YAPH was thinking very hard :
Not possible. Where are you going to recover the heat from? The cold air
is coming in to replace air being burned in the appliance and going up the
flue as Carbon Dioxide and Water (as steam). There isn't a corresponding
flow of warm air out of the room, as there is with mechanical heat
recovery ventilation units.

There is, but it goes up the chimney or flue.

One old solution, was to have an under-floor air vent where the air
supply came in close to the point where it was to be burned - rather
than at the back of the room, where the cold air would travel past the
room's occupants.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk