Water heater expansion tank size

Sep 10, 2006
Charlestown Indiana
I have a 50 gallon electric water heater in my house. After a pressure spike (100 psi) from my municipal water company my T&P valve began leaking. I added a pressure reduction valve and got the pressure to 70 psi. The valve continued to leak so I replaced it. The new one leaked so I reduced the temperature of the water from 140 degrees (which it’s been for 5 years) to 130 degrees, it leaked again. At this point I can only assume that the bladder in the expansion tank (6 years old) was damaged by the pressure spike. The existing expansion tank is 2 gallons, should I get the same size or bump it up to a 4 gallon? Thanks in advance for your help!
I can not help w/ the size but you can call Supply House.com and they can answer your question.

The expansion tank pressure inside needs to be adjusted to match the incoming house pressure to work properly.

If it is leaking press on the shredder valve and if you get water, it is bad.
The expansion tank is probably upside down with an air fill at the bottom. Push a nail into the air fill valve and see if water comes out. If it does replace the tank. If not maybe use a compressor to put more air into the expansion tank.

Remember a 2 gallon water tank (expansion or well pump) does not hold 2 gallons of water. Much less.
With no water pressure (turn off the supply to the heater and open a hot water faucet), fill the tank with air to a few psi less than the PRV pressure.
do you have a water pressure gauge .. what is the pressure when its leaking?

If I read this right you think the expanding water from heating is over pressurizing because possibly the expansion tank has failed?

The right way to check/fix the expansion tank is to remove pressure from system and pressurize it with air to your normal water pressure (or slightly less) but not to exceed 80psi for most tanks.

Edit: I see Mk378 got his reply in while I was refilling my coffee :)

What is the orientation of the expansion tank if it is "upside down" you can check as Donald described.

If you have room a 4 gallon wouldnt be a bad idea... esp. if you have teens etc and you end up using all your hot water routinely, but a 2 gallon should be sufficient.
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The expansion valve is oriented so that the air fill valve is on the top. I pressed on the valve with a small nail and nothing happened. I put a tire pressure gauge on it and it shows no pressure