# Thermal storage & controls



## Jersey Bill (Feb 4, 2008)

Hi folks,
I am helping out my neighbor on a project. He doing a major remodel of his house and he wants a wood boiler and thermal storage. The house heat will be mostly staple up radiant. There is no room for an indoor tank, but we were thinking about sinking one in the ground in his crawl space (new construction).
First ideas were either field fabricated concrete, or prefab. A 1000 gallon precast "septic" tank is about $1000 delivered. (they will drop it in a hole @ delivery).  Then it has to be insulated and needs some heat-x coils. 

I just spoke with STSS. They recomend 2- 180 ft 3/4" coils, plus a DHW preheat coil. The 180 ft coils are $735 ea, and the dhw coil is $432. (ouch).  I am getting a price on copper, but I dont think that I can beat that price.

Next, I have to figure out how to control it. Usually I build control panels with a Tekmar controller. Tekmar makes a "solar storage" control, but its $700.  STSS uses an EMC control, for about $160 which thay claim can be integrated to another system. I am getting info on it and will check it out.

BUT, in general how are these things controlled? I can envision a 3 way valve on the primary loop, either diverting the water thru the heat-x coils, or bypassing. Is this too simplistic? Do I need to MIX or will on/off be good enough. Also, Sven @ STSS mentioned that in "their" design , they charge from the top, and discharge from the bottom to facilitate stratification. Is this necessary?

Attached is the proposed system design. 

Thanks for your help.


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## Bob Rohr (Feb 4, 2008)

I like it! 

 A couple suggestions  I'd move the boilers side by side, at least on the primary loop connection.  Put the most important, highest temperature load next.  The Indirect tank would be my choice. 

 Simple controls could be.

A Taco pump relay box, with the DHW priority  feature.  This would drop all pumps off while the DHW recovers.  A solar controller, aka a differential controller, and a setpoint controller would watch and charge or unload the buffer.  The setpoint allows the buffer circ to start and chage the tank when the loop temperature, downstream of the second boiler connection increases to a setpoint.  I use 140F for my setting.

For unloading the tank the differential control allows the buffer circ to run whenever the tank is warmer than the primary loop temperature.  They are a bit more money but the digital solar controllers offer additional features like data logging and btu calculations.  Nice displays also.

With Siggy's software, or by hand you can calculate the temperature drop around the primary loop as various loads start or stop.

I will be visiting the folks at Resol next week, www.resol.de  They have some nice controls for solar with wood fired boiler backup.  Not sure if it could run two boilers in addition or instead of the solar.

I think you are on to what i consider the ideal piping arrangement.  A Grundfs Miximiser or any circ with the tekmar VS control makes for great boiler circs.  They speed up or slow down according to the return temperature to the boiler.  Makes more sense then a 3 way thermostatic and manual balance valve, in my mind.  You will see how sweet they work when you get it up and running.

 hr


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## Jersey Bill (Feb 5, 2008)

What was on that sheet was all controlled by the tekmar 364, including dhw priortiy.
I realize that the piping is less than ideal. The boilers are physically next to each other, but I also try to minimize piping in the boiler room by connecting where convienient. I will take a look and see what I can do about that.

I did some research and spoke w/ Sven of STSS about buying his heat-x coils for a 1000 gal tank. for a 150 mbh boiler, he recommends (2) 120 ft, 3/4" copper coils, about $700 each, and a al/pex coil for the dhw , for $400. He claims that most of the money is in the price of copper. I checked it out and he is right.

Their magic however is in the counter-flow heat exchanger, according to Swen. Since the tank stratifies, hot on top, you have to charge from the top, and discharge from the bottom. The diagram that they produce does this, but the problem is that it doesn't fit my system. I spent some time on this today and came up with a pump and a 4-way valve. See the attached diagram.

The coils deserve their own pump because thay need 15 gpm (total) @ 14 ft head. This is no small pump ! The 4 way valve reverses the flow depending....

1. If the wood boiler (stack?) is up to temp, and the main loop is up to temp, then turn on the storage pump and energise the valve for "store" mode
2. when the wood boiler goes below, de-energise the valve for discharge mode.
3. when the main loop goes below, shut off the storage pump

I have to work out how I am going to do the control. I wasn't planning on putting in a PLC. I will have to try relay logic first.

