I hadn't paid close attention to the method, just the general idea of a 3 purpose furnace. Air entry must be isolated from exhaust. One solution is to draw the air through pipes into the shop -- a simple heat exchanger.
Another possibility is to route a metal chimney through the interior and back out.
In one of my sheds, made of concrete block, I have a Listeroid diesel generator outside, with its exhaust pipe routed through the shop and back out. Its cooling water and radiator are also located in the shop. The radator is an auto radiator and angled in two planes to allow convective circulation by both the coolant and air inside the shop. A carbon monoxide alarm is mandatory in that kind of setup.