Waste Heat Recovery

Has anyone attempted to heat the inside of a TM by recovering the heat that is normally wasted?

While we were boondocking in cool weather and trying to conserve energy, I noticed that there was quite a lot of heat coming from the flue gas of the water heater and refrigerator. This heat associated with the flue gas is being exhausted to the atmosphere.

I'm thinking if one could somehow reroute these flue gases so that the ducts ran through the TM before exhausting to atmosphere, a small amount of heat could be radiated for warming in moderately cool weather without the use of a fan motor.

Just a thought. Any comments?