I am about to have my original 19-year old Atwood GCH10A-3E water heater replaced with a new one. Atwood says that their GCH10A-4E is the recommended replacement unit. I have already bought the new water heater. The old and new water heaters have a heat exchanger (motoraid) that heats the tank while driving. Under the water heater there is a valve that varies the...