Maybe this was too obvious for me to think of, because when I read it, I thought, "WOW! What a clever idea!"

I am currently reading Tom Brown's Field Guide to City and Suburban Survival (1984). His suggestion for safe indoor heat without danger of carbon monoxide or oxygen depletion:

He says even if you can't build a fire indoors, you can build one outdoors and bring the heat in. Build a roaring fire outdoors and drop several large rocks, bricks or chunks of concrete into it until they are very hot. Then carefully shovel them into a large stainless steel or other non-coated cooking pot (don't use aluminum, galvanized or painted containers).

Carry the pot indoors, maybe running a broom stick through the handle and having a person at each end. Place the bucket on a noncombustible, insulating platform, like an oven pan or oven rack supported by bricks. He says a 2-gallon bucket of hot rocks will heat an average sized room for hours.

Sue