In this scheme, when the wood boiler finishes and the storage takes over, the storage pump is on all the time and the whole loop and the storage come down together. Since this is radiant heat, there will be a continous load (constant circulation)


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## Bob Rohr (Feb 5, 2008)

Is it the 120 foot coil that presents that 15 foot of head?  I know a HX design with rungs, like this, really drops the head but is more time consuming to build.

 hr


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## Jersey Bill (Feb 5, 2008)

yep, that one has a lot more parallel paths for the water so the drop is lower.  I wonder if straight tubing will cost less?
if i went to 4- 60 foot coils, the pressure drop would be about 2' across the coils, 4 ft with the valve to the pump @ 15 gpm. its still a pump bigger than an 007

I might have figured out how to simply control the storage tank pump and valve.
see attached 

T-1 is a thermostat, CR is a control relay

I haven't figured why this won't work yet. let me sleep on it.


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## jebatty (Feb 5, 2008)

I considered the STSS tank and also the concrete septic tank. Right now I have a used fuel oil tank that I got real cheap.  There was a note in another thread that STSS is having some issues as water temp approaches 180. Talk to them about this.

Did you consider a flat plate hx rather than copper coil? The hx described below cost about $200 new on Ebay, name brand.

I use a flat plate hx, 5 x 12 x 30 plate. Minimal resistance (Taco 007 on the tank loop and Taco 009 on the boiler loop, the 009 only because I had one) and high heat transfer. The hx may be installed very close to the boiler to reduce physical head and pipe resistance.

Ex 1: cold boiler start, "cold tank" -- boiler supply to hx 155, hx to boiler 130; tank to hx 80, hx supply to tank 130.

Ex 2: boiler up to op temp, warm tank -- boiler supply to hx 180, hx to boiler 160; tank to hx 120, hx supply to tank 173. At higher tank to hx temps, hx out to tank approaches temp of boiler supply to hx.

In each case about 6 GPM through hx to tank through a 50 micron filter (because tank was used and sloughs off some crud from time to time).


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## jebatty (Feb 5, 2008)

Correction of STSS - problems may occur at 170. See link:

https://www.hearth.com/econtent/index.php/forums/viewthread/14864/


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## Nofossil (Feb 5, 2008)

Jersey Bill, looking at your schematic I'm struck by how quickly things get complicated in a pump / loop based system. I've always shied away from that approach, partly because I quickly get confused about what's happening and partly because I've never understood the advantages of that approach. I humbly ask for enlightenment.

Being ignorant of all of this when I designed my system, I went for the simplest approach. In my setup, I simply have a pump for each heat source and a zone valve for each heat load. The pumps draw from a common cold return and pump to a common hot supply manifold. Since the tank is both a source and a supply, it has a pump and a zone valve in parallel. Flow during charging is then top to bottom, and flow during drawdown is bottom to top.

Only one pump is running at a time. Zone valves are controlled by thermostats. I use a more complex controller, but the zone valve for charging the storage tank could be controlled by an aquastat and a relay. This seems really simplistic compared to some of the approaches I've seen, including yours. 

What are the drawbacks to what I've done? What are the system benefits of having a primary loop with a pump for each load an source? I'm in a position where people ask me for advice, and I feel very inadequate at explaining this particular tradeoff.


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## termite (Feb 5, 2008)

Jersey Bill said:
			
		

> The 4 way valve reverses the flow depending....
> 
> 1. If the wood boiler (stack?) is up to temp, and the main loop is up to temp, then turn on the storage pump and energise the valve for "store" mode
> 2. when the wood boiler goes below, de-energise the valve for discharge mode.
> ...



Jersey Bill:
I have a similar setup for tank storage in operation now.  While the boiler is the heat source the mixing valve is set for storage and I use a Steca differential controller to run the pump that feeds the tank.  When the boiler is down the 4way is set to retrieve heat from the tank.  The tank pump is then operated when there is a call for heat.  I was using the boiler's circulator line for deciding if the boiler was up or down.  Now I'm using a combination of that and the controller's logic to decide when and what to do.  Its working very well.  I posted schematics of the piping and controls here https://www.hearth.com/econtent/index.php/forums/viewthread/12887/P15/.  I've modified the controls but the schematic shows how I'm using the differential contoller and valve.


