Speculating a bit, but the reason they might put the front gage lower than the supply port  ....



Just a note - front gauge is higher than the supply port.  If you use the front gauge to determine to fire, you may actually be firing too late if the thermocline is at the lower supply port ...

Tom


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## Gooserider (Nov 22, 2009)

foxt said:
			
		

> Gooserider said:
> 
> 
> 
> ...


OOPS!!   :red:  Me bad....

Gooserider


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## Jim K in PA (Nov 23, 2009)

Interesting.  I would think that with Steve's customer's high-flow setup that there would be enough mixing to disturb that pocket of warm water up at the front wall temp gauge.  Apparently not!  That front bung is about as far away from the return bung as you can get.

Jim - your project with the 3200 - keep in mind that like any storage system, you are NOT going to have a constant supply temp like you would have in a constant firing appliance like an oil or gas furnace.  You can set up a mixing valve to reduce output temp to something below the max temp, but that only works until your tank drops below that point.

I think the lesson here is that no matter what piping arrangement you have, circulator controls should be based on temps in the *PIPING*, not on what the temps are at the front gauge or in a well attached to the tank.  I have a temp well installed in the second bung on my WHS2000, but I only use that for remote temperature display.  I have Tees with temp wells installed in the primary piping for (as yet unused) control circuits.


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## heaterman (Nov 23, 2009)

Jim K in PA said:
			
		

> Interesting.  I would think that with Steve's customer's high-flow setup that there would be enough mixing to disturb that pocket of warm water up at the front wall temp gauge.  Apparently not!  That front bung is about as far away from the return bung as you can get.
> 
> Jim - your project with the 3200 - keep in mind that like any storage system, you are NOT going to have a constant supply temp like you would have in a constant firing appliance like an oil or gas furnace.  You can set up a mixing valve to reduce output temp to something below the max temp, but that only works until your tank drops below that point.
> 
> I think the lesson here is that no matter what piping arrangement you have, circulator controls should be based on temps in the *PIPING*, not on what the temps are at the front gauge or in a well attached to the tank.  I have a temp well installed in the second bung on my WHS2000, but I only use that for remote temperature display.  I have Tees with temp wells installed in the primary piping for (as yet unused) control circuits.



I thought it would upset the apple cart a bit also Jim. That 1400 is moving some serious GPM through the system and the thermocline in the Garn seems oblivious to it as the temp readings from bottom to top showed. I was really surprised that more blending wasn"t taking place.

I'll ditto the comment regarding the control setup and system being designed to work with variable water temps coming from any type of storage. That's why the method of heat transfer is just as important as the unit supplying the heat. The lower a person can use the better.


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## foxt (Nov 23, 2009)

Jim K in PA said:
			
		

> ... circulator controls should be based on temps in the *PIPING*, not on what the temps are at the front gauge or in a well attached to the tank.



Jim,
   Thanks for that, but I have a question: my piping is not p/s, but rather a single loop that feeds the house via fphx directly; if I put the circ control sensor on or in the supply piping, won't I see a drop in indicated supply temps when the system is idle because the water in that pipe is cooling down?  Or is there enough conduction between the water in the tank at the supply port, and water in the supply pipe say 24" away?  How close to the supply port itself do I need to get with that sensor?  

If I had known that the front of the tank is an unreliable location for a temp sensor (who woulda thunk it, especially since the Dectra folks supply you with a temp gauge for that same position), I would have rigged it so that I could put a deep thermowell right into the tank at the supply port and be done with it.  Next time ....

In addition to the other options suggested by folks (1-wire, etc), another option I am considering is to submerge a sensor in the tank, at the manhole, at the same elevation as the supply port.  I recall reading the details on how to accomplish what I need somewhere on this forum ...

Tom


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## heaterman (Nov 23, 2009)

foxt said:
			
		

> Jim K in PA said:
> 
> 
> 
> ...



