# Therma-coil: to heat water with a wood stove



## Beno (Jun 22, 2007)

Hi there,

Two dealers of solar systems recommended the use of a coil to preheat the water using a wood stove. One mentioned also this site: http://www.thermacoil.com/description.htm .
I wonder what do you guys think about such a system, and if there are any risks with using it. Would I be able to use it in a VC cat fireplace? Will this affect the warranty?

Thanks,
Beno


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## webbie (Jun 22, 2007)

Here is one article I wrote on it....
https://www.hearth.com/what/woodstovedhw.html

I think there is a good article on it at www.woodheat.org also.

The main thing is that such a project is not a job for the average DIYer.  You have to consider factors such as whether it will work by thermosyphon or whether controls and a pump will be used - also the placement and type of the coil. 

I have used some coil over the years with success.....most of mine were actually outside the firebox, like on the rear of the stove. This requires more heat exchange area - such as a finned coil. If you do a forum search, you might find a couple earlier threads about this. Search on DHW or wood stove coil....


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## R&D Guy (Jun 26, 2007)

Interesting - I like the green part about it too.  I wonder how interested people would be in having their dealer supply a kit approved by the MFG???   Anyone?


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## webbie (Jun 26, 2007)

R&D Guy said:
			
		

> Interesting - I like the green part about it too.  I wonder how interested people would be in having their dealer supply a kit approved by the MFG???   Anyone?



IN my experience this has not worked in the US, although it is very popular in Europe. The problem is that such a small % of stove buyers have interest in installing such a thing - perhaps 1%, that testing and approving such a device may be a loss leader.

VC and others have had such items in the past and dropped them from their lines due to either slow sales or liability.


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## R&D Guy (Jun 26, 2007)

Craig - Do you know how long ago VC did that? 5, 10, 15 years ago? Like most, I'm finding it hard to come up with that new, or perhaps re-newed green idea that people would gobble up. What I like about this is that I don't think anyone is really looking for a more efficient wood stove as far as heat into the living space. In fact I know from my wood stove days that they have 3 settings - Too hot, a little warm, and Wow it's cold in here! :bug: Not too much I can do about the "it's cold in here" part unless you have a big ole (400 gallon plus) insulated water tank, and not many have the room or want for that.

I have noticed a lot more commercials about the tank-less heaters and what a saver they are (although I wonder), and if a simple mod to a wood stove, or it's flue could help people get more out of their stove without making the room even hotter. Perhaps I could make the heat output into a room more consistent.


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## cbrodsky (Jun 26, 2007)

I've thought about doing DHW with my system, but I am not at all comfortable with putting something like this in the firebox.

If/when I do this, it will be using something like a transmission cooler placed between the stove and rear heat shield.  This will provide much greated heat transfer area to help offset the fact that it's "only" about 400-500 degrees in that space versus the much higher internal firebox temperatures.  I also expect this would be a much more reliable safe installation as anything inside a firebox is going to be subjected to immense stress that I am not comfortable with.

Finally, the other way I'd do this is an indirect heat exchanger using a non-pressurized water loop.  That will give extra protection against a steam explosion.  Since we already have a solar hot water system with a solar heat exchange tank connected to the main hot water storage tank, I will just add a second loop up to the stove, and use the "Panel 2" temperature sensor for the solar controller.  This way, when the fire is on, it'll sense it and pump water up to the transmission cooler exchanger, and loop back to dump the heat to the main storage tank.  When the fire is out, or we lose power, it'll just drain back to the basement and stop the exchange loop.  Of all the automatic hot water systems I've seen for stoves, I think this would be the safest by far.

-Colin


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## webbie (Jun 26, 2007)

VC did this approx. 1986-1988.

It was an internal solid cast or steel tank - maybe stainless, although it was painted black.

I agree with NY soapstone in that on my stoves (and some solar installation), I would have a local shop (which made DHW coils for boilers) create multiple S curves out of finned copper and then mount these on the rear of the stove - with a heat shield behind them. This, IMHO, cuts vastly down on the possibility of explosions, etc. when not installed correctly.

An internal model that is pretty good (smaller, and only one hole needed) is the Yukon HotRod.
http://www.yukon-eagle.com/hotrod.htm

Problem with the external model is that many stoves have built in convection chambers now - but it would work great on VC, Soapstone and similar products. In other cases, you might be able to snake it between the stove and an existing convection chamber in the rear.


