On Mar 2, 6:10*pm, Nate Nagel wrote:
a wrote:
Replaced the filter after doing some renos and noticed that the exhaust
(flue?) that runs through the cold air return portion (and thus heating
it) is just regular round pipe. *It occurs to me that it would be *much*
more efficient if there were thin metal vanes epoxied/welded to the
outside and thus giving a much larger surface area for the exchange of
heat.

Are there such vanes? *Can't I just make them myself?

a

You could shove in a flue from an old water heater, if it isn't too small....

nate

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Its not code or safe to have a flue run through a return