What is an Expansion Vessel?

An expansion vessel is a small tank with a rubber sheet (diaphragm) inside. One side has water, the other has air or nitrogen. When water heats up and expands, the vessel gives it somewhere to go.

Over time, water can leak through the diaphragm into the air side. This is called 'waterlogging'. When this happens, the vessel stops working and the system pressure goes wrong.

How to Purge It

1. Turn off the vessel from the system using the isolation valve.
2. Drain the water side using the drain valve.
3. Check the air pressure at the Schrader valve (like a tyre valve).
4. If water comes out of the Schrader valve - STOP. The diaphragm is broken. The vessel must be replaced. Do NOT pump it up.
5. If the pressure is low (and no water): Pump it up with nitrogen or air to the right pressure (usually 1.0-1.5 bar - check the system paperwork).
6. Close the drain valve.
7. Slowly open the isolation valve to put it back in service.
8. Watch the system pressure over the next heating cycle to make sure it works.
9. Write down the old pressure, new pressure, and condition.

When to Do This

• Once a year as part of planned maintenance
• When the system pressure keeps dropping
• When the pressure relief valve keeps letting water out
• As part of the BG50 closed system programme