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## Bob Rohr (Feb 5, 2008)

Jersey Bill said:
			
		

> yep, that one has a lot more parallel paths for the water so the drop is lower.  I wonder if straight tubing will cost less?
> if i went to 4- 60 foot coils, the pressure drop would be about 2' across the coils, 4 ft with the valve to the pump @ 15 gpm. its still a pump bigger than an 007
> 
> I might have figured out how to simply control the storage tank pump and valve.
> ...



Bill, that Grundfos 15-58 on speed 3 should cover you.  Grundfos also has 3 speed 26-99's out now.  The 15-58 is a bit cheaper, and less power consumption.  

I'll think about your ladder diagram.  it should be a simple solution.

On one of my trial and error plastic pallet tanks, I used a bunch of old copper fin tube baseboard element for a hx.  Tons of HX surface, easy to find.  The HX worked great, but the tank got very soft at 160F.  not worth the risk of a meltdown.

Even in an open system that fin tube should last many years.  AC condensors do, outside.  No reason you couldn't use galvanized, aluminum, maybe even schd 40 steel pipe to cut costs.

 hr

 hr


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## Jersey Bill (Feb 5, 2008)

nofossil, 
Did you call your system simple ?
I don't understand why you think that this system is complicated. The primary loop moves energy to/from all devices, either in or out. secondary loop devices all do their own thing. primary and secondary are isolated except for energy transfer. 
I use the primary/secondary for 2 basic reasons. 1- this is what I was taught by Siggy, through his books and classes. the main issue is to be able to match the pump and the load flow characteristic. for instance, the storage requires 15 gpm @ 15 ft of head. this would have to be added to another circulator if it didn't have its own circulator, and if we were not using storage, the pump would be oversized. 2-this method is easily implemented with a standard "off the shelf", no programming, controller, like the Tekmar. All the features that you had to program- outdoor reset, dhw priority, mixed loops, etc. was all canned by tekmar. If the controller blows up, gets fried by lightning, whatever, a replacement is easy to get and install. I do not know what it would take for you to replace your controller board. What would happen if you weren't around? (and it was cold). If you go to the Tekmar website, there is a list of recommended contractors that are familiar with the products, can get and install a replacement.

So you were able to optimize the piping, because you have unlimited control. This is similar to my home system where I have a PLC, and I have to maintain it. Luckily, I am in a business where these things are common, i know folks that can troubleshoot in the event that I am away. But when I design a system for a customer, custom controllers are generally ruled out because of complexity and price. 

Why dont you post your system diagram and we discuss the pros/cons.

Termite, I checked out your diagram. I haven't figured out what the 2nd 4-way valve does yet. Where did you get the 4-way? Tekmar was the only supplier that I found so far. I searched on the net and found only 1 other mfgr- Termomix (www.acaso.se) but I don't know where to get it in this country, and the website was no help.
I am reluctant to use the differential controller for the tank because it seems like overkill, and its $$ besides. These controllers are targeted for a solar system where the energy input might be finicky (night time, clouds, etc), when a wood boiler is on, its ON!


MOS (Master of sparks) - I don't usually use Grundfoss because of supply lines. Taco pumps are easily available here from my local plumbing supply. While speaking w/ STSS, he did mention the upper limit of 180 storage temp. This is the limit of their liner, but to get heat into a 180 deg tank, you have to supply 200 deg water. this is where they hit the wall.
As far as constructing the heat-x out of a different metal, its an interesting idea. I will have to think about it. threaded steel would really suck putting together. Why can't I use twice the length of PEX tubing?
I like that heat-x that you built. probably less copper that a 120 ft 3/4" coil. I will look into that. Those 1/2" vertical tubes will have less surface area than the 3/4" tube, but with the parallel paths, the residence time is long.

The STSS heat-x is nice, but is it a value for $1400? For half that I could probably get a plate heat-x, and 2 pumps. I would still need the 4-way valve to reverse direction in the tank. I would still go with their dhw coil because it will give a really good drawdown especially in summer.


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## Eric Johnson (Feb 5, 2008)

More like 3x the length with pex, Bill.

I think Honeywell makes a cast iron 4-way mixing valve for less than $100 for a 1 1/4-inch version, as I recall. You have to buy the actuator separately--something like another $70.


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## Jersey Bill (Feb 5, 2008)

no luck w/ honeywell. I called my sales guy, the biggest size 4-way valve they had was 1/4".
I also checked my catalog.
Do you have a honeywell model no?


