Plumbing Questions? Ask a Plumbing Expert.
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Does the pressure rise when the boiler is cold?
Ok, then turn off the water feed to the boiler. 20 psi is fine if the boiler is warm/hot. Relief valve should discharge at 30 psi. There are only 2 possibilities; either the feeder isn't properly regulating the pressure let into the boiler and is letting too much pressure in (so you shut the water off to check this) or you have an expansion problem. Since you replaced the tank the only expansion issue would be if the tank is too small.
There is one other possibility if you have a domestic hot water coil in the boiler or you have an indirect fired water heater with a coil in the tank. If either one of these coils has a leak then it's letting water into the boiler at full water system pressure.
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