[Greenbuilding] stealing heat from a woodstove for water heating

[conservation architect]

Corwyn writes:
Figure out what you want for a flue temperature to ensure no creosote.
It is probably best not to think of 'stealing' heat, but rather as
burning longer to heat both house and water.

My comments:

If the creosote was to occur directly above the firebox, would it not 
provide additional fuel.  The hazard of creosote is that build up in the 
chimney creates the hazard of chimney fires and eventually clogs the 
opening.  As a liquid, this can be a sticky mess if it drains into chimney 
seams that can leak.  However, if located directly above the fire box where 
it would drain directly into the firebox, it would be burned for additional 
heat.  If a chimney fire was to occur in stove pipe where water jacket was, 
it would similar to a wood stove where the fire is contained.

The one issue with using a chimney to capture heat from wood stoves is that 
it will dampen the draft, similar to masonry chimneys that often backdraft 
before the fire can heat it up.  The additional cool mass of water will 
simulate this.  Some kind of device for a forced draft would be appropriate. 
If combustion air is a closed duct from outside, might a fan on the supply 
air provide the forced draft. With a forced draft, some attention to the 
burn temperature is needed, in that forced air would increase the burn 
temperature.  A balance is needed.  This is the opposite of dampening the 
draft.  Forced draft from air supply side would need to be cut off while 
loading wood from the room.  A forced draft from above the fire would be 
better to prevent backdraft while loading wood.  However, this creates 
additional difficulty of running a fan in the combustion gases.  I fooled 
with this a few years ago with a fan motor located above the top of the 
chimney with a shaft that extended just below the top that would allow the 
gas to escape and not go through the motor.  I had a wired on off switch 
located next to the stove to control the run times according to conditions. 
I think this is a good concept.  However, I could not work through the 
mechanical issues.

Eli

-----Original Message----- 
From: Topher
Sent: Tuesday, December 31, 2013 5:41 PM
To: Green Building
Subject: Re: [Greenbuilding] stealing heat from a woodstove for water 
heating

On 12/31/2013 3:40 PM, Reuben Deumling wrote:
> The further from the firebox one chooses to steal the heat the fewer
> + negative repercussions for the combustion process, and
> + BTUs per hr transfer into the water

> My inclination is to have him make the water jacket taller (more surface 
> area), and find a smaller tank (less cold water volume to dilute the 
> heat), but in the absence of some empirical work, or others' experiences, 
> this would be somewhat of a crap shoot .

Sounds like it is time for some calculations, to get you started:

Wood runs around 8000 BTUs per pound.
1 BTU raises 1 pint of water 1 degree F.
Figure out what you want for a flue temperature to ensure no creosote.
It is probably best not to think of 'stealing' heat, but rather as
burning longer to heat both house and water.

Thank You Kindly,

Corwyn

-- 
Topher Belknap
Green Fret Consulting
Kermit didn't know the half of it...
http://www.GreenFret.com/
topher at greenfret.com


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