steve86 may lack a little tact but Dr. Heat Pump would have to come down on his side on this one.
Though, I can imagine a vapor-compression refrigeration device used to capture waste heat from the compression of a gas. This would be an auxiliary unit used for heat reclaim and would certainly be secondary to the main process.
So if you happen to be running a 75HP air compressor constantly, you may be able to heat your house for free.
Central Services heating engineers may be able to help!
http://movieclips.com/smFA-brazil-movie-harry-wastes-central-services/