If I understand what your saying, is if I need the heater to keep me warm I would open "the" valve and the hot water would flow normally, and when not needed (summer) I simply close off the valve.
Sorry for being dense, but this is a first for me.
In this picture, you can see where the PO had installed a Prestone flush and fill devise.
http://rides.webshots.com/photo/2144250750054402330lWbUcu But the correct valve waould go somewhere on the hose?
