Central heating expansion vessel: when to replace?

What does an expansion vessel do?

Water expands when it gets warm. In a sealed central heating system, that extra volume has nowhere to go, and without a buffer the pressure would rise sharply with every heating cycle. The expansion vessel absorbs this. It's a round vessel, usually red or grey, that hangs next to or above the boiler. In many combi boilers it's inside the boiler itself, behind the casing.

Inside is a rubber membrane. On one side is water from the system, on the other side air or nitrogen under pressure: the pre-charge. When the water heats up, it pushes the membrane in and compresses the gas. When it cools, the gas pushes the water back. This keeps the pressure in the system within limits.

Symptoms of a faulty expansion vessel

• The pressure fluctuates strongly. When the system is cold the pressure is normal, but as soon as the heating comes on it rises sharply, sometimes to the maximum. When the system cools down, the pressure drops back down again.
• The pressure relief valve is dripping or running. That valve opens when the pressure becomes too high (around 3 bar in many systems) to protect the system. If it drips mainly when the heating is on, the expansion vessel is a prime suspect.
• You have to keep topping up water. The water that runs off via the pressure relief valve has to be replenished somewhere. This creates a cycle of topping up, rising and running off.
• The boiler goes into breakdown due to pressure that's too high or too low.

Note: pressure loss can also be caused by a leak, or by a filling tap that is not fully closed (then the pressure slowly rises). Read about this in boiler losing pressure.

Causes

• The pre-charge pressure has dropped. Over the years, gas slowly leaks from the vessel. The membrane then no longer absorbs the expansion.
• The membrane is broken. Then water enters the gas side and the vessel no longer works.
• The vessel is too small for the system, for example after expansion with extra radiators or underfloor heating.
• The pre-pressure doesn't suit the property. The correct pre-pressure depends on the height of the system. An incorrectly set pre-pressure gives the same complaints.

What can you do yourself?

1. Keep an eye on the pressure gauge. Note the pressure when the system is cold and after half an hour of heating. Also write down whether the pressure relief valve is dripping. This information helps the technician.
2. Check the filling loop. Is it really closed and is the filling hose removed?
3. Don't overfill. Keep the pressure of a cold system within the range stated in your boiler manual.
4. Place a container under the pressure relief valve if it's dripping, so that no water damage occurs. Never close the valve and do not block the discharge: it's a safety device.

You don't open the vessel, adjust the pre-pressure or replace it yourself. For that, the system must be depressurised and the pre-pressure must be matched to your property. If the vessel is in the boiler, the casing must also come off.

Top up the pre-pressure or replace?

If only the pre-charge has dropped and the membrane is intact, a technician can often restore the vessel to the correct pre-charge. If the membrane is broken, water comes out of the valve on the gas side, or the vessel is rusted on the outside, replacement is the only proper solution. If the vessel turns out to be too small for the system, replacement with a suitable vessel is also necessary.

Replacement is in many cases a straightforward job. With a built-in expansion vessel in an older boiler it may be a reason to look at the rest of the boiler too; for that, read replace or repair central heating boiler?

Do you smell gas? Then do not touch the boiler, do not use light switches or open flames, open windows and doors, leave the property and call the national gas and electricity fault number: 0800-9009.

Having an expansion vessel checked or replaced: costs

When there's a pressure fault, the technician starts by measuring. First we check the pre-charge of the expansion vessel with the system depressurised; then the filling pressure of the central heating system and the pressure relief valve. Only when it's clear that the vessel itself is leaking or worn do we replace it.

We size the new vessel to match the water content of your system. In a larger home with many radiators or underfloor heating, a bigger vessel than the old one may be needed. After installation, we fill and bleed the system and check the pressure both warm and cold.

If you have to top up more than once every few months, have it checked. Constant fresh water in the system causes more limescale and rust.