Expansion Tank

Water expands as it heats. In a sealed system that expansion has nowhere to go, so pressure climbs until the relief valve opens. An expansion tank holds a cushion of air, separated from the water by a diaphragm or bladder, that compresses to absorb the change.

A relief valve that weeps or discharges every heating cycle is usually reporting a failed expansion tank, not a faulty valve. Replacing the relief valve without checking the tank leaves the real problem in place and removes a safety device from service unnecessarily.

The tank is pre charged with air to match the system fill pressure. As the loop heats, water pushes into the tank and compresses the air. As it cools, the air pushes the water back out. The connection point is the point of no pressure change, which is why circulator placement is referenced to it.

1. 1Note the cold fill pressure on the boiler gauge.
2. 2Isolate and drain the tank of water, or remove it from the system where the piping allows.
3. 3Check the air charge at the schrader valve with a tire gauge, with no water pressure on the tank.
4. 4Compare to the required fill pressure and add air if low.
5. 5If water comes out of the schrader valve, the diaphragm has failed and the tank must be replaced.
6. 6Recheck system pressure hot and cold after service.