[Greenbuilding] stealing heat from a woodstove for water heating

[Norbert Senf]

Also might be a good idea if you have fire insurance to check with your
agent as to whether it is OK or not to mess around with a stove if it has a
U.L. label. Normally the insurance requires the label, and the label only
applies to a stove installed "according to the terms of its listing".

Creosote (tar) can only be created from a smoldering fire, by the way. A
flaming fire can only make soot. With a flaming fire, you can drop your
chimney temperature as much as you want, and will not get creosote. With a
smoldering fire, you want as high a chimney temperature as possible to
minimize flammable deposits. The ideal creosote creator is an unlined water
cooled metal firebox, like you see in outdoor boilers. Some of them are
able to convert 10% of the weight of the wood into tar droplets
(smoke)..................N


On Sun, Jan 5, 2014 at 3:58 PM, conservation architect <
elitalking at rockbridge.net> wrote:

> 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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-- 
Norbert Senf
Masonry Stove Builders
25 Brouse Road, RR 5
Shawville Québec J0X 2Y0
819.647.5092
www.heatkit.com
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