How to Install an Expansion Vessel Safely
An expansion vessel is a small component with a major job: it gives heated water somewhere safe to expand. If you need to know how to install expansion vessel equipment on an unvented hot-water cylinder, the first priority is identifying the system and using a vessel specified for potable water. A poorly sized, incorrectly charged or wrongly positioned vessel can cause regular discharge from the pressure-relief valve and unnecessary strain on the system.
For most homeowners and landlords, fitting or replacing a vessel on an unvented cylinder is work for a competent installer. In England and Wales, work on an unvented hot-water system must comply with Building Regulations Part G and is normally carried out by an installer with the appropriate G3 qualification. This guide will help you understand the correct process, specify the right part and check that the installation has been completed properly.
What an expansion vessel does
Water increases in volume as it heats. In an unvented cylinder, the incoming cold-water supply passes through a check valve, which prevents that expanded water from flowing back into the mains. The expansion vessel accepts the extra volume instead.
Inside the vessel is a flexible diaphragm. One side contains air or nitrogen at a set pre-charge pressure; the other receives the expanding water. As the cylinder heats up, water compresses the air cushion. When hot water is drawn off and the system cools, the vessel returns that stored pressure to the system.
Without a working vessel, pressure can rise until the expansion relief valve opens. A little discharge during a fault investigation may be expected, but repeated water from the tundish or discharge pipe is a warning sign that needs attention.
First, confirm which vessel you need
Do not assume that a vessel from a sealed central-heating system is suitable for domestic hot water. The two look similar, but their intended use, internal lining and approval can differ. For an unvented cylinder, use a potable-water expansion vessel rated for the pressure and temperature of the installation.
Check the cylinder manual and the inlet control set specification before ordering. You need to confirm the vessel capacity, maximum working pressure, connection size and required pre-charge pressure. The cylinder manufacturer’s stated vessel size takes account of cylinder capacity and the operating pressure set by the pressure-reducing valve.
Larger cylinders generally need larger vessels, but capacity alone is not enough. A system supplied at a different regulated pressure may require a different vessel size. If there is any uncertainty, follow the cylinder manufacturer’s data rather than selecting the closest-looking replacement.
How to install an expansion vessel on an unvented cylinder
The vessel is normally connected to the cold-water inlet pipework on the cylinder side of the check valve. This is essential. If it is installed upstream of the non-return valve, it cannot absorb the expansion created as the stored water heats.
Many unvented systems use an inlet control group incorporating a pressure-reducing valve, check valve and expansion relief valve. The vessel connection is typically made at the designated take-off point or through a correctly located tee after the check valve. Follow the component and cylinder instructions, as connection arrangements vary between models.
1. Isolate the correct services
Switch off the cylinder’s electrical supply at the local isolator and confirm it cannot energise while the cylinder is drained or partially drained. Isolate the cold-water feed to the cylinder using the service valve or inlet control set.
Open a nearby hot tap to release pressure. If the vessel is being replaced rather than added, drain sufficient water from the cylinder or affected pipework to remove it without flooding the cupboard. Never rely on an expansion relief valve as a routine drain point unless the manufacturer specifically permits it.
2. Check the vessel pre-charge before connection
With the water side fully depressurised, measure the air pressure at the vessel’s Schrader valve using a reliable pressure gauge. Set it to the pre-charge specified for the system, normally matching the regulated cold-water pressure after the pressure-reducing valve.
Do not check or adjust the pre-charge with the water side pressurised. The water pressure will give a misleading reading and can prevent the vessel from accepting the required expansion volume. If water comes out of the air valve, the diaphragm has failed and the vessel must be replaced.
3. Mount the vessel securely
An expansion vessel becomes substantially heavier when it contains water. Support it with a suitable wall bracket or mounting arrangement rather than allowing pipework or a flexible hose to carry its weight. Choose a location that remains accessible for future pressure checks and replacement.
Follow the manufacturer’s approved orientation. Some vessels can be installed in more than one position, while others have a preferred mounting direction. Keep the connection clear of sharp bends, excessive strain and areas where freezing is possible.
4. Connect it to the cold-water expansion point
Use fittings compatible with the vessel connection and existing pipework. The connection must be on the cold feed after the check valve, with no isolation valve that could accidentally separate the vessel from the cylinder during normal operation. If an isolation valve is present for servicing, it must be a lockable type that is kept open in service, in line with the manufacturer’s requirements.
Use appropriate sealing materials for potable-water installations and tighten fittings correctly. Over-tightening can damage threads or seals, particularly on compact brass fittings. Where a flexible hose is supplied or specified, make sure it is pressure-rated, undamaged and not twisted.
5. Refill, vent and inspect
Close the hot tap, reopen the cold-water supply slowly and allow the cylinder to refill. Open hot taps one at a time until water runs smoothly to clear air from the pipework. Inspect every new connection for leaks while the system is cold and at normal incoming pressure.
Once the cylinder is full, restore the electrical supply only when the manufacturer’s filling instructions have been met. Never energise an immersion heater in an empty or partially filled cylinder, as the element can fail quickly.
Commissioning checks that matter
A correctly fitted vessel should not be judged by the pipework alone. Its performance needs to be checked as the cylinder heats and pressure rises.
Confirm that the cold-water pressure after the inlet control group matches the system design pressure. Then heat the cylinder through a normal cycle and inspect the tundish and discharge arrangement. There should be no continuous discharge from the expansion relief valve. A small, short-lived change in pressure is normal; frequent lifting of a safety valve is not.
Also check that the vessel bracket remains firm, all joints are dry and the air valve cap is fitted. Record the vessel capacity, pre-charge setting, installation date and any replacement parts in the cylinder’s service record. This is particularly useful for landlords, managing agents and future maintenance visits.
Common faults after fitting an expansion vessel
If water still discharges from the relief valve, do not immediately assume the new vessel is defective. The issue may be incorrect pre-charge pressure, a vessel that is too small, a blocked or wrongly positioned connection, a failed check valve, or excessive incoming mains pressure.
A vessel can also lose charge gradually over time. Testing it with the water side depressurised should form part of routine cylinder servicing. If the vessel is sound but the relief valve continues to drip after operation, the valve may have been fouled by debris or may need replacement. Safety valves and discharge pipework should never be capped, blocked or altered simply to stop visible water.
When to call an installer
Arrange professional help if you are changing the inlet control group, pressure-relief valve, tundish or discharge pipework; if the cylinder has no accessible isolation point; or if you cannot verify the regulated pressure. The same applies where a cylinder is leaking, the discharge pipe is regularly running hot, or the pressure-temperature relief valve has operated.
An expansion vessel is inexpensive compared with the cylinder and safety controls it protects. Choosing the specified potable-water vessel, mounting it securely and setting its pre-charge correctly will give the system the best chance of providing reliable hot water without nuisance discharge.