robala
Temperature and pressure valves use temperature and/or pressure to open. On a water system that has backflow preventers, no thermal expansion tank, and a line pressure close to the T&P pressure, as the water heats it expands and increases the pressure enough to open the valve. Also opening a faucet or other fixture in the house brings the increased pressure back to line pressure. You need to install a thermal expansion tank for the system. Good luck
