I have a Pleasant Hearth with 6 inch flue and I want to heat some hot tub water with the wood stove. I thought about the stainless steel tube in the box, but that voids the warranty and insurance would frown. I thought about heating water using the exhaust gases. I have a 6 foot run of single wall pipe that I might put some 3/4 copper tubing either inside or out. Out is safer but I wonder if it would heat the water enough. I also wonder if inside the exhaust would get too hot for the copper? Any thoughts?