Not exactly fossil fuel heat, but recently I was staying at a place that had a wood burning furnace which is explicitly designed to work even if the power goes

Not exactly fossil fuel heat, but recently I was staying at a place that had a wood burning furnace which is explicitly designed to work even if the power goes out. The hot air naturally rises and pressurizes the vents slightly, and the only constraint is you have to set the damper in a way that keeps the fire small/cool. I don't think it would be possible with natural gas or anything like that, as you really do need a thermostat...I assume a continuous burn without the blower is not something they're designed for.