Expansion tank

Where a backflow preventer or pressure-reducing valve sits on the water supply, the heated water cannot expand back into the street main, so pressure climbs every time the tank fires. Without an expansion tank the temperature-and-pressure relief valve becomes the release point, dripping during every heating cycle and wearing out early. The expansion tank gives the expanded water somewhere to go, keeping the system pressure stable and the relief valve seated.

An expansion tank is sized to the system volume and pressure; an undersized tank simply fills and stops absorbing, returning to the same drip cycle.

An expansion tank that has lost its air charge (its pre-charge pressure) behaves like no tank at all, and the relief valve starts weeping again. The tank's pressure is checked against the supply pressure at the service visit, and the tank is re-charged or replaced as needed.