Any thoughts on what the problem is? It is leaking about a 2 cups or so a
day. Do I have to replace the water heater which is now over 15 years
old? is your water extremely hot ? if so your thermostats are not shutting
off causing temp to go above preset level of relief valve (150) replace
therms.
You may have a lot of mineral deposits causing the valve to stick partly
open. You should change the valve and completely flush out the tank. I
wouldn’t bother changing the tank unless it springs an internal leak
somewhere.
Do not replace the tank as yet. Is the thermal tank doing its job? Is
the temperature of the water between 120 and 140? Do you have abnormal
pressures in your main water supply? From your description the expansion
tank is the more likely suspect.
you could try turning the heat down on the water heater. if it’s a brand new pressure relief valve it shouldn’t be leaking.
If you have a pressure reducing valve on your main from the street it
may be defective and allowing the house pressure to slowly increase when
the water is not in use. Home Depot sells a pressure gauge you can
install on the water heater drain to monitor pressure.

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