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Water expansion tank pressure

Last year, I had a leaky T&P valve on the water heater and after replacing it I found that the real problem was a broken water expansion tank. After replacing it, the leak was fixed. Unfortunately, the T&P is leaking again and I found the water pressure to be 62 psi and my tank is only about 51 psi.

It is my understanding that the water expansion tank should be about the same pressure as my water pressure. What's the exact method of relieving system pressure so that I can pump up more air into the expansion tank?

Re: Water expansion tank pressure

There are several issues involved here but you can start by attaching a bicycle pump to the schrader valve (tire valve) at the bottom of the expansion tank & pumping up the pressure to 62 psi to equal the system pressure & see if that solves the problem.

Unfortunately, a leaky T&P valve could be caused by any one of 5 things: before even doing the above take your thumbnail & quickly press the schrader valve on the ET & instantly let go---if you get any moisture on your thumbnail, you have a leaky internal expansion tank bladder & your air charge won't last.

Since you've been having problems recently, check the hot water pressure by attaching a screw-on pressure valve to the drain valve at the bottom of the tank & opening the drain valve---check the water psi when the system is not making hot water, & when the system IS making hot water---turn the t-stat on the hot water heater up, or open a hot water faucet for a few minutes, then close when the heater comes on---the psi should not exceed 80 psi in both cases.

If you get a high spike above 80 psi in the water pressure, you may need a pressure reducing valve on the incoming cold water supply (if you don't already have one), otherwise, it's probably a leaky ET, or, then again, it could be the T&P valve is shot, or its sensor is caked up with calcium deposits---did you have a lot of deposits on the one you replaced previously??

Typical values for a T&P valve is to open at 125/150 psi or open at 210 degrees---ET's usually pre-charged at 40 psi---check the values stamped on your valve labels & ET to verify maximum pressures/temperatures.

Re: Water expansion tank pressure

I have a gauge that I attach to my utility sink to get the pressure. I've also attached it to the bib on the water heater.

I have the PRV where my water comes into my house set to 60 psi.

I still have to get a better air pressure gauge because the one I have for the car only goes to 51 psi. I've filled enough air into the expansion tank to be a little above 51 psi (maybe 55ish). Again, I need a new gauge.

The ET does not spit water out when I release air.

I ran a test this morning where I turned off the water heater, put my water pressure gauge on, and didn't run any water for four hours. The gauge remained at a constant 60 psi.

When I fired up the water heater, the pressure was about 75 psi in an hour or so. I fear that this pressure continues to rise while no water is being used, and, apparently, after long enough amount of time, it will trip the T&P valve.

I have the water heater set at the recommended temperature setting and my expansion tank is nearly matching the psi of the supply.

I assumed that since the water expansion tank exists in the system, that the pressure would never increase over 60 psi?

Re: Water expansion tank pressure

Yes, get a psi gauge that will measure at least to 100 psi & attach it to the drain valve hose bib---be aware that you have to pump enough air into the ET to match the water pressure when the HWH is making hot water.

Water EXPANDS 5% when it is heated (thermal expansion)---this is normal, and constitutes an increased water volume of a gallon or so when the HWH burner is firing---but since you have a PRV valve at the cold water inlet, this increased volume has no where to go----the ET internal bladder acts as a safety spring that absorbs this increased gallon volume, providing the air pressure is high enough so the ET bladder doesn't bottom out against the inner ET housing due to too low air pressure.

If the increased pumped air doesn't do the trick, it's a good idea to put in a new T&P valve as a next step, since they only cost about $10.

To put in a new T&P, you would have to shut off the power to the HWH, turn off the incoming cold water supply & vent any existing water pressure out by holding open the T&P lever & temporarily removing the copper overflow tube that goes to the floor so you can unscrew the old T&P valve---use pipe thread compound or thread sealant tape on the new T&P.

Re: Water expansion tank pressure

At this point, I've reduced my water pressure to 50 psi and have pumped up the water expansion tank to 50 psi. The T&P valve on the water heater is new and functioning properly.

What I've been seeing is that overnight the water pressure can build from 50 to over 110 psi.

I did repeat a test where I turned off the hot water tank and didn't run the water for over four hours and the water pressure remained at a constant 50 psi. When the hot water tank is on, the pressure slowly builds. That hot water temperature is about the same so it doesn't at all look like a run-away burner or something like that.

I'm really surprised that the system could go from 50 to over 100 -- doesn't that indicate the water expansion tank isn't doing it's job? Even though my water pressure is 50 psi, should I put more pressure into the water expansion tank to counteract the pressure built up when the water heater is doing it's thing? Or, am I looking at a water expansion tank (it's two years old) that has failed (I can still put in and take out air pressure)?

Re: Water expansion tank pressure

Does the pressure in the line rise above the setting of the pressure regulator with the water heater off? If so the PRV may be faulty and what you are seeing may be pressure creep. The higher pressure in the main line may be slowly passing through the regulator.

Re: Water expansion tank pressure

With the water heater off, the pressure does not increase at all.

Re: Water expansion tank pressure

Where exactly do you have this ET installed? Incoming/cold water side or outgoing/hot water side of the WH?

If on the incoming/cold water side......and your WH has an integral check-valve/heat-trap valve .......the ET won't be able to help/absorb/eliminate the increase in the pressure caused by the water expansion when it is heated. The ET should then be installed on the hot water side....... or the integral check valve removed...... or some modification to the setup such that the ET can be installed on the cold water side between the WH tank and any installed check valve.

Re: Water expansion tank pressure

I've included a picture showing my ET is on the cold/supply line, it is hung above the water heater, and the only thing that appears between it and the water heater is a ball valve. I have also traced the water lines and this is indeed the supply and the other line is the hot water that runs into the house.

Since I have run tests with the water heater off and no increase in water pressure happens, I assume this is thermal expansion. When measuring the water pressure from the nearby utility sink I use the cold water. I assume this indicates that the thermal expansion in the tank is expanding on both the hot water and the cold water lines. The nominal pressure is 50 psi and overnight it will work itself up to 110.

The expansion tank is pumped to 50 psi.


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