GARN standby loss?

[foxt]

Since I have been running a WHS 2000 flat-out for two months, I haven't had a good chance to assess how much standby heatloss I have, until this week. Since I have been scratching my head with the short circuit issue that I've posted about, I was doing some rework yesterday and I didn't pull any heat out of the GARN. Over the course of about a 12 hour period, I lost somewhere between 5-7 degrees in the tank (depending on which sensor/gauge I read, I have three monitoring the tank). Tank temp started around 135, and I think the ambient air temp range was 25-32 over that same time period.
If I extrapolate, I could be losing as much as, if not more than, 10-15 degrees over the course of an average winter day just from the tank, which was a surprise. And since I try to go as high as 200*, I've got to be losing even more than that 10-15 when I am at higher temps.
The unit is insulated similar to what I sense others have done. Unit sits in a corner of a shed that was constructed for it, with the two outside walls insulated with about 4" of spray foam. The third interior wall is insulated with R-21 pink. There is a framed 2x10 ceiling at about 6-1/2' over the top of the unit, and that is insulated with R-30. All walls are covered with firecode rock, seams taped. The empty spaces between the tank and the walls are stuffed with more pink stuff. I do have about 30" between the back of the unit and the rear wall. I had to make a 90* turn with the exhaust and since, per Martin, that needs to be inspected and cleaned annually I left myself a little bit more room back there to work in. Since I do have that space, and since I've got heatloss in the space due to the cold air intake duct and the airgap around the class A exhaust thimble, I went ahead and steel-stud framed a wall right up against the rear of the unit and insulated that with 3-1/2' pink stuff too. There is an access hatch for that space, but that is insulated fairly well too.
The only area that is not "finished" is the front of the unit - it is insulated with 3 1/2 " of pink stuff, but I don't have the firecode rock on it yet (yeah, I know, that's a hazard and I will get to it). So I am sure I am losing more there than any other area (lowest r-value, and drafts due to the lack of the sheetrock). The manway cover is also a little bit of an issue, I've been fighting with it to keep it from letting small amounts of water vapor escape into the insulated cover I made for it.
Aside from those two things, I thought I had this all wrapped up nice and tight. I was expecting standby loss, just not so much. 10% or more over the course of a day seems high ... what do other GARN owners see?

Tom

 

[heaterman]

OK. Let's see here..........2000 gallons @ roughly 8#'s per gallons times a 10* drop in 24 hours would be about 160,000 btu standby loss per day or about 6,500 or so per hour. For comparison, I did a little experiment much the same as you a few years ago using a CB 5036. Ran it up to 200*, unloaded the remaining wood, shut everything off and let it set. The heat loss from that came to about 16,200 / hour IIRC.

So to sum it up. Yes you have some heat loss and you can probably make some changes to eliminate some of it, but you will soon be up against the point of diminishing returns.

 

[Jim K in PA]

Tom - I dont have any hard numbers, as I have not really tried to gather them, but I don't think mine are much worse than yours, and I think yours is better insulated than mine. I also still have to install the s/r on the front wall, and my metal studs are acting like cooling fins at the moment.

BTW - you don't have to install Fire Code rated rock (5/8) and I don't think you will be able to get it under the front lip of the fire box. In fact, my front wall is slightly concave, and I am going to have to use 3/8 rock to get a clean install.

I do notice a higher rate of loss when the storage temp is above 195, as you might expect.