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## webbie (Jun 26, 2007)

Silver solder and 180 degree copper U's, plus pipe would also probably do the job. A good DIY'er or R&D'er might also get some copper baseboard interiors (with the aluminum fins) and then braze or silver solder the 180 degree els onto that.....

Small rectangle tanks are tough due to the relatively high pressures of DHW.


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## Rhone (Jun 26, 2007)

I read an article once talking about the problems of putting preheater pipes inside the firebox.  Water sucks SO much heat from the fire it doesn't burn properly and todays stoves are so efficient they only waste as much heat as to sustain draft.  When you retrofit water pipes they suck so much heat your "newer" stoves will no longer draft properly.  External though, I wouldn't see why it wouldn't work it won't affect the firebox.  You then have to worry about explosions and things like that, and what happens during a power failure if you depend on pumps.  

My first stove had water pipes in it, that was back in the early 70's.  Both sides of the stove had a sideways "W" channel of metal pipe running along the sides which were behind a metal shield.  The stove was in the basement, and when you lit a fire it would heat the water in it which flowed naturally through the baseboards in the house, or with a valve you could direct it to heat the water tank instead.  It ended up being no replacement for a boiler, worse the pipes would break in the stove every couple years and flood the stove/basement, it's a harsh environment in there.  They were then a groan to try to get out and replace, sort of like trying to remove the bolts of the headers of your car.  After 6 years and replacing 2 sets of pipes a plumber was called to redirect the plumbing so it didn't go through the stove anymore.  

For pre-heating your hot water, take a look at a graywater heat recovering system like http://gfxtechnology.com/ it recovers the heat from the the hot water going down the drain.  The passive is only good for simultaneous stuff like showers, or if you a spouse like mine who likes to do dishes with the hot water constantly running and doesn't use a dish pan (but I also must say at least she does them and not me).  Otherwise, the active one sounds interesting, it works as long as there's a temperature difference.


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## R&D Guy (Jun 27, 2007)

Thanks for all the great information and links guys! I especially like reading install manuals; they give an abundance of info. 

I originally envisioned a heat exchanger or a simple "W" that mounts to the back of the unit, but Craig bought up the obvious issue of the forced convection chamber which is on most wood stoves. It may be just as easy to sneak something along the sides if the unit has additional side shielding, but as mentioned this would likely have to be part of the EPA emissions testing if its anything that mounts to the stove itself. I can see the advantage that the aftermarket guys have here since they don't have as many 3rd party testing requirements if any at all.  I noticed one of the links took me to a low-buget home.att.net/homepage site.

I also like the idea of adding the heat exchange to the flue pipe.  If the grey water system can claim significant money savings than hot flue gases should be at least as good, and the EPA is likely less interested in gagets added to the flue pipe since it's post combustion.


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## cbrodsky (Jun 27, 2007)

R&D Guy said:
			
		

> Thanks for all the great information and links guys! I especially like reading install manuals; they give an abundance of info.
> 
> I originally envisioned a heat exchanger or a simple "W" that mounts to the back of the unit, but Craig bought up the obvious issue of the forced convection chamber which is on most wood stoves. It may be just as easy to sneak something along the sides if the unit has additional side shielding, but as mentioned this would likely have to be part of the EPA emissions testing if its anything that mounts to the stove itself. I can see the advantage that the aftermarket guys have here since they don't have as many 3rd party testing requirements if any at all.  I noticed one of the links took me to a low-buget home.att.net/homepage site.
> 
> I also like the idea of adding the heat exchange to the flue pipe.  If the grey water system can claim significant money savings than hot flue gases should be at least as good, and the EPA is likely less interested in gagets added to the flue pipe since it's post combustion.



I would be concerned that you're risking problems with maintaining consistent draft and creosote buildup.  If the stove is being run properly, there should not be significant excess waste heat going up the chimney - just enough to maintain a consistent draft and ensure nothing precipitates out at the very top of the chimney.

I would suggest focusing on something that does not interfere with the firebox internals or chimney performance to reclaim heat, as well as using a "drainback" type design that requires active power and control to have water anywhere near the stove.  Such a system empties the tubes of water automatically upon power failures or when a maximum tank temperature is achieved.  You could easily use off the shelf solar hot water components to construct such a system and this would greatly alleviate the risk with such systems.  This requires having a hot water tank in your basement or somewhere lower than your stove.  (probably not that uncommon)

The thermosiphon idea is not bad, but most people don't have their hot water tanks higher than their stoves.  If you don't, it's a dangerous system IMHO because you now run the risk of boiling and exploding the water line if you have a pump failure, control system failure, or general power failure.  You have to bank on a relief valve blowing open that will have to be at the highest point of the system (likely inside your house).  And if it has your full pressure house supply running directly through the exchanger, versus an isolated heat exchange loop, you now not only have a flood from the heat exchange loop - you've got an unlimited source of water to continue the flooding.  At minimum, you'd probably want to plumb that to a high flow capacity drain and consider some redundant failsafe controls.

