The T&P valve on our HW heater starting leaking, so I replaced it, and new one still leaks. Getting about 6-8 oz of water every other day, sometimes every day.

Checked my system pressure and am getting around 74psi when HW heater is not running, but 100-115 psi when it has been running for awhile. (And I assume, close to 150psi, since that's the max pressure on the T&P valve?)

Temp on HW heater is set at 120. Heater is 50 gallons. 14 years old, A.O. Smith.

We do have a backflow preventer valve, so from what I've been reading, it sounds like I need an expansion tank, but am confused as to how to calculate what size tank I need. For sizing, do I use the lower system pressure reading of 72 (when the heater isn't causing a spike?), or the maximum pressure I'm seeing when the heater IS causing a spike? Does my question make sense? The calculators ask for a "supply pressure or maximum system pressure."

Also, should I install a pressure reducing valve? Or is the expansion tank itself all I need?