Expansion vessels: what they do on sealed heating and unvented hot water

Water expands when it gets hot. In an old open-vented heating system, that extra water has somewhere to go: up a pipe into a small tank in the loft. Many modern homes have no loft tank. The heating is a sealed system, and the hot water may come from an unvented cylinder fed straight from the mains. In both cases, the extra volume has to go somewhere else. That is the job of the expansion vessel.

This guide explains what the vessel does and how to spot that it may not be working. It is an explainer only. It does not cover checking, adjusting or replacing a vessel. On a heating system that work is next to the boiler, and on an unvented cylinder it is part of a safety system, so it belongs with a professional.

What an expansion vessel is

An expansion vessel is a sealed metal container, shaped like a small drum or a flattened ball. Colour is often a clue (heating vessels are commonly red), but the label is what tells you what it is for. Inside is a flexible rubber diaphragm or bag that divides it into two parts:

• one side connects to the water in the system;
• the other side holds air or gas at a set pressure, called the charge.

When the water heats up and expands, it pushes into the vessel and squeezes the air side. When the water cools and contracts, the air pushes it back out. The vessel works like a cushion, so the pressure in the system rises and falls gently instead of jumping every time the heating comes on.

Where you will find one

Sealed heating with a combi or system boiler. The vessel is usually built into the boiler casing, so you will not see it. Some systems also have a separate vessel on the wall nearby. Our guide to combi, system and regular boilers explains which homes have sealed heating.

Unvented hot water cylinders. These usually have their own vessel on the cold supply near the cylinder, or an air space built into the cylinder itself. It takes up the expansion of the stored hot water. It is one part of a set of safety devices described in our unvented cylinder basics guide.

Open-vented systems. A traditional system with a feed and expansion tank in the loft may have no vessel at all.

The heating vessel and the drinking water vessel are different products. One made for heating water should not be used on drinking water, and the reverse. That is one of the reasons a professional should choose and fit them.

Why the charge matters

Over time, the air side of a vessel can slowly lose pressure, or the diaphragm can fail and let water into the air side. Either way, the cushion gets smaller. The water has less room to expand into, so the pressure in the system climbs further each time it heats up.

On sealed heating, that extra pressure has to go somewhere, so the safety valve on the boiler opens and lets water out. The pressure then drops when the system cools, because water has been lost. On an unvented cylinder, the relief valves are there for the same reason, and repeated discharge is a warning sign.

Signs a vessel may have lost its charge

None of these proves the vessel is at fault. They are reasons to have someone look.

• Boiler pressure swings a lot. The pressure gauge on the boiler rises noticeably when the heating is on and drops back further than it used to when it cools.
• Pressure keeps falling and needs topping up often. On sealed heating, a frequent drop can mean water is being lost through the safety valve because of expansion. It can also mean a leak, so look for damp patches and drips under radiators and pipes too.
• Water dripping from a pipe outside. A small pipe through the wall near the boiler, or a discharge pipe from an unvented cylinder, that drips or runs when the system heats up. Do not confuse this with the boiler's condensate pipe, which is a plastic drain that runs as normal when the boiler is working.
• Hot water from an unvented cylinder spurts or discharges into the tundish (a small open funnel on the discharge pipe) when the cylinder heats up.
• Taps spit or pressure varies at the hot taps on an unvented system, alongside the signs above.

What you can safely do

• Note the boiler pressure reading when the system is cold and again when it is hot, so you can tell the engineer what you have seen.
• Look for leaks on radiators, valves and visible pipework. Our radiator bleeding and balancing guide covers air in radiators, which is a different problem.
• Read the boiler's user manual for what its pressure gauge should show. Follow only the user-level instructions in it. Our boiler pressure guide explains reading the gauge.
• If water is discharging from an unvented cylinder's tundish or a relief pipe, especially if it is hot, switch off the heat sources to the cylinder if you can do so safely and call for help.

Who should look at it

On or next to a gas boiler: a Gas Safe registered engineer. The vessel is often inside the boiler casing, and the boiler should only be opened by someone on the Gas Safe Register. A check of the vessel is commonly part of a boiler service, so mention any pressure problems when you book.

On an unvented hot water cylinder: a competent person, usually a plumber or heating engineer qualified to work on unvented systems. Building Regulations in England and Wales treat unvented hot water storage as safety-critical, as our unvented guide explains.

On an oil or other non-gas boiler: an engineer qualified for that fuel. OFTEC is the scheme many oil engineers belong to.

Restoring a vessel's charge, or replacing a vessel, involves isolating and draining part of the system and setting pressures to the maker's figures. We do not give a method for it, and we do not give pressure figures, because the right numbers depend on the system and the maker.

If your mains pressure is high, the professional may also talk about a pressure reducing valve, which works alongside the vessel on unvented systems.

Is an expansion vessel the same as a pressure reducing valve?

No. A pressure reducing valve limits the pressure of incoming mains water. An expansion vessel absorbs the extra volume when water heats up. An unvented cylinder usually has both.

Can I check the expansion vessel myself?

You can watch the boiler pressure gauge and look for leaks or discharge, and pass that on. Checking or adjusting the vessel itself is a job for a Gas Safe registered engineer on a gas boiler, or a qualified plumber on an unvented cylinder.

Why does my boiler pressure go up when the heating is on?

Some rise is normal, because water expands as it heats. A large rise, or water discharging outside, can point to a vessel that has lost its charge. Ask an engineer to check it.