-Colin


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## stoveguy2esw (Jun 28, 2007)

Webmaster said:
			
		

> VC did this approx. 1986-1988.
> 
> It was an internal solid cast or steel tank - maybe stainless, although it was painted black.
> 
> ...



we did a similar line about the same time , early to mid 80's only the water jacket was on the side of the unit rather than the rear, the coil was wound around inside the jacket which was filled with a medium to trap heat , vermiculite or sand i think (was before my time at ESW. i got the feeling when i asked about it when we were reworking one for a local customer that they were not all that popular in relation to our "normal" line. i have thought about building a similar unit as a pool heater, but its just ink on paper (or bytes on screen) havent figured out what kind of heat  output it'd need to heat the pool to reasonable temps for early or late season extension. im not even sure i can make it work but its a fun theroetical exercise.


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## littlesmokey (Jun 28, 2007)

I remember reading a thread recently about a solar system that had a woodstove hooked to it. Can't find it, but sounded really good. 

I have seen kitchen heaters from the 30's that were plumbed. We used to call them trash burners, but they did supply DHW. 

This isn't a new thought, but one that should be revisited. With Pex lines and an abundance of heat transfer units, seems like a good idea. 

I even remember a water heater made in Ireland that was pellet heated, but the company is not on the Web, so I will try and cull the printed lit to find it. I thought about buying one, but the shipping, taxes and fees were twice the price of the unit, as I remember.


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## tuxnjack (Dec 31, 2009)

I talked to 2 friends who are hunting me down transmission coolers.  I think I can sodder them together to build a heat exchanger that encompasses 3 sides of my parents old wood stove.  I think i can scrape up some scrap sheet metal to make a heatguard, and an old circulating pump as the stove is about 30 feet from the furnace and boiler buddy..
what do you think??


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## WES999 (Jan 2, 2010)

I just finished a wood stove DHW pre-heater, my system is similar to what 
NY soapstone mentioned in his post.
"indirect heat exchanger using a non-pressurized water loop."
You can see some pics and read about in this thread:
(see my post #6)
https://www.hearth.com/econtent/index.php/forums/viewthread/49253/

The system has been running for 5 days now, seems to be working well.
The electric water heater witch normally runs a bit over than 1 hr/day has only run .7 hr in 5 days.

One thing to consider is you really need a lot of heat, I would put the heat exchanger on the hottest part of the stove, that is why my HX is on the top/ front of the stove.


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## cbrodsky (Jan 4, 2010)

Very cool!

I haven't actually gotten this done myself... it looks like you have really good heat transfer with that setup.  If I do this, with a soapstone stove in a living area, my wife would demand that it be "invisible" hence my thought on sandwiching something on the back of it.  Woodstock makes a stock heat shield for the rear of the stove that leaves about an inch or so of space... my thought was the transmission cooler could fit in between.  I'd probably place some sheet metal around the top/sides of the transmission coils as well, so that room air can't flow up through that space and cool it.  Of course this would mean less convective heat transfer into the room - but more heat into the water.

Still, I wonder if it would really get enough heat in to make it worthwhile.  In your case, you're getting 150 which is great... I'd be happy even if I could just get to 100-120 since we have a 120 gallon solar storage tank w/electric backup element.  Straight from the well, we start at about 53, so I would expect we could get a lot of benefit using our same solar controller, fooling it to think the woodstove is a second solar array that the sun is hitting whenever the woodstove gets warm.

-Colin


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## peakbagger (Jan 4, 2010)

My friend has a home brew set up that consists of a heat exchanger built out of short sections of slantfin baseboard that sits on top of a wood stove. He circulates water from a heating zone through the coils and out to the house. If the temperature of the water exceeds a certain temp, another zone dumps the heat to a unit heater mounted in a cold area of his basement. He also has a pressure relief on the unit and has the coil set up so that he can swing the unit up and out of contact with the top of woodstove if the power fails. He used to heat his Ranch type home with just a woodstove, since he switched to this system he has cut down his wood usage 30 to 50%.


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