I think i have a way to heat water in a separate tank using the descending gases. Oil fired boilers in the UK often blow the fire up through a square steel water jacket tube (30cm x 30cm x 100cm) with baffles. I found one at the scrap, removed the baffles and the base and have what could be a great riser chimney that i think would work as a water heater if lined with insulation and then with the thin heat bricks as the actual riser chimney. As the secondary combustion gases descend the outside of the water jacket will pick up the heat and once hot set up a thermal siphon to an adjacent hot water tank. The chimney encapsulation will be made from brick and a fire mortar mix to allow the flow and return pipes to be built in using 300C silicone sealant. The tank is vented. The top will be a steel plate from an old range for cooking on. There will also be a small radiator on the return loop to shed excess heat. What do you think?