Feed and expansion tanks: the small heating tank in the loft

If you have a regular (heat-only) boiler, open the loft hatch and you will often see two tanks. The big one is the cold water storage tank. The small one, usually a fraction of the size and sometimes tucked beside the big one, is the feed and expansion tank, often shortened to the F&E tank or the header tank. It belongs to the central heating, not to your taps.

This guide explains what it does, how to tell the two tanks apart, and what it means when the small tank looks wrong. It is an explainer. Anything that involves the boiler, or draining and refilling the heating, is a job for a heating engineer.

Which homes have one

A feed and expansion tank is part of an open-vented heating system. That is the traditional British setup: a regular boiler, a hot water cylinder in an airing cupboard, and tanks in the loft. Our guide to combi, system and regular boilers explains the three common setups.

A combi boiler or a system boiler normally uses a sealed heating circuit instead. A sealed system has a pressure gauge on or near the boiler and an expansion vessel, and it does not need a tank in the loft for the heating. If you have a gauge and no small loft tank, this guide is not about your system. See boiler pressure for sealed systems instead.

What the tank does

The water in your radiators and boiler is a closed loop, often called the primary circuit. Water Regs UK's page on hot water and heating systems uses that term for systems that supply heating, and notes that all hot water and heating systems must be able to accommodate thermal expansion.

The F&E tank does two jobs for that loop:

1. Feed. It keeps the heating circuit topped up. A small amount of water is lost over time, for example when radiators are bled. A ballvalve in the tank lets in fresh mains water to replace it.
2. Expansion. Water expands when it is heated. On an open-vented system, the extra volume is pushed back up into the F&E tank, so the water level in the tank rises when the heating is on and falls when it cools.

There is usually also an open vent pipe from the heating circuit that rises above the tank and turns down over it. It is a safety route on an open-vented system. Leave it alone: it should not be blocked, capped, cut short or moved.

How it differs from the cold water storage tank

	Cold water storage tank	Feed and expansion tank
Size	Large	Small
Feeds	Hot water cylinder and often bath, shower, toilet and basin cold taps	The heating circuit only
Water inside	Should be clean	Heating water, often discoloured and treated with inhibitor
Level	Fairly steady	Rises and falls with the heating

They both have a ballvalve, a lid and an overflow, so they look similar. They are separate systems. Heating water must not get into the water you wash and drink with, and Water Regs UK says drinking water supplies need appropriate separation from fluids in primary systems.

Our guide to loft cold water tanks covers the large tank, including what the water fittings regulations say about its lid, insulation and overflow.

The water level and the ballvalve

On a cold system, the water in the F&E tank should sit low in the tank. It needs room above it for the expansion when the heating comes on. The ballvalve is normally set so the cold level leaves that space. If the tank is filled close to the overflow when cold, it can overflow every time the heating runs.

Because the ballvalve controls fresh water into the heating, it matters which way it is set and that it shuts off properly. Adjusting it is a small job, but if you are unsure which tank is which, or the arm is stiff or corroded, leave it for a plumber or heating engineer.

The overflow

The F&E tank has its own overflow (warning pipe), usually running out through the eaves or a wall. Water from it is a sign the tank is too full. Our guide to overflow pipes explains how to trace which tank or cistern a dripping overflow comes from.

Common reasons the F&E overflow runs:

• the ballvalve is not shutting off fully, so mains water keeps creeping in;
• the cold water level has been set too high;
• water is being pumped or pushed over from the heating circuit, which points to a fault in the system;
• on some systems, a fault in the hot water cylinder's heating coil can let water pass between the heating and the hot water. That needs a heating engineer to diagnose.

Warning signs

Look at the F&E tank now and then, as you would the big tank. Things worth reporting to a heating engineer:

• The tank is empty or nearly empty with the heating cold. The heating may be losing water through a leak somewhere, or the ballvalve may have stuck shut.
• It keeps overflowing, or the level climbs after the heating has been on.
• The water is constantly moving, bubbling or spurting from the vent pipe when the heating is running. This is often called pumping over. It can be linked to the pump, a blockage or the way the system was piped. See heating circulator pumps.
• Lots of air in the radiators. Needing to bleed radiators often can be a sign of air being drawn in. Our radiator bleeding guide covers the routine side.
• The water is black or sludgy. Some colour is normal. Thick sludge suggests corrosion inside the system. See heating inhibitor and sludge.

Any of these can involve the boiler, the pump or the cylinder. Diagnosis is a job for a heating engineer. Work on a gas boiler must be done by a Gas Safe registered engineer; our guide on when to call a Gas Safe engineer explains why.

What you can safely do

• Check the tank has a lid that fits, so debris and insects stay out.
• Check it and its pipes are insulated against frost, without blocking the vent.
• Note the water level cold and with the heating on, so you can describe it to an engineer.
• Watch the overflow outside and note when it runs.

Draining down or refilling the heating, adding chemicals, changing the pipework or converting to a sealed system are jobs for a heating engineer. We do not give a method for them.

Is the small tank in my loft for drinking water?

No. The small tank is the feed and expansion tank for the heating. Its water circulates through the radiators and boiler and is not for drinking or washing. The large tank is the cold water storage tank.

Why does the water level in the small tank go up when the heating is on?

Because water expands as it heats. On an open-vented system, the extra volume goes back into the feed and expansion tank. That is why the cold level should be set low enough to leave room.

Why is my feed and expansion tank overflowing?

Usually the ballvalve is letting in too much water or is set too high. If it keeps happening, or the water is moving when the heating runs, ask a heating engineer to check the system. It can point to a fault with the pump, pipework or cylinder.

Does a combi boiler have a feed and expansion tank?

Normally not. A combi usually uses a sealed heating circuit with a pressure gauge and an expansion vessel. If you have a combi and still see a small tank in the loft, it may be left over from an old system. Ask a heating engineer before removing anything.