My current boiler burns 24 hours on one filling. It has 350 CDN gallons water storage. I built it from large bullet style propane tanks and metal from scrap yards. I heat a 135 year old home, about 2500 sq feet. If you can weld and scrounge, you can build it yourself.
Manufacturers have cutaway pictures of their designs.
I've read you have to put the fire out in a hot air furnace in the event of a power outage, because the fire would melt the furnace. With my hot water stove, there is no danger of melting if the power goes off. It just takes electricity to operate the pump and fans to transfer heat to the house. I suppose that would be true of any unit with a water jacket. When the power is out, the fire shuts down and smolders.