[Greenbuilding] stealing heat from a woodstove for water heating

[Clarke Olsen]

Many years ago, we had a fireplace with a grate made of steel tubes. A fan would blow air through the tubes into the room to extract heat from the fire. It was so effective, turning it on would just about put out the fire! A graphic lesson that
nothing is free. Later, having learned nothing, I built a concrete stove with a water heat exchanger in the top of the firebox. The fire was undiminished, and the concrete radiated heat, but my exchanger was essentially a creosote condenser. Of course, if you cool the exhaust enough, you will collect that creosote right there in the chimney...
Clarke Olsen
clarkeolsendesign.com
373 route 203
Spencertown, NY 12165 
USA
518-392-4640
colsen at taconic.net




On Jan 1, 2014, at 10:25 PM, Reuben Deumling <9watts at gmail.com> wrote:

> from the woodheat.org article linked on your website:
> 
> "The biggest obstacle to adding DHW systems to wood stoves is that EPA emissions certification virtually eliminates the possibility of taking heat directly from the stove. Any DHW collector that robs heat from the combustion process would almost certainly create a dirty burn that would fail the stove. That is why manufacturers of EPA certified wood stoves don't offer DHW collectors as optional accessories. There may be one or two available, but we are aware of none at all."
> 
> 
> On Wed, Jan 1, 2014 at 7:10 PM, Reuben Deumling <9watts at gmail.com> wrote:
> Thanks, Gary. 
> 
> On Wed, Jan 1, 2014 at 5:59 PM, Gary Reysa <gary at builditsolar.com> wrote:
> Maybe you are already aware of this, but there are coils that fit inside the fire box: http://www.hilkoil.com/product.htm
> 
> These are elegant. My only reservation (and I have this about all systems installed inside the firebox) is that due to their proximity to the fire--which to promote complete combustion we'd prefer to burn very hot--the coils are going to continuously reduce the temperature in the firebox by bringing cold/cool/warmer water right into the fire. It is my understanding that this is the reason why (EPA-certified) domestic wood stoves are no longer permitted to be designed around/preplumbed for/sold with firebox mounted water jackets. The amount of heat per hr you can extract from the system and use for DWH purposes is certainly highest doing it this way. 
> 
> 
> You do have to be very careful to have pressure/temperature relief valves at the right points in the plumbing with these to avoid steam explosions:
> http://www.builditsolar.com/Projects/BioFuel/biofuels.htm#FireboxCoils
> 
> Thanks for those links. I so enjoy your whole webpage and have recommended the solar water heating portions to many folks.  
> 
> Seems like this would be a more efficient water heater and reduce the chance of cooling the flue gases too much?
> 
> faster yes. I'm just trying to figure out a way to trade off incomplete combustion and speed. 
> 
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