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## Nofossil (Feb 5, 2008)

Jersey Bill said:
			
		

> nofossil,
> Did you call your system simple ?
> 
> Why dont you post your system diagram and we discuss the pros/cons.



I'm not attacking you! I'll be the first to admit that my system is anything but simple ;-)

However, the controller is not an integral part of the system. Everything except storage still works if the controller is shut off. You would have to manually select your heat source, and some advanced features like oil boiler heat scavenging wouldn't work. To get storage working, you'd need a couple of relays and an aquastat or two. I've even implemented heat scavenging using only thermal switches and relays. The controller is because I can, not because it's necessary. In fact, I didn't have it for the first year.

Like you, I started with the basic configuration that I have based on the advice of a professional. Different professionals give different advice! Hmm..... Ever since, I've been trying to get my head around the pros and cons of each approach. I'm NOT arguing that mine is superior, I'm just trying to figure out the advantages and disadvantages of each, and understand circumstances that would make one or the other a clear choice.

I understand the issue of flow rates for the heat loads - if you had a heat load that needed a flow rate radically different than what the primary circulator provided, especially if there was a lot of flow restriction. I assume that the primary loop pump has to be higher flow rate than any load, and that any heat load that absorbs a significant portion of the boiler output should be later in the loop.

Are there other considerations that would lead you to the pump-based approach?

I've posted my schematic a bunch of times. In consideration of all the people who are sick of seeing it, here's a link.

I'm genuinely interested in understanding this, both to improve my own system as well as the advice that others pry out of me. Thanks for any help on that path.


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## Eric Johnson (Feb 5, 2008)

Jersey Bill said:
			
		

> no luck w/ honeywell. I called my sales guy, the biggest size 4-way valve they had was 1/4".
> I also checked my catalog.
> Do you have a honeywell model no?



This is what I'm talking about, Bill. Not sure if it's what you're looking for, but hopefully so.

http://customer.honeywell.com/honeywell/ProductInfo.aspx/V5442N1031

http://www.partsguy.com/cgi-bin/PartsGuy/V5442N1031.html

Honeywell doesn't make it easy to locate their products. I searched their site in vain, but was able to dig up an old spec sheet that I downloaded last year to get the model number. Note that this is available in sizes up to 1.25 inches.


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## Bob Rohr (Feb 5, 2008)

Here is a flow reversal 4 way I tested.  the idea was Siggy's.  Paxton assembled the valve with off the shelf parts.  the valve itself is available from Paxton, Danfoss, Tekmar, and a few other hydronic specialty shops.  Most are re-badged ESBE brand 4 ways.

The control is a basic honeywell damper motor.  The mount and shaft adapter was simple.

The intent of this device was to fix excessivly long radiant loops.  Like the 1000 footers homeowners sometimes install   Every 15 minutes the flow changes direction to try to warm the loop from both ends.  I tested it in my shop, data logged it on some 800 foot loops.

Contact Leif at Paxton if you want to try his version.s  www.paxtoncorp.com

hr


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## termite (Feb 5, 2008)

Jersey Bill said:
			
		

> Termite, I checked out your diagram. I haven't figured out what the 2nd 4-way valve does yet. Where did you get the 4-way? Tekmar was the only supplier that I found so far. I searched on the net and found only 1 other mfgr- Termomix (www.acaso.se) but I don't know where to get it in this country, and the website was no help.
> I am reluctant to use the differential controller for the tank because it seems like overkill, and its $$ besides. These controllers are targeted for a solar system where the energy input might be finicky (night time, clouds, etc), when a wood boiler is on, its ON!


Jersey Bill:
2nd valve is for control of return temp to boiler loop.  The valves and actuators are ESBE 1 1/4".  I got them on ebay (where else?).  I can see how that might be a problem for a professional installer though.  While the Steca controller has the ability to deal with the nuances of solar supply I'm using it only as a differential controller for the time being.  My primary loop satisfies the heat load in the house prior to being available to the tank.  The controller seemed the simplest and cost most effective way to determine if the temperature of the return water would be useful in the tank and control a pump accordingly.  I think tekmar has a differential controller specifically for heat accumulation tanks.  I believe it is model 155.  That's what I wanted but the Steca was way cheaper.


