Waterlogged Expansion Vessel on an Unvented Cylinder

What does the expansion vessel do on an unvented cylinder?

Water expands as it heats. On a vented cylinder, the extra volume simply rises up the vent pipe into the loft tank. An unvented cylinder is sealed, so that extra volume has to go somewhere, and the expansion vessel is where it goes.

Inside the vessel a rubber diaphragm separates the water from a pocket of pressurised air. As the stored water heats, the diaphragm moves and the air is compressed to take the extra water. As the cylinder cools the air pushes the water back. A healthy vessel does this every day without anyone noticing.

Many stainless cylinders have no separate vessel. They hold an air gap inside the cylinder itself, which is called internal expansion. Cylinders with an external vessel show it as a squat tank, often blue or white and marked for drinking water use, connected to the cold inlet pipework or fixed to the cylinder side. If you cannot find one, check the data plate before assuming it is missing.

This page is about the cylinder vessel. A boiler has its own expansion vessel on the heating circuit, and the symptoms there are different.

How do I know the expansion vessel is waterlogged?

The tell is timing. A waterlogged vessel cannot absorb the expansion, so the pressure in the cylinder rises sharply every time it heats. The expansion relief valve in the inlet group is the valve meant to release that, and in the usual design it opens before the temperature and pressure relief valve (TPRV). The ratings are printed on the valves and in the manual.

• Water drips or runs from the tundish a few minutes after the cylinder starts heating, then stops once it cools.
• The expansion relief valve discharges on heat-up but not when the system is cold and idle. A discharge that also comes with hot or steamy water belongs to the TPRV article, not to the vessel.
• Hot taps spit or surge briefly while the cylinder is heating.
• A pressure gauge on the inlet group, if fitted, swings noticeably between heated and cold states.
• Water, rather than air, comes out when the vessel valve is pressed. That last test is for the engineer, not you.

Why does a vessel fail?

Almost always the diaphragm. It is flexed through heating and cooling cycles for years, so it eventually cracks or splits. Once it splits, water enters the air side and the vessel can no longer cushion anything. This is the waterlogged condition, and recharging the air side does not cure it because the air escapes through the split.

The other failure is slow air loss. Vessels leak air gradually through the valve and the diaphragm, and a vessel that was charged correctly at installation can drop below its setting after several years. This vessel is not waterlogged but undercharged, and it still cannot absorb the full expansion. It is the more recoverable fault.

Corrosion is the third, particularly on older steel vessels where moisture and a small air leak combine to rust through the shell. A vessel with a weeping seam or rust bloom is replaced without further testing.

Hard water does not split the diaphragm directly, but scale from repeated small discharges can mark the seat of the expansion relief valve that was opening because of the vessel problem, which is how a vessel fault often becomes a valve fault as well.

How is the vessel pressure checked?

The check has a fixed order, because the reading is meaningless unless the water side is at zero. The engineer isolates the cold supply, relieves the water pressure on the vessel side by opening a hot tap, and only then measures the air charge with a gauge at the vessel valve, which looks like a tyre valve.

The reading is compared with the charge on the vessel label and with the inlet pressure setting. The usual aim is for the vessel charge to match the pressure reducing valve setting, but the vessel label and cylinder manual give the real figure for your set-up, and it differs between models.

Three outcomes follow. A reading close to the right figure with no water at the valve means the vessel is fine, and the fault is elsewhere, which is why the check moves on to the valves in the inlet group and, if the water was hot, the TPRV. A low reading with no water at the valve means the vessel is undercharged and can be recharged. A reading of zero, or water at the valve, means the diaphragm has failed.

The test is quick, which makes it a standard step in an unvented cylinder service check as well as in a fault visit.

Repressurise or replace the vessel?

Recharging is sensible when the vessel is only low and has no water on the air side. The engineer tops up the air with a pump to the right figure, with the water side at zero, and checks again some weeks later. If the pressure holds, the job is done.

If the vessel loses its charge again within a few weeks, or if any water comes from the valve, recharging is wasted effort. The diaphragm has gone or is going, and the vessel is replaced. On an external vessel this is a short job: isolate, drain down the section, swap the vessel, refill and re-check.

On a cylinder with internal expansion, there is no vessel to swap. The air gap inside the cylinder can be lost over time, and the usual response is a recharge or re-establishment of the air gap as part of servicing. If the cylinder is old and scaling up, it may be a sign that replacement is nearer than repair.

A relevant point on replacement parts: when a vessel is replaced it is usual to check the relief valves and the pressure reducing valve at the same visit, because a vessel that has been failing for a year has usually been making those valves discharge repeatedly. The part list for that visit is set out in TPRV and expansion vessel repair. Recharging a vessel is not a householder job on an unvented system, and neither is adding a spare vessel. The TPRV and expansion vessel cost article covers what the parts typically cost.