I always put my domestic expansion tank on the cold inlet to my boilermate with a check valve behind it. I see a lot of guys just placeing a tee in the domestic cold in any old location with the tank there. they will have the check valve located back at the meter. I put 2 check valves, one at meter and one at boilermate. Tanks sweat here when cold water goes through them big time. I don't like the idea of a tank of water sitting at a different temp in the cold line. On the cold inlet to hot water tank it is going to be heated anyway so let it preheat there. If the tank sweats it will be dried off by the heat generated. Any comments/sugg. welcome.