[DangeloInspections]

Another common reason for this is perhaps a waterlogged thermal expansion tank. This is the tank that is installed just above your hot water tank. Not all homes have this, but many do. If that tank has a worn or bad bladder inside, it can become waterlogged and no longer expands properly for expansion.
One way to check this is to "tap" the tank...the lower part (if installed with nipple down) should sound solid, indicating water in the tank, while the top part should sound hallow indicated proper air in the top.
If it sounds the same it usually indicates a bad bladder. Sometimes, but not often you could have a bad shrader valve on the tank and if you know what you are doing you can try to add some air to the proper psi to see if that helps. Usually you just turn off the tank, drain down the tank a bit, (never below the elements if electric) and replace the expansion tank. The cost of a new expansion tank is usually around $50.
If any of this sounds confusing, one would want to call a licensed Plumber.

Hope this helps, Frank D'Angelo