Water expands when heated
A closed hydronic system needs a way to accommodate the change in water volume as temperature rises. The expansion tank provides that cushion.
Pressure is a system clue
Abnormal pressure can relate to expansion-tank condition, fill-water controls, temperature or other system conditions. A pressure reading by itself is not enough to diagnose the cause.
Relief-valve discharge is not normal maintenance
A relief valve that is actively discharging is responding to a condition that should be evaluated. Do not plug, cap or defeat a boiler safety relief valve.
What homeowners can do
Observe the boiler gauge, visible leakage and whether the relief discharge pipe is wet. Diagnosis or adjustment of pressure-control and safety components should be left to qualified service personnel.