how adjust expansion tank - hot water baseboard heating system - has fitting with set screw?

Question

water pressure gradually gets up to 30 psig and pop off valve starts releasing - must have worked for many years - seems to be a trick to reducing water level in expansion tank to get right amount of air/water mixture

Answer 1

With the water shut off both on the entry and the exit side of the tank and, the heater off. slowly drain the tank until it's empty. Let off all pressure through valve. then shut off exit valve and open entry water valve. The tank should be at a level you want when full, and pressure. Unless you have a leak.