Ideally, the best location for that sensor would be to install a Tee right on the supply port and use a well in either the side or the bull of it depending on how your particular piping dictates.


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## foxt (Nov 23, 2009)

Ok,  thanks for that.  That's what I expected to be the right way to do it, and probably would have done from the start if I had known I shouldn't use the front tap.  Those two easily accessible  front taps and the Dectra-supplied gauge for that location lulled me into just going with what seemed to be the intent and using that location for the control sensor.

Given where I am at, I think I'm inclined to live with it the way it is  until the end of this heating season.  I was hoping I could avoid draining down the tank and the little bit of repiping that would be required.  Current setup is not ideal, but at least now I know what is going on and that there's nothing I can do to draw down those last btu's above the supply port.  

If you get a chance, I would be interested in Martin's thoughts on the location of that front temp gauge.  Is it placed in that particular spot vs any other spot on the front wall for any reason other than easy line of site?  Given our exchange here over the past few days, would he be inclined to relocate it so that it's at the same elevation as the supply, or did he consider that and reject it for some reason?  I know that the design has been refined over the years, and I assume that at this point everything has a reason for the way it's designed ...

Tom


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## Jim K in PA (Nov 23, 2009)

Tom - Getting at those "last" Btu's may not be worth the effort.  Given Steve's estimate that the layer above the upper port is only about 200 gallons or so, even with a delta T of 60, you only have about 100k Btus there to draw down.  Depending on your heat load, that may give you a coupla-three more hours of draw time, but that's about it.

You also may not need to change the piping next year.  Live with the system for this season, and then see how comfortable you get with it.  Honestly, the only thing you really need to do is get rid of the 24/7 pumping.  Use a Zone Valve Controller (assuming you have ZVs) with an end switch and have the pump turn on when a zone calls for heat.  Mine works fantastic this way.  Pretty simple too, and you do not need a temp sensor in the tank, piping, or anywhere else.  Just use the front temp gauge for reference when determining if a burn is needed.


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## heaterman (Nov 23, 2009)

foxt said:
			
		

> Ok,  thanks for that.  That's what I expected to be the right way to do it, and probably would have done from the start if I had known I shouldn't use the front tap.  Those two easily accessible  front taps and the Dectra-supplied gauge for that location lulled me into just going with what seemed to be the intent and using that location for the control sensor.
> 
> Given where I am at, I think I'm inclined to live with it the way it is  until the end of this heating season.  I was hoping I could avoid draining down the tank and the little bit of repiping that would be required.  Current setup is not ideal, but at least now I know what is going on and that there's nothing I can do to draw down those last btu's above the supply port.
> 
> ...



I doubt if there will be any change in future model in regard to the location of that tapping. I'm guessing the new control being worked on right now is going to either eliminate it or make it a moot point. Just guessing but based on what we talked about this past spring in Minnesota, that's my gut feeling. 
I have no idea if there is a specific reason for the current location that is operation related. Personally, I tell my customers that it's just there for reference and not meant to be anything other than a rough idea of where your tank temp is.

Jim brings up a good point and that is to go the winter with it and learn a few more things about your system and the garn before you make any changes. There are many wrong ways to do hydronic piping and control but there are also many different ways that will work very nicely.


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## RowCropRenegade (Nov 24, 2009)

I can't wait to get into piping.  

Brings up a thought, how about two supply ports.  one front, one back?  An extra 100k of btu is at least one more standby hour.


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## foxt (Nov 24, 2009)

Jim K in PA said:
			
		

> ... the only thing you really need to do is get rid of the 24/7 pumping.


Whoops Jim, I'm not pumping 24/7 in the GARN loop.  The tekmar that runs my distribution system decides when to generate a boiler demand.  When it does so, the aquastat on the front of the GARN determines whether the demand can be met by the GARN, or if the backup fossil boiler needs to fire.  If the GARN temp is above setpoint (which is really set 30* above my actual setpoint due to this thermocline thing), the GARN circ is enabled upon a boiler demand, otherwise the oil burner is enabled.  I would imagine that this is the logic behind many controls that include a backup boiler?




