Heat-exchanging venntilators
From: DanR510 (DanR510aol.com)
Date: Fri, 4 Nov 94 08:48 CST
Several years ago I gave considerable thought to wood stove design. 
I thought about doubled-walled stovepipes, not for fire safety 
purposes, but to bring cold outside air in and recover the heat 
going up the stovepipe, rather than pulling warm air in from the 
room, and cold air into the room from outside. One problem here is 
that you'd need a very clean burning fire to prevent chimney 
condensation of creosotes and tars, much of which would normaly be 
carried away as smoke. You'd also need maybe a stainless steel 
stovepipe to keep water condensation and other chemicals from 
rusting it away very quickly. You might also need a blower to 
replace thermal convection draft.

While on that tack, I also thought of this type of ventilator. 
(Perhaps unfortunately, I went on to other things.) Frosting up is 
one problem I hadn't considered, living in western Oregon at the 
time. Getting dirty is one I did consider. Either one reduces heat 
exchange. To be efficient, it must must have a lot of heat- 
exchanging surface, which would be hard to keep clean. Do present 
versions do anything about this?

Another important factor, which I assume commercial versions 
incorporate, partly since it would be more complicated not to, is 
the counter-current effect. Two bodies of still air, or air currents 
moving in the same direction, of different temperatures, with a heat 
conducting surface between them, will both approach a medium 
temperature. 0 degree air and 70 degree air will both approach 35 
degrees. Two air currents moving in opposite directions will each 
approach the original temperature of the other. The 70 degree air 
will approach 0 degrees, and vica versa. Just how much heat is 
exchanged depends on how long they share the heat-conducting 
surface. 

To Roger Diggle:

I've always assumed that carbon dioxide is a major pollutant in a 
"tight house", but you don't mention it. Not so? 

>Air-to-air heat exchangers, or, as the industry seems to want to call them
these daze, heat recovery ventilators,...

"Air-to-air heat exchangers" says nothing about the fact that they 
are venilators, which seems rather important here. 

>They also offer an impermeable heat exchanger core (polyethylene, I
think) that would not be appropriate for Northern winter weather.  

Is aluminum (a very good heat conductor) or other metal a problem 
because it corrodes too fast?

I like the idea of the "heat wheel", but preferably with flat 
surfaces instead of honeycombs. Then brushes could isolate the two 
air streams, and also keep the surfaces clean.

Now that I've said my piece, and hopefully impressed you all, I must 
also say that it seems like this subject would be more appropriate 
under something like "building environmental design".

Dan Robinson

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