An expansion vessel gives heated water somewhere to go. Water expands as it warms, and in a sealed heating system or an unvented hot water system that extra volume has to be absorbed, or the pressure climbs until a safety valve opens. There are two main kinds: heating vessels, usually red, for sealed central heating circuits, and potable vessels, usually white or blue, for drinking water systems such as unvented cylinders. They look similar but are built from different materials and are not interchangeable.
How an expansion vessel works
Inside the steel shell is a flexible rubber diaphragm or bag. One side connects to the system water. The other side holds air or nitrogen at a set pre-charge pressure, which you can check through a tyre-style valve on the vessel.
As the water heats and expands, it pushes against the diaphragm and compresses the air. When the water cools, the air pushes it back. That cushion keeps the system pressure within a safe working range. If the vessel loses its air charge, or the diaphragm fails, there is nothing to absorb the expansion. The pressure then rises sharply when the system heats up, and the pressure relief valve or expansion relief valve opens to let water out.
Heating vs potable expansion vessels
| Heating vessel | Potable vessel | |
|---|---|---|
| Typical colour | Red | White or blue |
| Where it is used | Sealed central heating circuits, including inside most combi and system boilers | Unvented hot water cylinders, booster sets and other drinking water systems |
| Materials | Diaphragm suited to treated heating water | Materials approved for contact with drinking water |
| Interchangeable? | No. Never fit a heating vessel on a drinking water supply. | |
Colour is a convention, not a rule, so always check the product description and markings before you buy. A potable vessel should be suitable for use with drinking water, and fittings on the water supply must comply with the Water Supply (Water Fittings) Regulations 1999.
Vessels inside boilers
Most combi and system boilers have a heating expansion vessel built into the casing. Replacing it, or checking and recharging it, means working inside the boiler, so it is a job for a Gas Safe registered engineer. Our guide to boiler GC numbers explains how to identify your boiler so the correct manufacturer part can be ordered.
External heating vessels
Larger sealed systems, or systems where the built-in vessel is too small for the volume of water, may have an additional external vessel fitted to the heating pipework. These are sized and installed by the heating engineer as part of the system design.
Vessels on unvented cylinders
An unvented cylinder stores hot water at mains pressure, so it needs a potable expansion vessel (some designs use an internal air gap instead). Unvented hot water systems over 15 litres must be installed by a competent person under Part G3 of the Building Regulations, and the safety devices, including the expansion arrangement, should be checked at the service intervals set by the cylinder manufacturer.
Signs an expansion vessel has failed
- On a heating system: boiler pressure that shoots up when the heating is on and falls when it cools, water dripping from the pressure relief valve discharge pipe outside, and repeated low pressure lockouts after topping up. Our guide to boiler losing pressure covers these symptoms in more detail.
- On an unvented cylinder: water discharging through the tundish (the open funnel on the discharge pipe near the cylinder) when the water is heating, or noticeable pressure surges at the hot taps.
These are symptoms, not a diagnosis. A failed vessel, a worn relief valve and an incorrect pre-charge can all look similar, so the cause should be confirmed by a qualified engineer before parts are changed.
What sizing depends on
The right vessel size is not a guess. It depends on:
- the total volume of water in the system (or the cylinder's capacity);
- the working temperatures;
- the cold fill pressure and the relief valve setting;
- the vessel's pre-charge pressure.
Vessel and cylinder manufacturers publish sizing guidance based on these figures, and an unvented cylinder's instructions will state the vessel it needs. If you are replacing a vessel, matching the original specification from the manufacturer's documentation is the safest route.
Pre-charge pressure explained
The pre-charge is the air pressure in the vessel when the water side is empty or at zero pressure. It is set to suit the system, usually in relation to the cold fill pressure, and the correct figure comes from the boiler or cylinder manufacturer. Checking it properly means isolating and relieving the water side first, which is why it is normally done during a service. Do not add or release air from a vessel on a live system unless you are qualified to do so.
Where to find expansion vessels
Replacement boiler vessels are listed in boiler spares by manufacturer part number, and related parts are in domestic boiler accessories. For hot water systems, browse unvented cylinders and the wider cylinders and water heaters range, and check each product page for its intended use.
Frequently asked questions
Can I use a red heating vessel on my unvented cylinder?
No. Heating vessels are not made for drinking water. An unvented cylinder needs a potable vessel suitable for contact with drinking water, as specified by the cylinder manufacturer.
How often should an expansion vessel be checked?
Follow the boiler or cylinder manufacturer's service schedule. On many systems the vessel charge is checked during the annual boiler service or the unvented cylinder's routine maintenance.
Can I recharge my boiler's expansion vessel myself?
No. The vessel inside a gas boiler is part of the appliance, and checking or recharging it is a job for a Gas Safe registered engineer.
Why does my unvented cylinder drip into the tundish?
Discharge into the tundish means a safety device has opened. A failed expansion vessel is a common cause, but it can also be a relief valve fault. Have a competent unvented installer investigate.