What are and how to use expansion tanks

A layer of air in the expansion tank of a hot-water heating system provides a cushion as the water expands and contracts. If water spurts from the pressure-relief valve on the boiler and the expansion tank feels uniformly hot all over, the tank needs recharging with air.

On some tanks a combination valve lets in air as water drains out through a hose. Turn off power to the boiler; allow the boiler to cool. Attach a hose, turn off the shut-off valve between the tank and the boiler; open the combination valve and empty the tank. Reverse procedures; turn on the power.

If a tank lacks a combination valve, drain the water into buckets. If your system has a diaphragm tank (usually suspended from a pipe) leave the job of recharging it to an expert: the diaphragm is easily ruptured.