Expansion Vessel Installation

A typical expansion vessel for a sealed heating system has a steel casing with a rubber membrane inside. Heating water sits on one side of the membrane and gas, usually air or nitrogen, sits on the other. When the water heats up, it pushes against the membrane and compresses the gas. When the water cools, the gas pushes it back into the system. The result is that the system pressure only changes slightly.

Older houses sometimes still have an open vented system with a feed and expansion tank at the highest point of the house, often in the loft. This tank is open to the air, so water evaporates from it and oxygen gets into the system, which speeds up corrosion. During a refurbishment, the system is usually converted to a sealed one with a membrane vessel.

The heating system will usually show you when the vessel needs attention. Typical signs include:

• The pressure gauge rises quickly as the system heats up and drops noticeably once it cools
• You have to top up the system again and again, even though there is no visible leak
• The safety valve drips or discharges water while the heating is on
• Water comes out when you press the small valve on the gas side of the vessel, which means the membrane has split

Why not just replace the safety valve? The valve only reacts to excessive pressure. If the vessel is not doing its job, a new valve will discharge water just like the old one.

The exact procedure depends on whether this is a new installation or a replacement. In general, the heating engineer works through these steps:

1. Check the water volume of the system, the type of heating and the height of the building, and choose the vessel size to suit.
2. Let the system cool, switch off the boiler and lower the system pressure, draining part of the water if needed.
3. Remove the old vessel if it is being replaced.
4. Set the pre-charge of the new vessel before connecting it, based on the system height and the boiler manufacturer's instructions.
5. Mount the vessel on the wall or a bracket and connect it through a service valve that cannot be closed by accident.
6. Refill the system with treated water, bleed the radiators and the boiler, and set the fill pressure.
7. Start the heating, watch the pressure as the system warms up and check all joints for leaks.

The vessel is usually connected to the return, the cooler pipe running back to the boiler, because the membrane suffers less from heat there. The exact position still depends on the boiler instructions and the way the system is laid out.

How is the pre-charge set? As a rough guide, it is based on the static height of the system, meaning the height from the vessel to the highest radiator. Allow roughly one bar for every ten metres of height, plus a small margin, usually around 0.2 to 0.3 bar. In a typical family house this works out at roughly 0.8 to 1.2 bar. The system fill pressure is then set slightly above the vessel pre-charge. The pre-charge can only be measured accurately when no water pressure is acting on the vessel.

The choice depends on the type of heating and the amount of water in the system. Underfloor heating holds far more water than a few radiators, so it needs a larger vessel. A solid fuel boiler with a buffer tank needs an even bigger reserve.

• Built-in vessel inside the boiler: most wall-hung gas combi and system boilers have one. It can be enough for a small flat, but when the system is extended or underfloor heating is added, an extra external vessel is often fitted
• Separate wall-mounted vessel: the usual solution in houses, and easy to reach for checks
• Floor-standing vessel on feet: used for large volumes, for example with buffer tanks

Watch out for mix-ups. Vessels for domestic hot water, often blue or white, and vessels for solar circuits have different properties from heating vessels, which are usually red. They are not interchangeable. The volume should come from a calculation, not a guess. An undersized vessel causes the same problems as a faulty one.

If the boiler has a built-in vessel and that is the one being replaced, the work involves opening the boiler. On a gas boiler this is a job for a qualified, registered gas engineer who is trained on that boiler. Otherwise you risk losing the warranty and creating a safety hazard.

The simplest case is swapping a vessel for the same type in the same place when a service valve is already fitted. The engineer can isolate it without draining the whole system, and the job is done fairly quickly.

These situations make it more demanding:

• There is no isolating valve and the whole system has to be drained
• The vessel is hard to reach in the boiler room or sits high up near the ceiling
• Pipework needs altering, or fittings, a pressure gauge or a safety valve need adding
• The old system is full of sludge, which gets stirred up when it is drained
• A low loss header (hydraulic separator) is being fitted at the same time, or an old tank is being removed

A low loss header separates the boiler circuit from the heating circuits. It helps when a house has several circuits, for example radiators and underfloor heating. If the system is being reworked anyway, it makes sense to fit the vessel and the header together.

In a block of flats with a central plant room, individual owners do not deal with the expansion vessel. If your pressure fluctuates or the heating does not warm up, contact the building management or the owners' association.

In practice, problems are often caused not by a faulty vessel but by poor installation. These mistakes come up again and again:

• The pre-charge was left at the factory setting and never adjusted to the building
• The pre-charge was measured while water pressure was acting on the vessel, so the reading was wrong
• The vessel is isolated by an ordinary ball valve that someone closes, leaving the system unprotected
• The vessel is connected at a poor point where the pump or high temperatures affect it
• The volume was chosen by eye
• The vessel's air valve faces the wall, so the pre-charge cannot be checked

Manufacturers and the relevant EN standards for heating systems expect the system to stay protected at all times. That is why the vessel must not be isolated by a standard valve without a safeguard against accidental closing.

When the job is handed over, ask the engineer to show you how the vessel is connected and check these points:

• The vessel is firmly fixed and its air valve is accessible
• It is connected through a service valve protected against accidental closing
• The engineer has told you the pre-charge and the system fill pressure
• All joints stay dry even after the heating has warmed up
• Pressure rises only slightly as the system heats and returns to its original level once it cools
• The safety valve does not drip while the heating is running
• Radiators are bled and warm across their whole surface
• You have received a record of the work and the vessel's instructions