I use an "in line" valve to control the water flow through my heater. I have the "heat" control of my 57 GMC deluxe heater control connected to it. Typical use is to cut open the heater hose that supplies hot water to the heater and insert the valve. Typical cost is around $10 at FLAPS. Valves can be found in "push to open" and "push to close". I've seen them that fit 3/4" or 5/8" hose on both sides as well as 3/4" on one side and 5/8" on the other. Some are plastic case, some metal. If your parts person can't figure it out, ask him to let you look at his (I think Wagner) book that lists heater valves. You'll find what you need in it. If he doesn't have a book, you are in the wrong parts store.