I saw a very clever wood stove built on the cheap in a muffler shop a while back.
Imagine a 55 gallon drum built for the firebox, with another 55 gallon drum on top with 2 very large exhaust piping pieces allowing the smoke to go into the upper drum (which is sealed off) then a generic stovepipe attached to the upper drum heading outdoors. I asked the muffler shop owner about it (it's a one man operation) and he said he recorded a 90 degree F temperature on the output (I assume with an infared thermometer.) Apparently he kept his underinsulated shop comfortable with this rig.
I've also read how Russians have very convoluted chimnies to capture as much heat as possible. I suppose they invented the geo mass concept (go ruskies)
My point being the simplest heat extractor would use heat from the smoke. If you downsize the pipe as you extract more and more heat you can keep creosote down to a minimum, or you can design everything assuming creosote is going to build up and make it safe to have an occassional chimney fire (insulated pipes in the house, a long pipe on top that vents the fire safely.)
Or you could have a planned cresote burn where the exchangers are turned off and you put in some dry oak and hickory to make the chimney fire happen when you expect it.
Just some random thoughts, dispose of the useless info