 

[TCaldwell]

tom, is your flue horizontal or discharge vertically, mine discharges vertically and i noticed heat waves from the flue long after finish of burn, you could actually feel a heated flow with your hand, and conversly a slight induced draft at the combustion air inlet, i took some readings with my anemometer combined with the gas temp heaterman calculated about 4200btu/hr heatloss through the flue,coupled with a small radiation factor of the vessel dependant upon insulation, your calcs seem in the ballpark. i installed a 8 inch motorized zone damper on the combustion air inlet. it is adjustable for percent of closure set to about 80%. opens on fan activation and closes 2 hrs after fan stops, it is power on and power close so a power outage will not change its position. i will post changes in cfm when i get a chance to get on the roof

 

[foxt]

OK. Let's see here..........2000 gallons @ roughly 8#'s per gallons times a 10* drop in 24 hours would be about 160,000 btu standby loss per day or about 6,500 or so per hour. For comparison, I did a little experiment much the same as you a few years ago using a CB 5036. Ran it up to 200*, unloaded the remaining wood, shut everything off and let it set. The heat loss from that came to about 16,200 / hour IIRC.

So to sum it up. Yes you have some heat loss and you can probably make some changes to eliminate some of it, but you will soon be up against the point of diminishing returns.

Thanks heaterman. It's good to know I am in the ballpark. Funny how we work so hard to eek out every btu from these systems, but eventually you have to yield to that brutal point of diminishing returns ....

Tom

 

[foxt]

BTW - you don't have to install Fire Code rated rock (5/8) and I don't think you will be able to get it under the front lip of the fire box. In fact, my front wall is slightly concave, and I am going to have to use 3/8 rock to get a clean install.

Jim,
When framing the front of the tank, I wasn't able to find the 3-1/4" or whatever crazy dimension steel stud the manual called for. Around here, it's 3-5/8 or 2-1/2, so I went with the 2-1/2 and spaced it so that I'd have enough room for 5/8 rock between the steel and that flange. That also left a little gap between the stud and the tank, at least a thermal break, although I am sure I am losing heat by not having the front buttoned up yet.
I too noticed that both the front and rear walls have a little bow to them, and I didn't notice it before adding the water although I probably wasn't really paying attention to it at that point. What I am not sure about yet is, when I do get to the rock, whether I will try to work around the areas where I need a penetration for the temp gauge, the thermowell, and the level gauge (thanks again for that great idea), or if I drain the tank a little and just yank that stuff off, rock it, and then cut the holes and put everything back. It will be neater that way, and maybe even less work in the long run, just kicking myself for not getting the rock on before that other stuff - I was in a hurry to get up and running ....


Tom

 

[foxt]

tom, is your flue horizontal or discharge vertically ...

Tom,

Thanks for the idea, but I'm horizontal. I do wonder if I am losing heat via the supply duct, since I have about 3-1/2 feet of it and its been hanging out in a slightly less well insulated part of the install. I'll have to check for a draft with a lighter in the morning (just finished a burn a little while ago ...).

Speaking of the flue, I noticed in another thread that you were working on a means to kill the blower based upon process parameters other than just run time. I'm sure I run the blower too long (better safe than sorry), but this idea makes alot of sense to me so I'll drop you a pm about it ....

Tom

 

[foxt]

tom, is your flue horizontal or discharge vertically, mine discharges vertically and i noticed heat waves from the flue long after finish of burn, you could actually feel a heated flow with your hand, and conversly a slight induced draft at the combustion air inlet, i took some readings with my anemometer combined with the gas temp heaterman calculated about 4200btu/hr heatloss through the flue,coupled with a small radiation factor of the vessel dependant upon insulation, your calcs seem in the ballpark. i installed a 8 inch motorized zone damper on the combustion air inlet. it is adjustable for percent of closure set to about 80%. opens on fan activation and closes 2 hrs after fan stops, it is power on and power close so a power outage will not change its position. i will post changes in cfm when i get a chance to get on the roof

Checked for flow through the flue today at least 12 hours after the last burn, and even though I am horizontal there is definitely a flow there. I don't have a means by which to measure it so I have no idea what the heatloss is from this. I'll try a little experiment tonight and manually do what your motorixed damper does - close the flue 2 hours after the fan stops, and see what I have in the am (seems like temps will be about the same tonight as they were last night, so it'll be almost apples-apples).

