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The feed and expansion 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 often informally known as the header tank. It is typically made out of plastic, although 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 – often near the cold water storage cistern – and forms the highest point of the central heating system. It is supplied by the rising main, and is generally the smaller of the two cisterns: in a typical home, it will have a capacity of around 4 gallons (18 litres).
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, and its exit outside should be clearly visible so as to alert the homeowner 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.
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, and 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.
Like the cold storage cistern, feed and expansion cisterns are generally made from plastics – either polyethylene, polypropylene, or polyvinyl chloride (PVC), although glass-reinforced polyester (GRP) may also be encountered.
Older cisterns may be made from galvanised steel, which is susceptible to corrosion. Some cisterns may also be made from asbestos cement, and just like cold water storage cisterns, 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, because they may vent extremely hot water into the cistern even as part of their normal operation.
Modern plastic cisterns are generally black in order to resist algae growth, and 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.
Base and location
The feed and expansion cistern is obviously not as heavy as the cold water storage cistern. Nevertheless, it must conform to exactly the same regulations, and should be situated on a flat, stable, continuous base that extends at least 50mm beyond each side of the cistern and will 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, you may find that it is suspended directly above the cold water storage cistern via a base or joists across the larger cistern. In some cases, the overflow and/or the vent pipe may simply discharge into the cold water storage cistern, risking 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 adjusted 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, and the elevated water level must be at least 25 mm / 1 inch below the overflow.
An inhibitor solution must be added to the central heating system in order to prevent internal corrosion and the formation of oxides. This is easily added via the cistern, and can be done once the heating system has been drained down and flushed out. Bacteria and mould spores can cause the formation of a foul smelling jelly or skin on the surface of the water; this can be prevented by adding a biocide to the cistern.
Loaded with anti-corrosion and biocide chemicals, the stale, recirculating water of the central heating system should obviously never come into contact with the domestic hot water . However, this will happen in the event that the coil in the hot water cylinder develops a leak, and of the two cisterns in the loft, will cause the one with the lower water level to overflow – usually the feed and expansion cistern, as its water level tries to balance out with the higher water level in the cold water storage cistern.
The cistern and all its adjoining pipes must be lagged in order to prevent them from freezing during the winter. Loft insulation should not be placed underneath the cistern, as heat rising from the household below during the winter will help prevent this.
A small backing plate, either metal or plastic, and located on the outside cistern wall, is secured via the ballcock’s rear nut. This reduces stress to the cistern wall from the upward force of the float.
A close-fitting lid is required, and will prevent dust, insects and rodents from getting inside, as well as bits of loft insulation and other debris which could cause a blockage or damage in the central heating system.
Water byelaws require the fitting of certain components in order to minimise the risk of contamination. These are:
- A rubber grommet, to create a sealed admission point for the vent pipe.
- A screened breather, to keep the cistern at atmospheric pressure.
- A screened warning pipe unit, to prevent anything outside, such as insects, from entering the cistern.
These parts can be bought as a kit. Byelaw 30 kits, as they are known, are often supplied with a new cistern, but they can also be purchased separately if you are looking to upgrade an older cistern. They will also include a dip tube which allows the warning pipe to dip into the water so that cold drafts cannot enter the cistern, although this is less likely to be of much use, as unlike a cold water storage cistern, the default water level in a feed and expansion cistern will usually be well below the overflow.
Where to buy
Brand new feed and expansion tanks for new installations or to replace an existing cistern are available 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.
- Polytank 4 gallon feed and expansion tank with fittings (free click-and collect)
- Wickes 4 gallon feed and expansion tank (free click-and collect)