How To Drain the Boiler Expansion Tank
If you have a boiler based heating system, it will have an expansion tank. However, only closed-vent systems require the expansion tank to be drained periodically.
The expansion tank is fitted in-line with the heating system. The expansion tank is normally filled with air, which is easily compressible. As the water in the system is heated, it expands. As it expands, the pressure in the system rises. In order to balance that increased pressure, the expansion tank relieves the system pressure by pressurizing the air.
When the expansion tank fills with water, which it naturally does, it reduces the size of the air cushion. Water is not very compressible and only a small air cushion is available to absorb the system pressure. It cannot absorb as much pressure from the system and so the system PSI rises, possibly triggering the pressure relief valve.
Draining the expansion tank should be done periodically, usually once or twice a year. We recommend doing this maintenance before the cool season so you don't have to work with hot water and risk getting burned.
Turn off the boiler and close the water supply valve. Allow the tank to cool if necessary.
Connect a hose to the purge valve on the expansion tank or place a bucket beneath the tank. Open the purge valve and allow the tank to drain completely. Occasionally a vapor lock can occur in the tank if air cannot enter the tank as it drains. If this happens, you may be able to blow air into the tank by blowing through the hose. Be careful as when you stop, the water may flow suddenly.
Once the tank has been drained, close the purge valve, open the water supply valve, and start the boiler (if needed).
Tip: One way to determine if the expansion tank is full, when the system is in use, feel the bottom of the tank. It may feel warm from the heated water. Now feel the top of the tank. If it is full of water, the top will also feel warm. If it is cool, then there is some air cushion. You may be able to find the water line by feeling for where the temperature change occurs. Tapping lightly on the tank with a metal object may also help to determine water level. Listen for the pitch change above and below the water line.