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## Jersey Bill (Feb 5, 2008)

nofossil, I am sorry if I offended you in any way, as that was not my intention. I just thought that it was funny that you called your system simple, and even funnier out of context. 
I did look at your system diagram and I really liked it. I thought it was the best designed system that was not primary-secondary. I like that you minimized pumps and put the mixing valve on the DHW tank for more storage. It seems like all the bases were covered- minimum boiler flow, preventing excessive flow, and preventing backflow. and most important of all is that it is working well. But I dont see how it is much simpler than the primary-secondary system.  1 thing that makes your system easier is that you are only dealing with 1 supply temperature to your baseboard heat. And baseboards are really forgiving on temperature, flow rates, and pressure drops. Primary-secondary does look like overkill for your system.

The system would get very complex if you had to add a bunch of radiant, with 2 different supply temperatures.  That is why I prefer to use an off the shelf tekmar controller, like the 374, which is not only a control for the gas boiler and DHW, but also does partial reset on the boiler loop, and full reset on 2 more radiant loops, handling the set points and pump relays. 

Believe me that I am humbled with the knowledge and experience of the people on this board, and hope that I can contribute as much as some others.


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## EForest (Feb 6, 2008)

Eric Johnson said:
			
		

> Jersey Bill said:
> 
> 
> 
> ...



Hey All,
Check out this site http://www.blueridgecompany.com/ you'll find valves of every type including a 4 way Taco with outdoor reset and lots of controllers.
I'm still in the design stage of my system but plan to purchase from blueridge.
Best of luck!


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## Jersey Bill (Feb 7, 2008)

How do you guys paste in quotes from other posts?

I did not find those 4-way valves on the Blue ridge website, but the paxton corp had the full line.

Any way,
In the control schematic that I have shown before, in the storage discharge cycle, I will have constant circulation through the heat-x coils until the primary loop drops below the useable setpoint. 
Is this the way that its typically done? My concern is that the constant circulation will screw up the temperature stratification.

If I add another set point controller (now up to 3 just to control the storage) I can cycle the pump to "sip" hot water from the storage.
To make it more complicated, I can add a analog controller instead and a 4-way valve and mix in the hot water instead.

 I am really glad that I finally have a place to "air" these issues. In the past I would agonize over these kind of decisions for days. I have tried to get help from others, but could never find anyone who had even the slightest clue what I was talking about. Even on the RPA website, I would post something like this and get absolutely no response. So far, I know personally of NO other wood boiler or storage installations (except here).

Thanks for your help.


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## Eric Johnson (Feb 7, 2008)

If you hit the QUOTE button at the bottom of the post you want to copy, you can display the contents of the post in your reply. You can also edit out the irrelevant parts, as long as you don't disturb the coding at the beginning and end.

Was that Honeywell 4-way mixing valve the kind of valve you were looking for, Bill, or am I (typically) confused?


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## Bob Rohr (Feb 7, 2008)

Jersey Bill said:
			
		

> How do you guys paste in quotes from other posts?
> 
> I did not find those 4-way valves on the Blue ridge website, but the paxton corp had the full line.
> 
> ...



I'm not following the advantage of stratification.  Sure, in a solar or mod con buffer you want the coolest possible return to drive the efficiency up.  But with mine I charge the entire tank to 180F, sometimes higher with overshoot.  Then my setpoint control allows it to be drawn down to 130F where it turns off the buffer circ.  So I have a full 500 gallons of 180- 130F "capacitance" leveraging that 50 degree delta t for my "off fired" 

The circulator needed to move energy from the primary loop to the buffer tank could be very small.  I had a 15-18 on mine.  Now a 15-58 on speed 1.  I may move to one of the Alpha pros for even lower power consumption.

hr


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## Jersey Bill (Feb 7, 2008)

Eric Johnson said:
			
		

> If you hit the QUOTE button at the bottom of the post you want to copy, you can display the contents of the post in your reply. You can also edit out the irrelevant parts, as long as you don't disturb the coding at the beginning and end.
> 
> Was that Honeywell 4-way mixing valve the kind of valve you were looking for, Bill, or am I (typically) confused?



Thanks Eric. Yes, that was the honeywell valve that I wanted.


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## slowzuki (Feb 7, 2008)

I didn't quite follow the point of stratification either but it is important in systems with heat exchangers, not sure about pressurized or pumped circulation types.



