I may be lost. If you have a normal residential electric water heater, you really shouldn't need a thermal expansion tank, unless your water pressure is to high, or your water temp is to high. You say your water heater is set at 120 degrees. Does it feel scalding hot at times? Maybe try checking the temp of your water with a thermometer. This problem of the leaky temp/pressure relief valve may be as simple as a fualty themostat on the electric water heater.

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Originally Posted by mygirlsdad77

If you have a normal residential electric water heater, you really shouldnt need a thermal expansion tank, unless your water pressure is to high, or your water temp is to high. You say your water heater is set at 120 degrees. Does it feel scalding hot at times? Maybe try checking the temp of your water with a thermometer. This problem of the leaky temp/pressure relief valve may be as simple as a fualty themostat on the electric water heater.

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Mgd77,

You are incorrect on not needing an expansion tank. On city water, there is a back-flow prevention valve at the meter that prevents contamination of the water distribution piping network in case of a loss of pressure. When you turn on a water heater full of cold water, the expansion as it heats could drive the pressure on the house above safe limits because of that back check valve. Usually the T&P valve blows off in that case. Around here it is a code requirement to have an expansion tank on any house that has city water and a water heater, which most do! :p

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Originally Posted by Quantum62

Yup. i was referring to the expansion tank only having 8 PSI when everything is open. I would "assume" that it should read 40PSI. So you guys think the expansion tank has a ruptured bladder? Is there a way to check this w/o taking it off?

Is 80 PSI water pressure really that high that I need a PRV? What is a standard pressure?

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Q62,

To test your expansion tank, shut the water off at the meter, shut off the water heater, open a valve in a sink on the lowest floor and let the pressure go down to nothing. Measure the pressure in your xpn tank and if it is 8 psi, pump in some air until you get it back to it's 40psi precharge number. If the pressure just keeps leaking away (thru the open faucet), then the bladder is shot. Otherwise PUT THAT PRV ON and then precharge the xpn tank to the same pressure as the PRV setting. NO MORE and not much less. You don't want much water in there at all under normal working conditions. It should only have to take water to absorb the expansion of heating water. If you actually read the installation instructions on those things they will tell you to do what I just said.

EPM

I think I recommended a PRV in post #2. It's the logical thing at this point. I did however misunderstand KISS and Letme for a moment, I'm on board now guys.

Varies from state to state I guess. We are not required to install expansion tanks on potable water systems. Just boilers and the like. I guess I'm just saying that the expansion tank is most likely not the problem for pressure, temperature relief valve leaking from time to time. We have no idea where asker is from, or what codes they follow.

I may be incorrect, for the area you live, but please do not say that I am (in general) incorrect. Kind of hurts my feelings. Lol

If Check valve (Backflow preventer) on Main then install Expansion Tank

If Boiler Then install Expansion Tank

If incoming water pressure is too high (>~60 PSI) [Expansion Tank Irrelevant] then install Pressure Regulator (PRV).

OK?

The water pressure at the tap is 80 PSI. It almost sounds to me like the expansion tank has ruptured AND I have to hig of a pressure entering the house.

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Originally Posted by KeepItSimpleStupid

Let me think about it too. Air is compressible. Water is not.

This gage is measuring water or air pressure? If it's measuring air pressure I could see it being 80 PSI. If it's measuring water pressure then I don't see it.

But, on the same note if you have a PRV, you should not be able to get to 80 PSI at all. As say 50 PSI air is compressed by 40 PSI water, you'll end up with a 10 PSI difference.

The expansion tank works because it has a variable air volume and that volume is pressurized just below the normal water pressure. It cannot read higher than the water pressure.

Think about this: Can you inflate a tire to 90 PSI from a 40 PSI stream. No.

Water is a wierd fluid. It expands when cooled and it expands when heated, but we are concerned about the heating. Heating water below the boiling point will not raise the pressure 40 PSI.

Go buy a pressure gage that fits on a hose fitting and measure the house pressure at an outside tap. It won't be between 45-55 PSI.

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Get the pressure reducing valve installed first. There is no point troubleshooting the expansion tank now.

You do stand a chance of having a bad tank, but there is nothing pointing to that yet.

For lots of bedtime reading material about expansion tanks, see:

Learn About - What is thermal expansion, selecting an expansion tank, Potable vs. Non-Potable systems.

Sounds like kiss nailed it. Good luck and let us know what you decide.

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Originally Posted by mygirlsdad77

Varies from state to state i guess. We are not required to install expansion tanks on potable water systems. Just boilers and the like. I guess im just saying that the expansion tank is most likely not the problem for pressure, temerature relief valve leaking from time to time. We have no idea where asker is from, or what codes they follow.

I may be incorrect, for the area you live, but please do not say that i am (in general) incorrect. Kind of hurts my feelings. lol

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Sorry, I didn't mean to hurt your feelings, but I thought that was current code all over. You can see that I don't travel much! :p I will agree, an expansion tank is not needed on a city water system without any back flow prevention valves. However, I've seen leaking T&P valves that were "fixed" by nothing other than the addition of an expansion tank. But then I've also seen systems that worked fine with a back check and no expansion tank. A lot of piping helps. Around here one municipality I know of is going around and reinspecting homes and requiring back flow prevention and expansion tanks to be retrofitted. Other obvious violations are cited too, but they are bringing their systems up to date with the back flow prevention valves.

I don't travel much either, so I learn new things everyday on this site. We are not required here to have back flow preventers. No hard feelings. Thanks for the update.