Any other GARN owners out there have any experience with closing the flue to reduce standby loss? Seems like a wast to just let that heat fall out of the exhaust like that, but I have to wonder if it was a big enough deal wouldn't Martin have explored this over the last many years and either done something about it, or let it be? If my math is right , 4200btu/hr works out to 6-7 degrees of tank temp on a WHS2000, which is at least a double digit percentage of a single burn ....

Tom

 

[TCaldwell]

i think it was probabaly a liability issue, if a damper was installed that closed on no power there would be a carbon monoxide issue, also same circumstance if you underestimated the necessary burn time required with the timer.

 

[Swamp Yankee]

Hello, With our garn we get about 24 hours of runtime, sometimes longer. With the system being mostly in standby in a free-standing shed, this loss has been bugging me. Our garn has 1ft.fiberglass insulation on the sides, 18" rear, 24" on top, and the shed is very well sealed. Even with that if I had it to do over I would not have made the garn-shed freestanding, but either installed in the barn/workshop or as a lean-to structure on the house so at least one wall would be shared with the structure to be heated.

I was trying to estimate flue losses myself (post https://www.hearth.com/talk/threads/33960/). Not sure how these #s line up with others...

I've been considering making a fiberglass or some kind of non-flammable very lightweight plug that would just fit in the horz. flue. When a burn was done, plug the flue to stop the convection loss through the tank. I would remove the plug before starting the next burn a day or two later. I'm thinking making it lightweight enough that even if I forgot to remove it, the flow would pop it out. I don't like the idea of a motorized or any other kind of damper on the intake for reasons Tcaldwell outlined above.

 

[heaterman]

BTW - you don't have to install Fire Code rated rock (5/8) and I don't think you will be able to get it under the front lip of the fire box. In fact, my front wall is slightly concave, and I am going to have to use 3/8 rock to get a clean install.

Jim,
When framing the front of the tank, I wasn't able to find the 3-1/4" or whatever crazy dimension steel stud the manual called for. Around here, it's 3-5/8 or 2-1/2, so I went with the 2-1/2 and spaced it so that I'd have enough room for 5/8 rock between the steel and that flange. That also left a little gap between the stud and the tank, at least a thermal break, although I am sure I am losing heat by not having the front buttoned up yet.
I too noticed that both the front and rear walls have a little bow to them, and I didn't notice it before adding the water although I probably wasn't really paying attention to it at that point. What I am not sure about yet is, when I do get to the rock, whether I will try to work around the areas where I need a penetration for the temp gauge, the thermowell, and the level gauge (thanks again for that great idea), or if I drain the tank a little and just yank that stuff off, rock it, and then cut the holes and put everything back. It will be neater that way, and maybe even less work in the long run, just kicking myself for not getting the rock on before that other stuff - I was in a hurry to get up and running ....


Tom

That's the reason we use full 3-1/2" steel studs and put sheet metal over them. It's a little tougher to work with if you're not used to working with sheet metal but is solves the insulation problem and the dimensional issue as well.

 

[Jim K in PA]

That's the reason we use full 3-1/2" steel studs and put sheet metal over them. It's a little tougher to work with if you're not used to working with sheet metal but is solves the insulation problem and the dimensional issue as well.

Steve - do you use a thermal break between the steel studs and the face of the GARN? I just found that steel studs I have up tight against my tank are conducting heat to their front edges. Attaching a large surface area of more tin would increase the rate of conductive=>radiant heat loss, I would think, unless you have something insulating the studs.

 

[foxt]

I've been considering making a fiberglass or some kind of non-flammable very lightweight plug that would just fit in the horz. flue. When a burn was done, plug the flue to stop the convection loss through the tank. I would remove the plug before starting the next burn a day or two later. I'm thinking making it lightweight enough that even if I forgot to remove it, the flow would pop it out. I don't like the idea of a motorized or any other kind of damper on the intake for reasons Tcaldwell outlined above.