			
				Jim K in PA said:
			
		

> Just use the front temp gauge for reference when determining if a burn is needed.





			
				heaterman said:
			
		

> I have no idea if there is a specific reason for the current location that is operation related. Personally, I tell my customers that it's just there for reference and not meant to be anything other than a rough idea of where your tank temp is.



I don't think I will use the front gauge to determine when to fire.  It's reporting the water temp at the end of the probe, period.  I agree that, during a burn when the tank is mixing well, it does give a fair representation of the tank temp.  So I will use it to ensure that I don't OVER fire (coming up on the last half of a burn, if the front gauge is at 190* or higher I won't put in another half load of wood).   However, the actual supply temp is what I really need to use to determine when to fire.


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## heaterman (Nov 24, 2009)

_I don’t think I will use the front gauge to determine when to fire.  It’s reporting the water temp at the end of the probe, period.  I agree that, during a burn when the tank is mixing well, it does give a fair representation of the tank temp.  So I will use it to ensure that I don’t OVER fire (coming up on the last half of a burn, if the front gauge is at 190* or higher I won’t put in another half load of wood).  However, the actual supply temp is what I really need to use to determine when to fire. _

And there in lies the reason to use a tee at the outlet port of the Garn for the sensing location. In all honesty, it would be impossible for Garn or any manufacturer to cover all the different ways and methods of installation their product could be connected in the field. There are just too many variations in systems and control strategies. So, one does it by trial and error (like me) or you rely on or hire and import the expertise and knowledge from someone else's head.
Looking through the ongoing description of your system here, I don't think you did to bad at all.  You just have a slight mismatch of control vs product function parameters which can be corrected.


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## foxt (Nov 24, 2009)

Thanks for that Steve.  I'm appreciative of all of the help from you and others on this forum.

I don't want to give anyone the wrong impression - I am very happy with the GARN and the performance of the system.  This temperature issue has been the only question mark, more so because the reaction I was getting was that I was seeing something unique to my system.  Now that we've hashed it out and have a theory about what is going on, I am content.  I'll spend some time taking some more measurements/observations, especially since I am curious to see how long it takes that thermocline to develop and rise to the supply port.

And now that this is all captured in this thread, hopefully others who are in the research or design phase of their project will benefit from this ...

Tom


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## Jim K in PA (Nov 24, 2009)

foxt said:
			
		

> Jim K in PA said:
> 
> 
> 
> ...



Hmm.  Sorry- I looked back at your first post in this thread from January and you stated that the pump was running 24/7, and I did not recall that you had changed it.

Not sure which Tekmar you are using, and I am not fully versed in all their equipment, but I recall that some of their controls have a means of starting the pumps for a pre-prpgrammed run time in order to get stagnant water out of a line.  If you use a strap on sensor on the supply piping and let the Tekmar run the pumps for 30 seconds before going to logic control, you will have actual water temp reading to then determine if the oil furnace kicks on.  Just trying to save you the work of draining and adding the Tee with the temp well.

My backup system is simple, but not automatic.  I have two valves and a toggle switch to throw, and I am back to oil heat.  After going through a season I am now much less worried about/desirous of having an automatic switchover.  However, I will be incorporating such an automatic backup system to directly heat the GARN via propane once I add a greenhouse to the system.


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## foxt (Nov 24, 2009)

Jim K in PA said:
			
		

> If you use a strap on sensor on the supply piping and let the Tekmar run the pumps for 30 seconds before going to logic control, you will have actual water temp reading to then determine if the oil furnace kicks on.  Just trying to save you the work of draining and adding the Tee with the temp well.



That's a great idea.  I picked up a tekmar controller to dedicate to variable speed control of the GARN circ based on deltaT at the fphx, but ditched that when I ran into the thermocline issue.  It's been sitting on a shelf with the hope of one day going back into service.  I recall that it also had this purge feature, and may use it for that instead of the variable speed.  Have to think on that one for a while. Thanks for the idea ... 



