I have created a drawing for piping two tank DSH. A drawing that Bergy created has been posted here before. It shows how some manufactures recommend piping DSH with two tanks. I am showing another way to pipe this set up using another manufacturers suggestions. I am looking for opinions on which is better and why. You will notice an expansion tank which some locals require anyway. After all when you heat water it expands and has to go somewhere, and going out the pressure relief valve is not desirable.