If you don't like the idea of closing off the intake, I'd definitely proceed with that idea of the removable plug. For the last two days I have been plugging the outlet with just enough fiberglass to block the flow of heat. I think I cut the standby loss by maybe as much as 30-40% . Hard to tell exactly what I am saving, but there has definitely been a significantly smaller temp drop with the GARN offline and the flue capped over the same 12 hour period that I'm using as my benchmark.

And as you predicted for yourself, I didn't remember to remove the fiberglass this evening when I first turned on the fan, but sure enough about 2 mins later when I said "oh sh*t" and ran around to the back of the shed, the fiberglass had been blown out. I think I'll go the power-close, power-open route that Tcaldwell mentioned because I'm too lzay to place and replace the cap and I found a unit for around $60 that'll work. Now I just need to figure out the control for the two-hour delay after the fan cuts out ...

Tom

 

[sorethumbs]

I did a little experiment much the same as you a few years ago using a CB 5036. Ran it up to 200*, unloaded the remaining wood, shut everything off and let it set. The heat loss from that came to about 16,200 / hour IIRC.

heaterman, interesting. Two questions for you: 1) did you take precautions to prevent thermpsyphoning during the experiment? 2) how long did you let the test go (time and temp) and how much did the heatloss rate decrease as the temp in the boiler dropped?

 

[TCaldwell]

tom the motorized damper that i have is a ultrazone urd-8 made by ewc controls inc, before the plc i used a intermatic ff36h spdt 6 hr timer to control damper

 

[foxt]

Thanks for that Tom. The Intermatic looks like a good simple option. Alternatively I was thinking something simple like a delay relay that I could wire off of the fan switch so that the damper and the fan would run off of a single switch. WIth your seperate switch approach I'd be concerned that I might forget to open the damper (or someone else who didn't know would just skip it).

The other problem with either approach is that the power on/off dampers all seem to take 30-60 seconds to open, so if the blower and the damper are energized at the same instant I'm concerned that the force of the blower will shorten the lifespan of the damper. Ideally I'd have a single on/off switch that opens the damper, 60 seconds later enables the timer/fan, then 2 hours after the fan cuts out closes the damper .... can probably get close enough if I just use the intermatic to open the damper and enable the fan, and then first set the intermatic to my burn time+2 hours, wait 60 seconds (maybe load wood), and then start the fan timer ...

Here's the alternative damper I'm looking at: heyoka 8" motorized damper

Tom

 

[leaddog]

Thanks for that Tom. The Intermatic looks like a good simple option. Alternatively I was thinking something simple like a delay relay that I could wire off of the fan switch so that the damper and the fan would run off of a single switch. WIth your seperate switch approach I'd be concerned that I might forget to open the damper (or someone else who didn't know would just skip it).

The other problem with either approach is that the power on/off dampers all seem to take 30-60 seconds to open, so if the blower and the damper are energized at the same instant I'm concerned that the force of the blower will shorten the lifespan of the damper. Ideally I'd have a single on/off switch that opens the damper, 60 seconds later enables the timer/fan, then 2 hours after the fan cuts out closes the damper .... can probably get close enough if I just use the intermatic to open the damper and enable the fan, and then first set the intermatic to my burn time+2 hours, wait 60 seconds (maybe load wood), and then start the fan timer ...

Here's the alternative damper I'm looking at: heyoka 8" motorized damper

Tom

Maybe you could put a limit sw2 on the damper arm so when the damper opens it would turn on the timer for the fan. That way the fan couldn't run if damper was closed.
leaddog

 

[Rick Stanley]

I don't have mine running yet, but I have had the blower plugged in and it seems to have quite a lot of force. I wonder if a simple flap arrangement could be rigged up out on the end of the flue pipe that would be blown open by the fan and just flop down to the closed position when the blower shuts off. Kinda like a dryer vent. Just a thought.........................