			
				Jim K in PA said:
			
		

> My backup system is simple, but not automatic.  I have two valves and a toggle switch to throw, and I am back to oil heat.  After going through a season I am now much less worried about/desirous of having an automatic switchover.  However, I will be incorporating such an automatic backup system to directly heat the GARN via propane once I add a greenhouse to the system.



Automatic backup was a big deal for me.   I bet there were half a dozen times last winter when the backup kicked in for me, and I wouldn't have planned ahead for it.   But I was learning, and only had a half season to play with it.  Or maybe I just go skiing for the weekend more than you do  We run on the GARN the first day or so that we're gone, and then the oil kicks in until we get home.  Haven't been away for a long enough stretch where the GARN really cools down, but I've been thinking that I should look into the pool-heater backup option and ditch the oil backup.  Not to hijack my own thread, but which propane-based backup were you considering?

Tom


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## Gooserider (Nov 24, 2009)

Jim K in PA said:
			
		

> foxt said:
> 
> 
> 
> ...



No reason you can't add a backup system to heat the Garn directly, but IMHO it's a really bad idea...  The only reason a Garn or other boiler + storage setup uses that big tank of water is to deal with the difference in the intermittent and variable heating demands, and the short term, steady and high heat output of a solid fuel fire.  A fossil burner can turn itself on and off rapidly to deal with the difference, so in most cases it doesn't need or benefit from storage.  If one does the backup to heat the storage, then in order to deliver heat to the load, a large amount of energy has to be spent in bringing the storage up to temp, or keeping it there, before any heat reaches the load.

Consider the case where you get delayed for some reason, and don't get around to building a new fire for 2-3 hours after the Garn dropped below it's minimum working point - The backup will come on and work at heating all that storage mass back up to the minimum working point, for only a few minutes of actual heat delivery.  If you had the backup set to deliver the heat directly to the load, then the backup only runs enough to keep the load warm...  

This is especially the case in milder weather - presumably the backup control would need to turn on the heat to the Garn as soon as the Garn dropped below the minimum, just so that the Garn could supply loads as soon as they called.  But in mild weather they might not even call in a two-three hour period, so you would have heated the Garn for no benefit, where if you had been doing direct heating of the load, the backup would never have fired at all...

I really can't see ANY circumstance where there would be benefit to having the fossil backup warming the Garn instead of going directly to the load.

I think the best approach is to plumb the backup in parallel with the Garn, so that one or the other supplies heat to the load, but not to the other boiler.  (Possibly there might be a need to do a "last ditch" backup circuit to allow the backup to supply a small amount of heat to the Garn if needed to prevent it from freezing - but that should almost never be needed considering how long it would take that much water to cool...)

Gooserider


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## Jim K in PA (Nov 24, 2009)

Gooserider said:
			
		

> No reason you can't add a backup system to heat the Garn directly, but IMHO it's a really bad idea...



Goose - with all due respect, you are making assumptions about a system that has not been built yet, nor which I have provided specific piping or heat flow details about.  Based on my single sentence, your reaction may be understandable, albeit a bit over the top.  Rest assured, I will NOT try and heat the entire storage mass of the GARN before or during a heat call event via a propane fired backup source.  The capacity of the supplemental propane unit I will be using far exceeds the demand anticipated.  The loads will be satisfied first, and then "excess" heat will be sent to the storage tank, but only during a call for heat, and/or to protect the GARN from freezing.  Without completely hijacking Tom's thread, the control logic will be such that temp as well as dwell time will be considered before the actuation of the backup heat source.  It will not be an accidental or occasional function.

Tom - you are correct, I am no skier! :bug: That is not my idea of fun.  We rarely go away during the winter.  I will post up a description of my propane pool heater backup in a seperate thread at another time.


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