My pellet stove was throwing a lot of heat overboard! I would bug me to come home and watch hot gasses rippling out of the chimney. So I made my own heat exchanger out of 3 inch single wall vent pipe. The pipes are folded side to side behind the pellet stove. 12 feet of straight tubing plus another 5 feet or so for elbows, etc. I joined and sealed all connections with hi temp rtv. The parts nearest the stove are steel, about 8 feet, then switches to aluminum single wall for the remaining portions. I found that they don't radiate well when all shiny so I painted them flat black, with a silicone based high temp spray paint. All tubes come out annually for cleaning the ashes out. You can feel a lot of heat radiating from the tubing, and there is a big temperature drop in the tubing surface from the stove end of the tubes to the colder blower end.
I added a couple computer type fans blowing room air onto the hot tubes, controlled by the thermal switch on the exhaust port so the fans are on whenever the exhaust is hot.
My stove has an air pressure switch that will shut down the stove if the burn chamber is not held in vacuum by the exhaust blower. I removed the exhaust blower from the stove and placed it after all the heat exchange tubing so it still draws the burn chamber down into vacuum, but it also draws down the heat exchange tubing so never an exhaust leak!
This cut my fuel consumption by nearly half!! Now the stove is set on the lowest low-low setting and that is too much unless it is in low 40s outside or colder. If it is high 40's or 50s outside the stove cycles on/off by the wall mounted thermostat. If it is in the 20's outside It cycles up/down from 1 to 2.
Only rarely, like after a power failure, I'll run it at 3 but no matter how cold it is outside it only takes a half hour to take a cold house to 72 degrees. Never run it at 4 cause it's too hot and will damage the burn pot.
I think it would be worth adding another 4 aluminum sections 2ft each between the cold end and the blower. That would make a total of about 20 to 23 feet overall of the 3 inch tubing. So now you know how much tubing it takes to capture all that waste heat!
The biggest factors for thermal energy exchange are temperature differential and time. So I have tried to slow the passage of the exhaust gasses by limiting incoming combustion air, to give it more dwelling time inside the tubing. Also, having the least fresh air leaking into the burn chamber or exchanger tubes to maximize exhaust gas temperature.. I wanted only enough air to support clean combustion, but when I slow the air that much the fire pot holes get blocked every other day, so have to clean more often, but on the good side less ash is carried into the exchanger tubes with the slower incoming airspeed. So I get the most heat by reducing incoming air leaks and slowing combustion air to minimum for clean combustion. Another thing I tried was to block 1/2 of the holes in the burn pot and reduce the burn pot volume to 1/2 by fabricating a stainless steel insert that fits inside the burn pot. Now with half the holes blocked the air speed in the remaining half is doubled which keeps that half of the pot cleaner longer, cleaner hotter yet smaller fire. The firepot insert must be a snug airtight fit, but it must be removed if the stove is run above 2.
Well, that's about as far as I have taken it...