Your Garage Heating System
http://www.portageandmainboilers.com/
outdoor wood furnace ... but need the open area and supply of wood.
Pipe the hot water into house/garage/shop and neighbour house lol Then to radiator in house/garage.
In the shop I use an old outdoor propane tank cut in half and a door added to the front. That thing can take heat... literally melts your pants off if too close. Burn wood...garbage.. whatever fits.
I always found natural gas and oil burners cold. We just upgraded our house to a corn furnace. about a bushel a day in an old farm house. so at the price of corn $7.00 a day. Only flip the corn furnace on when its really cold out.