What the expansion vessel does, and how to tell it's failing

What the expansion vessel does

Heating water expands as it warms. That extra volume has to go somewhere: the expansion vessel (expansievat). Inside it a rubber membrane separates the water from a cushion of gas. Vaillant’s glossary puts it simply: as the water heats and expands, the surplus flows into the vessel and compresses the gas on the other side of the membrane. When the system cools, the gas pushes the water back out.

On most combi boilers it’s a flat disc inside the casing; on larger systems, a separate tank on the pipework next to the boiler. Intergas asks for it in the return pipe, and Remeha explains why: the return is cooler, which helps the vessel last longer.

The signs it’s failing

The vessel gives up quietly, so what you notice is the pressure misbehaving.

• Low when cold, high when hot. Flamco, the expansion vessel maker, lists this fault: the pressure is too low at low temperatures and climbs fast as the temperature rises, because the gas cushion is gone or the precharge is too low.
• The safety valve drips. With no cushion the pressure runs up to the safety valve’s limit and it lets water out through its discharge pipe. Vaillant names a vessel that has lost its air as a cause of pressure that’s too high.
• You keep topping up. Every discharge sends water down the drain, so the cold pressure drops and you fill again. Intergas says that needing to top up several times a year is a reason to call your installer.

One sign alone can have another cause. Low cold pressure by itself is often a small leak, and pressure that’s high both hot and cold usually means the system was overfilled. My guide to boiler pressure and fault codes covers those. All three together point at the vessel. Nefit-Bosch lists a faulty expansion vessel as one of the main causes of pressure problems.

Why they fail

Two things go wrong. The gas seeps out through the membrane and the valve over the years, until the cushion is too small to do anything. Or the membrane splits and the gas side fills with water. Heat speeds both up.

The topping-up cycle then makes it worse. Fresh water brings fresh oxygen into the system, and oxygen means rust and sludge in the radiators and the heat exchanger.

How it’s checked and recharged

This is what I do, and why it isn’t a five-minute job. Flamco’s procedure, in short:

1. Take the water pressure off the vessel, through its isolating valve or by letting water out at the filling and drain valve. The system doesn’t need draining completely.
2. Take the cap off the vessel’s valve and measure the gas pressure with a tyre gauge.
3. If it’s low, pump it up to the right precharge, with nitrogen for preference; Flamco notes that compressed air can cause rust inside.
4. Check the valve for leaks with a wet finger.
5. Refill to 0.2 bar above the precharge, cold; after heating through and bleeding again, Flamco tops up to 0.5 bar above.

If water comes out of the valve when it’s measured, the membrane has failed and the vessel is replaced, not pumped up.

On a combi the vessel sits behind the casing, which stays closed for anyone but an engineer. I check it at every boiler service, the natural moment to catch it.

Recharge or replace?

A vessel with a sound membrane just needs its charge topped up, and that’s part of the visit. It’s replaced when:

• water comes out of the gas valve
• it’s rusted through or weeping at the seam
• it holds a charge for a week and then loses it again
• the system has grown since it was fitted, with underfloor heating for example, and the vessel may now be too small

For a boiler’s own internal vessel the replacement is the maker’s part. Where there’s no room, an external vessel on the return pipe does the same job. Either way, it’s €114.95 incl. BTW per hour, with no call-out charge, plus the vessel. A typical repair with a replacement part comes to around €300-€400 incl. BTW, and I confirm the price before I start.

Don’t keep draining and refilling

It’s tempting to let a little water off when the pressure is high, top up when it’s low, and carry on like that all winter. Please don’t: each refill puts fresh water and air in, the safety valve keeps getting a workout, and the sludge builds. Vaillant is blunt about it: the vessel and the safety valve are parts for a qualified engineer to inspect and repair, not the homeowner.

What you can do: read the gauge when the system has been off for a few hours, then again after it has run hard for an hour, and write both down.

The safety valve, and what else drips

The safety valve (overstortventiel) is the vessel’s partner. Intergas specifies a 3 bar valve in the flow pipe, no more than half a metre from the boiler, with no tap in between; Flamco notes that 3 bar is the Dutch norm for buildings under 15 metres. Its discharge pipe usually shares a drain with the condensate, so a drip there could be several things; my guide to a leaking boiler sorts them out. Air that keeps coming back in underfloor heating is often the same story: cold pressure sagging because the vessel has gone.

Get it checked

Send me a message on WhatsApp with the make and model of the boiler, the pressure reading cold and hot, a photo of the safety valve pipe if it drips, and your postcode. I work by appointment, 08:00-16:00, seven days a week. If the vessel turns out to be fine, we look for the leak instead, and boiler repair explains how a visit works.

If you smell gas, leave the property and call 0800 9009 from outside. If a CO alarm goes off or you suspect carbon monoxide, get everyone outside and call 112.