- Some coal furnace manufacturers used to provide for that.
I used to have a furnace with two efficiency ratings. It was a coal furnace (modified-to-oil).
It was set up for gravity feed, and this involved large sloping airducts and a *huge* diameter return pipe. The heat from it was very gentle and uniform and quiet.
A fan on the return duct could be added to convert it to a 'forced air' furnace, with the higher efficiency rating. Some of my neighbours had systems like that.
So if you didn't mind the pipe octopus, it worked fine.
Notes: The gravity coal furnace required no electricity, but the oil-burner-modified version did.