 

[TCaldwell]

rick, it is a good idea,my concern is that the garn is tempermental to back pressure , i would try it with a flap or damper that offered mininal resistance

 

[Swamp Yankee]

A neighbor up the road tried the 'flapper on the flue' idea a couple years ago on their garn. It was similar to a tractor trailer exhaust cap. It worked fine until they had a load that 'puffed' and the thing was destroyed.

 

[heaterman]

If you look at the fan wheel in a Garn You can see that it's a very open design. It will easily move the required air volume and not clog with debris due to that configuration. The flip side of that type of design is that it will not "push" against a very high static pressure. Compare it, those of you lucky enough to have one, to a typical furnace blower with many closely spaced, curved "blades" and you'll see instantly what I'm talking about. Therefore, if a person introduces much in the line of air flow resistance in the flue, it will have the effect of cutting down the air flow.

 

[jklingel]

Question: I am almost certainly getting a GARN for my house and read this with interest. Is this really much of an issue, or is it micro-tuning to squeeze the old babe for all she's worth? I am just wondering how much wood you are saving by "tinkering w/ the engineering" and perhaps introducing another potential failure point. Thanks. j

 

[Sting]

I am simply a "wannabe" but how about a non pressure creating thermal drop in that barrel you all put under the exhaust vent ????

Drop the exhaust pipe in there then make it turn up and back down again -- or something

maybe several barrels of increasing size - something like an automotive muffler - Just random thoughts.

Would that be enough to stop the back draft ?

 

[foxt]

heaterman - yeah, I noticed that about the fan first time I looked at it. I chalked it up to more of a brute-force vs. elegant design approach by Dectra, since they make these things themselves and are probably looking at cost. I didn't consider the fact that they do need to plan for debris, however unlikely it is for something large to make it that far down the heat exchanged path ...

jk - we're talking about tuning here, at least for horizontal flues. This thread started with a question on my part about the standby losses I was seeing, which based on crude observations was in the ballpark of >10% of a burn over 24 hours. Turns out I can knock that down to single digits by simply closing the flue when not burning, so I plan to do that. On vertical flues, I think it was Tcaldwell who reported much higher losses and therefore more to be gained by closing the flue.

Sting - I don't know if the addition of one of more elbows at the end of the flue would stop the flow, and I wonder what it would do to performance while burning. It's easy and relatively inexpensive to just add a power damper to the intake side so that is the route I am going ....

Tom

 

[heaterman]

heaterman - yeah, I noticed that about the fan first time I looked at it. I chalked it up to more of a brute-force vs. elegant design approach by Dectra, since they make these things themselves and are probably looking at cost. I didn't consider the fact that they do need to plan for debris, however unlikely it is for something large to make it that far down the heat exchanged path ...

jk - we're talking about tuning here, at least for horizontal flues. This thread started with a question on my part about the standby losses I was seeing, which based on crude observations was in the ballpark of >10% of a burn over 24 hours. Turns out I can knock that down to single digits by simply closing the flue when not burning, so I plan to do that. On vertical flues, I think it was Tcaldwell who reported much higher losses and therefore more to be gained by closing the flue.

Sting - I don't know if the addition of one of more elbows at the end of the flue would stop the flow, and I wonder what it would do to performance while burning. It's easy and relatively inexpensive to just add a power damper to the intake side so that is the route I am going ....

Tom

As for the cost subject, there's actually a lot of engineering in that fan design. It would probably have been cheaper to buy someting that was ready made. Debris can easily make it to the fan as one customer discovered when he lit his fire with newspaper. Made an awful racket when it hit the fan.

There is no doubt some loss from the open flue via natural convective air flow so closing it when the blower is off would mitigate it. The question becomes is it worth it and is it safe. What if there is no "proving circuit" in the motorized damper and it stays closed when the fan is turned on. A person wouldn't notice it until the fire was well established. A person would want to consider all the scenarios and design for worst case.

I think a 90 down into a barrel with a foot or foot and a half of pipe would definitely have an effect on the off cycle losses.