PRV and Expansion Tank

The water pressure in my house was 120 psi so I installed a pressure reducer valve and thermal expansion tank today. I set the prv at 65psi and charged the expansion tank to 65 psi prior to installation. After taking a shower and running the washing machine, the pressure increased from 65 psi to 85 psi. I know that it is normal when the hot water heater is heating, but I was thinking that the thermal expansion tank would alleviate the increase in pressure. Is this normal and is everything operating correctly?

It shouldn't go over 65#s try turning off the water, let all the pressure out of the system and re set the tank pressure to 62#s...

expansion

The expansion tank does not alleviate the pressure buildup, it absorbs most of it. But since the air pressure will ALWAYS be the same as the system pressure, as long as it started equal to it or less, the pressure will HAVE to increase somewhat, (it has to go somewhere. It cannot just disappear because of the tank.), but to a much lesser degree than if the tank were not present.

All valves, especially PRV's have a certain amount of "creep". This means that even though they should close at 65 PSI, they let some pressure "creep" through and the pressure will increase. A larger expansion tank will give it more room to "creep", and keep the pressure lower. A 2 gallon size tank will only hold 4/10 of a gallon between 65 and 85 PSI. It doesn't take much "creep" to pass 4/10 of a gallon. Without that tank though, that much creep could have increased the pressure to 200 PSI.

Did you size the tank properly? http://www.watts.com/pro/divisions/watersafety_flowcontrol/support/support_DETsizing.asp has a worksheet that will calculate how big it should be for your tank and location.

creeping tanks

valveman said:

All valves, especially PRV's have a certain amount of "creep". This means that even though they should close at 65 PSI, they let some pressure "creep" through and the pressure will increase. A larger expansion tank will give it more room to "creep", and keep the pressure lower. A 2 gallon size tank will only hold 4/10 of a gallon between 65 and 85 PSI. It doesn't take much "creep" to pass 4/10 of a gallon. Without that tank though, that much creep could have increased the pressure to 200 PSI.

Click to expand...

that is about right, I have one in my home and the first thing in the morning ou can get a surge through the system till the extra pressure purges itself from the home.


the larger expansion tank is the only one i ever install..



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