Would it be possible that Thermal Expansion could be causing me a 100 lb pressure increase, even on the cold water side of my system?

I installed a new water heater and the T&P keeps relieving, with pressure spiking from 65 psi to 150 psi during reheat. I thought new water heaters had a check valve on the cold water inlet to prevent backflow into the cold water supply side?

Thermal expansion of water? Yes.

You need a place for the water to go when it heats up. The old way was some dead legs on copper pipe. I don't know what you have theses days (I think a plastic sphere thing).

~egon

10-4. I plan on installing an expansion tank to hopefully remedy. I never would have dreamed that thermal expansion could cause a pressure increase as high as 90 lbs. Thanks.

http://www.youtube.com/watch?v=rGWmONHipVo