Water heater problem. The T&P valve is leaking. It might go 2 weeks and then BAM... water on the floor. I have a thermal expansion tank installed. I replaced the T&P valve, but it still randomly leaks. The water pressure is 80 PSI. The temperature is set at 120 degrees. I checked both thermostats and they appear to be working correctly (electric water heater). I have NO idea what could be causing the problem. I was looking for information about the thermal expansion tanks (TET). They say that it comes pre-charged to 40 psi. When I have the water turned on, the TET is 76psi. If I turn off the cold water intake, and open the hot water faucets until they stop running (so the system is "open") the TET reads 8.5 psi. Is this normal? Any idea what could be wrong?

I think you could install a pressure reducing valve (PRV) right after your meter to get the house pressure down to about 45 psi. Your tank pressure gauge is likely just off by about 8 psi, it should read zero with the inlet closed and the hot taps open

One idea---If you tank should have a pressure of 40# with the water off and the pressure relieved then I think your bladder in the tank has a hole in it and needs to be replaced.

If this is not the case then try to add pressure to your tank with and bicycle pump or somethiing similar.

I'm confused by post #3, we are talking about a water heater, not a well pressure tank. There is no bladder in a water heater. With the supply closed and a hot faucet open the pressure should be zero.

Zero, no. Pressure is zero at the outlet of water of a pipe, but somewhere along that pipe there is a pressure drop.

Your not measuring the pressure at the outlet. Your measuring it somewhere up stream, thus frictional losses from pipe, elbows etc. show up. 8.5 psi would not be unusual.

Not sure what you meant. There is no supply to the tank and the tank is open at the other end via a faucet. That lets the tank pressure drain from 76 psi until the water stops completely, that should be zero. Are we looking at the same picture in our minds eye?

I think Quantum62 meant that expansion tank has only 8 lbs of pressure when water pressure is relieved. Indicating ruptured bladder in expansion tank. First thing to do however, is install pressure regulating valve regardless.

Yep, you really need to install a PRV.

KISS said it: Install Pressure Reducing Valve.

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?

Thanks for the help guys!

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Originally Posted by hkstroud

I think Quantum62 meant that expansion tank has only 8 lbs of pressure when water pressure is relieved. Indicating ruptured bladder in expansion tank. First thing to do however, is install pressure regulating valve regardless.

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Originally Posted by ballengerb1

I'm confused by post #3, we are talking about a water heater, not a well pressure tank. there is no bladder in a water heater. With the supply closed and a hot faucet open the pressure should be zero.

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I am sorry about the wrong answer that I entered but I thought he already had a thermal expansion tank. That is why I thought that he had a ruptured tank.

No, your answer was correct. I was talking about an expansion tank.

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Originally Posted by letmetellu

I am sorry about the wrong answer that I entered but I thought he already had a thermal expansion tank. That is why I thought that he had a ruptured tank.

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Originally Posted by Quantum62

No, your answer was correct. i was talking about an expansion tank.

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Then it is still possible that it is the bladder in the expansion tank. Water can not be compressed and so without a bladder pressure the pressure could build up past the rating on the T&P valve and when it opens it only drains a small amount of water because the pressure would drop very fast.

I posted this above:

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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Originally Posted by letmetellu

Then it is still possible that it is the bladder in the expansion tank. Water can not be compressed and so without a bladder pressure the pressure could build up past the rating on the T&P valve and when it opens it only drains a small amount of water because the pressure would drop very fast.

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Standard pressure is around 45-55 PSI. Your WAY above that. You need a PRV ASAP.

I figure 8 PSI is not unreasonable for reasons I said before. The closer the gage is to an open tap the lower the pressure is going to be. In the limit, i.e. when the pipe is at atmospheric pressure, the pressure is 0 PSIG.

Yes, but the 8PSI is on the expansion tank. Wouldn't I be measuring the pressure between the bottom of the tank and the diaphram? To me that SHOULD read 40 PSI, because isn't that what the tank is pre-charged to? Perhaps I'm missing something.

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Originally Posted by KeepItSimpleStupid

Standard pressure is around 45-55 PSI. Your WAY above that. You need a PRV ASAP.

I figure 8 PSI is not unreasonable for reasons I said before. The closer the gage is to an open tap the lower the pressure is going to be. In the limit, i.e. when the pipe is at atmospheric pressure, the pressure is 0 PSIG.

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Are you on city water or private well?

City water

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Originally Posted by mygirlsdad77

on you on city water or private well?

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If tank says its precharged to 40psi, and you are only reading 8psi, then yes, you may need to replace tank. However, if you instal a prv. Lower your pressure, you may not even need the expansion tank.

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 weird 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.