I have a 250,000 BTU gas forge in my shop that produces a LOT of waste exhaust heat. I have a hood over the forge ducted into a 12" pipe with a 1/2 HP in-line blower to pull the exhaust out of the shop. The temp in the pipe runs about 350 to 400 degrees F. Not sure on the CFM but it will blow your hat off. I want to use the exhaust to heat water in an 800 gallon tank for hot water heat. I thought of putting a truck radiator in the pipe. How could I best transfer the heat from the air in the pipe into water?