[Greenbuilding] stealing heat from a woodstove for water heating

Erin Rasmussen erin at trmiles.com
Tue Dec 31 16:54:12 CST 2013


Hi Rueben, 

There is some good math hidden in online conventional water heater
calculations, including some great averages for how much water typical tasks
like dish washing etc takes. You may want to download one of those and see
if the setup you're thinking of hits the right sorts of numbers.  The other
part of this is the heat transfer. I'm guessing that that's a coil around
the flue transfer system, there are other heat exchangers out there. The
pricey ones are pancake style and do a beautiful job of transferring heat,
but are not usually used residentially. The other part of this is making
sure that you don't have too much heat loss between the firebox and the
point of use.  

kind regards,

Erin Rasmussen

erin at trmiles.com 

 

From: Greenbuilding [mailto:greenbuilding-bounces at lists.bioenergylists.org]
On Behalf Of Reuben Deumling
Sent: Tuesday, December 31, 2013 12:40 PM
To: Greenbuilding
Subject: [Greenbuilding] stealing heat from a woodstove for water heating

 

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

But are there nonlinear solutions? Methods for stealing heat close to the
firebox that do not negatively effect the combustion process?

 

My ideal scenario relies on (1) below. http://www.axeman-fireflue.com/ My
reason for looking beyond that solution is that the average length of time I
burn a fire/day is a little over 2 hrs. The fellow who is advising me on the
flue water jacket (at link above) thinks this may not yield very much hot
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 . If moving things closer to the
firebox increases the rate at which heat transfers into the water, then
laying the 'water jacket' right on top of the stove (2) seems almost too
easy. Frank Tettemer has spoken up the most on this subject in the past.
Anyone else have thoughts on this? 

(1) external stainless flue water jacket
(2) external 'water jacket' laid on top of stove

Obviously the more effective this system is at robbing heat from the stove
for DHW purposes, the less heat goes into the room, which leads
(dynamically) to incrementally longer burn times to reach the same desired
air temperatures. This is not a bad thing. 

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