Subject: Re: Chimney air-air heat exchangers

From: [email-masked] (Phil Carden)
Date: 3 Nov 1995 11:39:25 GMT

In article <[email-masked]>, [email-masked] 
says...
>
>In article <[email-masked]> [email-masked] 
(Ed Ruf) writes:
>>In article <[email-masked]> [email-masked] 
(Nick Pine) writes:
>>>Does anyone use a woodstove or fireplace chimney as an air-air heat 
exchanger?
>>>
>>>Seems like this might work with a bare fluepipe running up through a 
masonry
>>>chimney, or a triple wall air-cooled "all-fuel" fluepipe with cold 
outside
>>>air coming down the from the outside, to a point lower than the air 
intake 
>>>for the fireplace, to supply combustion/ventilation air which is 
heated as
>>>it travels down the outside of the fluepipe. 
>>>snip

Consider using a short section (say 4 ft) of corrugated flue liner to 
increase exit gas turbulence and improve heat transfer between hot and 
cold streams. An increase of up to 250% may be possible given ideal 
flue gas and inlet air flow regimes. 

I have a recent research paper that suggests maximum heat transfer would 
be achieved in the transition between laminar and turbulent flow. The 
conditions sound right in this case given the gas velocities quoted. Heat 
transfer would be maximised with a corrugation depth of 1/4" in say a 6" 
flue. (e/D = 0.04)

An Excel spread sheet for predicting heat transfer and pressure drop in 
single corrugated tube-in-tube counterflow heat exchangers from 1/2" ID 
up to 6" ID is available. (Suitable for this application,SI units only). 
Email me if interested. 

Phil Carden   [email-masked]

---

From: Don Fugler <[email-masked]>
Date: 3 Nov 1995 20:42:16 GMT

[email-masked] (Nick Pine) wrote:
>Does anyone use a woodstove or fireplace chimney as an air-air heat exchanger?

Dangerous stuff.  Most modern woodstoves and chimneys do not have a great 
deal of excess heat, beyond that needed to drive the smoke out of the top 
of the chimney.  Once you start mining that heat, you can be looking at 
creosote, chimney fires, poor draft, backdrafting, CO poisoning, etc.

If you have an old system with loads of extra heat, a solid fluepipe, 
good monitoring and calculation skills, and you want to play around in 
the spirit of scientific curiosity, sure - try something out. Make sure 
you leave enough for venting. Evacuate your family maybe.  If your actual 
goal is to extract the most heat possible from the wood you burn, get 
yourself a good EPA-certified stove and chimney and trust that the 
manufacturers have done their own research. They have.

---

From: [email-masked] (Michael O'Brien)
Date: 6 Nov 1995 19:45:14 GMT

exchanger?
> manufacturers have done their own research. They have. 

Totally agree. Even if you can get some heat out, the risks just are not
worth it.

In Oregon, I think most wood stoves are operating in thermal envelopes
where there is a great deal of insulation, air sealing, replacment window
upgrading, etc. waiting to be done, and that would improve both comfort
and efficiency much more cost-effectively. Tweaking the stove efficiency
would be way down the list of priorities. And even then, using dry wood
would improve heat output more than any other single measure available to
the homeowner.

Mike

-- 
Republicans: Threat or menace? You decide.

---

From: [email-masked] (Richard Austin)
Date: Mon, 06 Nov 1995 23:39:35 -0600

In article <[email-masked]>,
[email-masked] (Michael O'Brien) wrote:

>> of the chimney...

One interesting approach I have seen is to run the stove pipe under an
adobe bench, heating the bench before going up and out the wall. The warm
bench worked like a Russian tile stove storing the extra heat from the
pipe.

-- 
  Richard Austin -- [email-masked]
   Institute for Planetary Renewal
    http://www.rt66.com/~richard/

---

From: [email-masked] (Sherman, Michael E. - Fiddlin' round.)
Date: Mon, 6 Nov 1995 23:08:32 GMT

>> deal of excess heat, beyond that needed to drive the smoke out of the top 

...snip

>Totally agree. Even if you can get some heat out, the risks just are not

Totally disagree.
Creosote is only present if the fire is not hot enough.  If the fireplace is
designed to burn the wood fast and hot, then most of the heat can be safely
extracted.  Check out the designs of Russian and other masonary heaters. 
They burn the wood quickly and at a high temperature, burning the creosote 
and other gasses at the same time.  The heat is then captured in a flue with
many switchbacks.  There are several of these on the market (like Tulikivi).

Most modern woodstoves burn at a low temperature, and then pass the gasses
through a catalytic converter to burn (most of ?) them.  

Also check out the heat recovery system on a gas furnace flue used in the
Waterloo, Ontario Green Home.  See the Book "Green Home".  See the web site
http://web.cs.ualberta.ca/~art/house/ for info on the book.

-Mike

Mike Sherman  <[email-masked]>

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From: [email-masked] (Nick Pine)
Date: 7 Nov 1995 08:49:17 -0500

Richard Austin <[email-masked]> wrote:
 
>adobe bench...

That sounds interesting. What are they called, bancas?

Reminds me of the traditional Chinese peasant "stove," combined with a
large hollow masonry sleeping platform, built like a Roman bath floor...

Nick