Feed and expansion tank

The feed and expansion tank, header tank, or feed and expansion cistern as it is technically known, is a cistern which supplies the central heating circuit with water and accommodates the expansion of the water when it is heated by the boiler. It is typically made out of plastic. However, like cold water storage cisterns, other materials have been used in the past, such as asbestos or galvanised steel.

The feed and expansion cistern is usually situated in the loft, where it forms the highest point of the central heating system. It is supplied by the rising main and is the smaller of the two cisterns. In a typical home, it will have a capacity of around 4 gallons (18 litres).

Feed and expansion tank – Contents

General overview

The flow of water into the cistern is controlled by a ball valve. Should the ball valve fail or water enter the cistern in the event of a broken hot water cylinder coil, an overflow pipe carries the water away, discharging outdoors. The overflow pipe should be fitted at a constant fall. Its exit outside should be clearly visible to alert residents to a fault. The overflow should be at least 19mm in diameter, and should be capable of evacuating all of the excess water under maximum fault conditions, such as in the complete failure of the ball valve. An isolation valve must be fitted to the mains pipe supplying the cistern.

A tank connector connects the cistern to a feed pipe which supplies the central heating system with water and allows for the water, when heated, to flow back up into the cistern as it expands. This pipe should be at least 15mm in diameter and must never be fitted with an isolation valve.

Vent pipe

Another pipe connected to the central heating system rises up above the cistern to form a ‘U’ shape, and points down into it without coming into contact with the water. This pipe is known as the vent pipe, and serves as an exit point for any air which may have got into the system or any gases which may have formed inside it, such as hydrogen.

The vent pipe should be at least 22mm in diameter, and should rise at least 450mm above the maximum water level in the cistern in order to prevent the system from pumping over. This fault may also occur when the pump is installed incorrectly or is set at too high a speed, causing water to discharge into the cistern through the vent pipe. This process enriches the water with oxygen, much like an aerator in a fish tank. Eventually,  the oxygen-rich water will quickly ruin the entire heating system via internal corrosion if left uncorrected. Similarly, the pump must not be installed in a position which will draw air into the system via the vent pipe.

Feed and expansion tank construction

Like the cold storage cistern, feed and expansion cisterns are generally made out of plastic. Common materials include polyethylene, polypropylene, and polyvinyl chloride (PVC). Glass-reinforced polyester (GRP) may also be encountered.

Modern plastic cisterns are generally black in order to resist algae growth. They are usually rectangular or loosely rectangular in shape. A lid is not always necessary, as some cisterns have an ‘enclosed’ style design with a built-in service hatch on the top.

Other constructions

Older cisterns may be made from galvanised steel, which is susceptible to corrosion. Some cisterns may also be made from asbestos cement. You should never attempt to cut up or even dispose of asbestos cisterns yourself. Even in tiny amounts, asbestos particles are extremely dangerous to human health. Skin contact alone can cause nasty dermatological problems. Contact a professional asbestos remover – it’s just not worth the risk.

Some cisterns may be made entirely from copper. Metal cisterns are required where water is heated by a device without any form of thermostatic control. This is because they may have to accommodate extremely hot water, even as part of their normal operation.

Base and location

The feed and expansion cistern is obviously not as heavy as the cold water storage cistern. Nevertheless, it should still be situated on a flat, stable, continuous base. It should extend each side of the cistern by at least 50 mm. The base must retain its structural integrity if it comes into contact with water. Marine-grade plywood of at least 19mm is recommended. Chipboard should never be used as it is liable to disintegrate if it gets wet.

In most households, the feed and expansion cistern will be located near or next to the cold water storage cistern. On older installations, it may be suspended directly above the cold water storage cistern via a base or joists. In some cases, the overflow and/or the vent pipe may simply discharge into the cold water storage cistern below. This risks the contamination of the domestic water supply with the dirty water that circulates through the central heating system. Such installations are as inadequate as they are dangerous.


The ball valve in a feed and expansion cistern is set so that when the central heating system is cool, the water level is only slightly higher than the outlet at the base of the cistern. This is so that the water in the central heating system has the space to expand when heated. The cistern must be able to accommodate the expansion of the water by approximately 4% of its original volume. The elevated water level must be at least 25 mm / 1 inch below the overflow.

Central heating inhibitor

Adding an inhibitor solution to the central heating system will prevent corrosion and the formation of oxides. This is easily added via the cistern, once the heating system has been drained down and flushed out. Bacteria and mould spores can produce a foul smelling jelly or skin on the surface of the water. Adding a biocide to the cistern should prevent this.

Loaded with these chemicals, water from the central heating system should obviously never come into contact with the domestic hot water. However, this will happen if the coil in the hot water cylinder develops a leak. In this situation, the two water systems – the domestic hot water and the central heating – effectively become one. Consequently, the water level in the tank tries to balance out with the level in the cold water storage cistern. A classic sign of this is the feed and expansion tank constantly overflowing, and the storage cistern constantly filling.

Extra fittings

The cistern and all its adjoining pipes must be lagged in order to prevent them from freezing during the winter. Heat rising from the household below helps to prevent this. For this reason, insulation should not be placed underneath the cistern.

A small backing plate can be secured to the cistern via the ballcock’s rear nut. This reduces the stress to the cistern wall from the upward force of the float.

A close-fitting lid will prevent dust, insects, rodents, and bits of loft insulation from getting inside. These things could damage your central heating system or cause blockages.

Where to buy

You can buy a brand new feed and expansion tank from most of the major UK hardware stores. B&Q sell a 4 gallon cistern from Polytank, which comes complete with an insulation jacket, a Part 2 ball valve and float, a ball valve backplate, a 15mm compression tank connector, a warning pipe elbow, and a lid. You can also purchase a feed and expansion tank from Wickes, although none of the extra components are included, and the Polytank cistern from B&Q is likely to be a better choice. B&Q offers a click-and-collect service, and free delivery on orders over £50.