Below is a posting I have made regarding high pressures on a potable water system. It was geared toward someone that had an initial burst of high pressure when opening a faucet. The concepts presented in the post apply to your situation as well and hopefully gives you the concepts needed to diagnose your problem. Post back once you have read it and applied the knowledge to your individual situation if you have any additional questions.
An actual measurement of the water pressure would be of great value to us. I would suggest a gauge with a lazy hand that records the highest pressure reached over a 24 hour period.
Testing products - PlumbingSupply.com
Pressure above 80 PSI. indicate a Pressure Reducing Valve (PRV) should be installed. Do you have an existing pressure reducing valve?
http://www.plumbingsupply.com/waterp...egulators.html
Of special interest to us is how and when the highest pressures occur.
1.) Lets say you have a pressure of 60 PSI. with the water flowing slightly. You shut off the flow and the pressure slowly creeps up to 120 PSI. and holds there. This would in most cases indicate a PRV is installed on the line and is set at 60 PSI. but it has failed and the pressure slowly leaks by until it reaches the 120 PSI in the water main. In this case the PRV should be replaced.
A second possibility for the above scenario is high municipal water main pressure with no PRV installed but has a restriction in the water supply cutting back the flow of water. In this case increasing the flow by opening additional faucets will further decrease the pressure.
2.) Another symptom is the pressure remains at 60 PSI. unlesss the water heater is operating. When the water Heater is operating after hot water is used the pressure creeps higher and higher. Many times the T&P valve on the water heater may discharge a small amount of water. This would indicate there is a closed system either from a check valve in the water meter, Back flow preverion device, or a PRV installed. In this case the pressure increase is caused by thermal expansion and a thermal expansion tank should be installed.
http://www.plumbingsupply.com/therma...ion-tanks.html
If a thermal expansion tank is installed it may not have a proper air charge in it or, the tank has failed. On the tank there is a schraeder valve like a tire valve. Depress the center of the valve and air should come out. If water comes out, the bladder of the tank has failed and the expansion tank requires replacement. If no water comes out, shut off the main water valve, open a faucet to bleed off the system pressure. Pump air into the tank to match the water system pressure then, reopen the main valve.
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