New T&P valves on water heater leaking, pressure driven from 65 to 165psi
Follow the manufacturer's installation instructions, and install an expansion tank. "Up" or "down" doesn't matter, only the pressure the tank is charged to matters.

When water is heated in a closed system, the pressure increases.  When you install a check valve or regulator on the water supply, the pressure increase cannot push back into the supply plumbing.