Old House. Your logic is good up to a point. But unless you already have a pressure reducing valve or a check valve in the water supply to the house or heater, the expansion that takes place will dissapate into the water service mains and not change the pressure in the water heater or house. You should test the water pressure with a gauge when the heater has finished heating the water and while it is heating. If there is a difference then you have some type of backflow preventer in the system and you need an expansion tank.
fourth year, I got your response after I sent the last one…
You describe how I THOUGHT the system should work, which is why I questioned the response from GE.
Before I installed the expansion tank, the cold water line to the water heater would get warm while the water was heating. This and a visual inspection (Yes, I took it apart) tell me I don’t have a check valve at the water heater. I know I don’t have a check valve or PRV anywhere else in the house, UNLESS it’s at the water meter out by the street.
Based on this, I didn’t think I needed an expansion tank. When the water expands, the extra pressure is supposed to be absorbed by the water supply out to the street. Right? I installed the expansion tank anyway, though I’m still not sure it’s serving a purpose other than to have something else to hit my head against.
Right now, the pressure on both the hot and cold is 90psi. The water heater hasn’t been heating in the last few hours. I want to test the pressure while the pressure relief valve is open, but it hasn’t happened again since I bought the gauge. I think this will tell me a lot.
Either way, based on earlier responses, it sounds like I still need the PRV. I wonder how much I can get for a slightly used expansion tank on ebay.
As always, I appreciate the responses!