> I'm not following the advantage of stratification.  Sure, in a solar or mod con buffer you want the coolest possible return to drive the efficiency up.  But with mine I charge the entire tank to 180F, sometimes higher with overshoot.  Then my setpoint control allows it to be drawn down to 130F where it turns off the buffer circ.  So I have a full 500 gallons of 180- 130F "capacitance" leveraging that 50 degree delta t for my "off fired"
> 
> The circulator needed to move energy from the primary loop to the buffer tank could be very small.  I had a 15-18 on mine.  Now a 15-58 on speed 1.  I may move to one of the Alpha pros for even lower power consumption.
> 
> hr


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## Jersey Bill (Feb 7, 2008)

standard domestic water heaters use stratification in a similar way. As the hot water is drawn from the tank, the cold water fills in from the bottom up. this gives hot water at the outlet for much longer. If the water in the tank was mixed, the outlet water temperature would start to drop right away as the cold water averages with the hot water.

In a similar way a large storage tank with a vertical heat exchanger coil  will be able to deliver, say, 130 degree water even though the bottom half  (?) of the tank is well below 130. basically it allows a better drawdown and increased delta t of the storage.

Another benefit is that if there is a layer of cold water on the bottom of the tank, there will be less heat loss from the bottom because the temperature difference is less.


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## Bob Rohr (Feb 7, 2008)

Jersey Bill said:
			
		

> standard domestic water heaters use stratification in a similar way. As the hot water is drawn from the tank, the cold water fills in from the bottom up. this gives hot water at the outlet for much longer. If the water in the tank was mixed, the outlet water temperature would start to drop right away as the cold water averages with the hot water.
> 
> In a similar way a large storage tank with a vertical heat exchanger coil  will be able to deliver, say, 130 degree water even though the bottom half  (?) of the tank is well below 130. basically it allows a better drawdown and increased delta t of the storage.
> 
> Another benefit is that if there is a layer of cold water on the bottom of the tank, there will be less heat loss from the bottom because the temperature difference is less.



_They tell me a typical 50 gallon water heater set at 130F with 55F incoming water will only proveid about 38-40 gallons of hot or warm water due to the mixing.  I wonder hw hot the top of the tank is if the aquastat trips off at 130?

These days I see most homes with DHW recirc pumps. I'd guess the tank would be blended well at 130F?

Now if the tank was blended at 180F it seems there are more btu then in a tank with a stratification of cool water at the bottom.

I need to ring up some folks in the water heater industry to get cler on the math to determine which would store more energy.  if the boiler or water heater is setpoint-ed at 130 or 180 for the boiler buffer isn't that the highest possible temperature with the rest of the tank progressivly lower as you go down?

 hr_


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## solarguy (Feb 8, 2008)

If you guys are dropping a septic tank in a hole, slinky a couple of rolls of pex instead of the copper. Do so going into it knowing that the heat transfer from pex will not be like copper but it's a hell of a lot cheaper.  You might also consider having someone foam the outside of the tank to help cut down on the heatloss.
We played around with this idea a couple of years but never found anyone willing to gamble on it.


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## Bob Rohr (Feb 8, 2008)

solarguy said:
			
		

> If you guys are dropping a septic tank in a hole, slinky a couple of rolls of pex instead of the copper. Do so going into it knowing that the heat transfer from pex will not be like copper but it's a hell of a lot cheaper.  You might also consider having someone foam the outside of the tank to help cut down on the heatloss.
> We played around with this idea a couple of years but never found anyone willing to gamble on it.



s-guy
I tried pex a number of years ago to replace some copper coils in an old Chiles Power System tank (pre Heatway days system).  Didn't work out so well.  An engineer at Watts Radiant later told me 3- 4 times as much pex would be needed to equal the heat transfer of copper.  My experiment proved him to be correct with that calculation.

Perhaps pex with a bunch of that UltraFin product fastened to it?

 hr


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## Jersey Bill (Feb 8, 2008)

master of sparks said:
			
		

> Jersey Bill said:
> 
> 
> 
> ...


_
Correct. A typical storage heater will dump 80% of its water at the stored temperature before getting into the mixed water.
It is possible and done (like on nofossil's system) where the water is stored very high, then mixed down to use. This will store plenty more btu's but acording to code it has to be mixed down to prevent scalding. Also, if there is a mixing valve like that, you can't recirculate from past the mixing valve.
In a typical gas water heater, the temperature is measured near the bottom where the cold water fills from. If the bottom of the tank is at 130, then the top is close. On an electric water heater there are 2 elements high and low, each with their own thermostat but the lower is the lead, bur the upper has priority. When the upper calls for heat, it locks out the lower when it comes on.

