If you live anywhere near some farmers who raise corn, wheat or rye, you might be interested in what I consider the cheapest heat available, with the exception of wood that you cut yourself.
Which requires quite a bit of largely specialized equipment and a lot of work (it heats you about 8 times between cutting the standing tree and throwing a block into the fire). Plus an available supply of raw material, whether appropriate trees or tops remaining after logs have been cut from a woodlot.
In this area around the Great Lakes, one can heat most of a 2,000 sq. ft. house (with good circulation, as most stoves have no ducting, and insulation) using about 150 - 200 bus. of corn per year, at about $3.00/bus. ... can you heat your living space that cheaply?
I sold such stoves, about 15 years ago, for a while.
A friend of mine, who'd sold a couple of models previously, one of them that he, quite mechanically capable, couldn't make work, said that he could build a better on ... and did.
One of the best ones available today, I think, though I'm somewhat out of touch.
As the fire requires power draft, one does not need a chimney - they can be vented though any exterior wall.
Two major problems - they must have electricity to function and one can never go and leave them for more than slightly under a day, as they require maintenance, or the fire goes out.
Environmentally friendly, into the bargain - testing agency had to re-test, as they couldn't believe the initial set of results!
If you're in need of hot air (more than your friends can provide directly), these systems of heating may be useful for you.