Solarguy, since you brought it up, the current plan is to put a pre-cast 1000 gal, square, storage tank under a slab in the house stairhall at the ground level. Its only a few feet from the boiler room. We will insulate the tank with 4" of foam. Any heat lost up will go into the house nicely. There is a closet below the stairs where the corner of the tank will be accessable. We will cut a hole in the corner of the concrete cover big enough to get the coils in. 
We thought about PEX, but by the time we figured out how to sink 3 times the linear footage at 1/3 the price per ft., we might as well go for the copper heat-x. The cost of 3/4" soft copper now is $3.35/lft, which is about $400 each for 120 ft, compaired to $700 from a vendor already formed.

Right now I am on the fence as for what do do about the controller. The tekmar boiler control is about $700, plus 2 more setpoint controllers for the storage tank control. I could put in a PLC and a small color touchscreen for as much money. The problem is that I would then have to write the ladder logic program to make it work.  There is an economy of scale. I could also add all the controls for the wood boiler into the same unit, so some money would be saved there._


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## Jersey Bill (Feb 13, 2008)

As the design matures, things change. 
I noticed that the primary loop, the storage, and the domestic hot water all require about 15 gpm. Previously there were 3 pumps doing the work here, so I changed the DHW and the storage so that there are 3-way diverting valves (2 position actuator) , and put a little more power in the primary pump, thus eliminating 2 pumps. 
The up front cost is about the same (I am working on getting pricing), but it will use less power and I believe that the valves will be more reliable than the pumps. 

As for the controls, its been tough. A tekmar boiler control (w/ 2 reset loops) is about $700. Their storage tank control is about the same price, and its really for solar and overkill for my application. I came up with a scheme to use 2 or 3 setpoint controls, plus some control relays for the storage tank control, and if I use electronic controllers (for a good temp display) its not much cheaper. 

For the  same $1,400, I am going to get a Siemens S7-200 PLC and a 6" color Maples touch screen. The plc will be set up with 8 thermocouple inputs, 16 digital inputs, and 26- 2 amp relay outputs. The controller will handle inputs from the t-stats, all the zone valves, pumps, and mixing & diverting valves.
I still need a few control relays for the pumps, a 24 vac power supply, 2 fuses, terminal blocks, and an enclosure to put all the stuff in.

It wont be cheap, but it will be flexible enough to do anything that needs to be controlled in the boiler system.


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## sled_mack (Feb 13, 2008)

HR,

I think stratification helps if you are not saturating your storage tank.  I only really saturate my tank to full heat right before going on vacation.  Even then, I'm heating from the top of the coils to the bottom, but as the top of the tank gets to the temp of the water coming out of the boiler, the heat gets pushed further down in the tank.  The advantage is that the coils for the house have the water flowing bottom to top.  This allows the water going to the house to get as close to the storage tank temp as possible.

It definitely requires more wood for me to saturate the tank to full temp.  Normally, I can run the tank between 170 and 180 (measured dead center) with no extra effort on the part of the boiler and plenty of heat for the house.  It takes a lot more wood to push the center of the tank to 200 deg.  Granted, I can shut down longer once it is saturated to that temp, but I still think it uses more wood that way, so I don't do that for normal operation.

My top and bottom thermometers have died, so I don't really know how much stratification I have now.  They actually died shortly after install, so I never got a good feel for it.  And it hasn't been a priority for me to replace them, either.


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## sparke (Feb 13, 2008)

sled_mack,  What are you using for a storage tank?


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## sled_mack (Feb 13, 2008)

It's an FRP (Fiberglass Reinforced Plastic) tank.  It was used as a holding tank for hydraulic fluid for a large printing press.  The hydraulic fluid must have come back hot, because it was rated for something like 300 deg F.

It was cheap ($300) off eBay, and works like a champ for me.  It's 10 ft across and about 5 ft deep.


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## sparke (Feb 13, 2008)

Thanks for the info.  You running pressurized with dip tubes?


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## sled_mack (Feb 13, 2008)

Boiler is pressurized with copper coils in the tank.  Is that what you meant by dip tubes?

Pic below.


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## sparke (Feb 13, 2008)

No I took it for granted you pressurized the tank and did not use internal Hex.  The pic answers all my questions.  Thanks!


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