Hot water cylinder

The hot water cylinder, often referred to as the hot water tank, is a vessel designed for heating and storing hot water for use in the home. It is typically made out of copper, and has a concave base for structural integrity. The top of the cylinder, or crown as it’s known, is usually bell-shaped in order to prevent air locks.

The hot water cylinder is usually situated in an airing cupboard. It is usually fed from the cold water storage cistern in the loft; however in some installations, the cold water cistern may be situated on a platform directly above it, or may even be part of the cylinder itself.

The water inside the cylinder is heated by water from the central heating circuit. Hot water from the boiler flows through the cylinder via a heat exchanger which consists of a coil of copper pipework. Such cylinders are also fitted with an electric immersion heater as a backup. This type of arrangement is known as an indirect hot water cylinder, due to the fact that the two bodies of water do not (and should not) come into contact with each other. Another kind of cylinder, known as a primatic hot water cylinder, separates the two bodies of water via an air bubble. Some cylinders are heated via electric immersion heaters only, and do not have an internal heat exchanger. These are known as direct hot water cylinders.

Cylinders fed via the cold water tank are classified as open or vented, due to the fact that the water is held at atmospheric pressure. However, unvented, mains-fed cylinders are becoming increasingly common, and do not require a cold water storage cistern.


The hot water cylinder is supplied with cold water via a 22 mm downpipe from the cold water tank. In larger households, a 28 mm pipe may be necessary.

It is gravity acting on the water in the cold water tank which effectively pushes hot water out of the top of the cylinder. Therefore the cold feed must be fitted with a gate valve so that the domestic hot water can be shut off if necessary. The cold feed must also be fitted with a drain-cock so that the cylinder can be emptied, as shutting off the cold feed will not empty the cylinder.

On an indirect cylinder, the flow from the boiler enters at the top of the coil, with the return exiting at the bottom. These are typically 22 mm fittings.

Water exits the top of the cylinder via a 22 mm draw-off. This pipe usually runs across the top of the cylinder towards the wall, at which point it is tee’d off, with the lower aspect supplying all of the domestic hot water services, and the other aspect rising up into the loft and bending over into the cold water tank, but without dipping into the water. This pipe is known as the expansion pipe. In order to prevent air from becoming trapped in the draw-off pipe at the top of the cylinder, it is important that it rises slightly as it runs towards the wall in the airing cupboard. This will prevent the formation of air locks.

When water in the cylinder is heated, air bubbles form.. The crown of the cylinder is usually in the shape of a curved bell so that air can rise up and out of the cylinder, instead of becoming trapped and forming airlocks in the system. Air bubbles should rise out of the cylinder and up through the expansion pipe. As the name implies, the expansion pipe also allows for the expansion of the water in the cylinder when it is heated, and for it to discharge harmlessly into the cold water tank if necessary. The expansion pipe and cold water tank act as a kind of safety valve for the cylinder, as it would be extremely dangerous to contain hot water without allowing for changes in its volume.

Immersion heater

On an indirect cylinder, the immersion heater enters the cylinder via the crown, pointing inwards towards the central body of water. A standard immersion heater consists of a 240 volt, 3 kilowatt element, and is secured to the cylinder via a 2 and a quarter inch BSP thread boss. The advantage of having an immersion heater on an indirect hot water cylinder is that even if the boiler fails, the household will still have hot water.

A direct hot water cylinder will have two immersion heaters, each entering the cylinder horizontally: one towards the bottom of the cylinder, and another around two thirds of the way up. A household with a direct cylinder will typically be on an Economy 7 electricity tariff, which offers cheaper rates for off-peak times.

Operated by a timer and located near the base of the cylinder, the bottom immersion heater will heat the entire cylinder using cheaper night time electricity. In contrast, the higher immersion heater heats mainly the top third of the cylinder, and will come on when water is drawn off during the day. It may also periodically switch on in order to keep the water in the cylinder at the desired temperature.


An immersion heater will have its own adjustable thermostat. This slides into the cylinder in its own pocket without coming into contact with the water. In order to minimise the risk of scalding and to prevent the growth of Legionella, the thermostat should be set to 60° C.

Construction & capacity

Hot water cylinders are typically made out of copper, and have a capacity of either 120 litres, 144 litres, or 210 litres. These are 90 cm, 105 cm, and 150 cm tall respectively. A 210 litre cylinder will be necessary for an Economy 7 tariff, ensuring that the water is heated cost-effectively.

The cylinder will be very heavy when full of water, and should be situated on a flat, stable, continuous wooden base – ideally ¾ inch plywood across three timber bearers. Air gaps between the timber bearers will permit the circulation of air under the cylinder, limiting the formation of condensation.

When replacing an immersion heater, it is good practice to leave water in the cylinder where it is possible to do so. The mass of water will support the cylinder wall when unscrewing the immersion heater, and help to prevent it from warping.

Due to their size and copper construction, old hot water cylinders may fetch anywhere up to £100 at a scrap dealer.


Preventing heat loss is one of the most effective ways of saving energy in the home, and the hot water cylinder is no exception. A specially made, glass fibre insulation jacket which is strapped around an exposed copper cylinder only costs a few pounds, and will almost certainly pay for itself within a few months. Most hot water cylinders available on the market today are sprayed with a layer of foam insulation, which is even more effective.


Not to be confused with the immersion heater thermostat, indirect hot water cylinders are fitted with a thermostat which is typically strapped to the side of the cylinder around a third or a half of the way up from the bottom.

When the water in the cylinder reaches the pre-set temperature, the cylinder thermostat will close the motorised valve, preventing the flow of hot water from the boiler through the coil.

In order to detect the temperature of the water accurately, a cylinder thermostat must come into contact with the copper wall of the cylinder. This will require any pre-existing foam insulation to be cut away where the thermostat is to be fitted.


Surrey flange

A Surrey flange creates an additional draw-off point from the top of the cylinder, which is typically used as a dedicated supply for a shower pump. It does this via a tube which dips down into the cylinder and draws water from the main body of water. This ensures that the pump is not damaged by air bubbles which form during the heating of the water – these are still allowed to rise to the highest point of the cylinder and out at the top through the main draw-off point before being expelled through the expansion pipe.

Essex flange

An Essex flange is another way of establishing a dedicated feed for a shower pump and protecting it from air damage. It draws water from the side of the cylinder, slightly below the crown.

One of the advantages of an Essex flange over a Surrey flange is that there is no need to modify the existing pipework above the cylinder. However, fitting an Essex flange will require a hole to be cut in the side of the cylinder. Some cylinders come with an Essex flange pre-fitted by the manufacturer.

Secondary return

Hot water cylinders can also accommodate a secondary return. This is a configuration in which hot water is pumped from the cylinder and around a circuit, before returning to the cylinder.

By drawing water from this circuit, the advantage of a secondary return is that hot water is always available near the point of use, preventing water from being wasted when e.g. a hot tap is used, and the cold water has to be run off before hot water is delivered.

Efficient insulation must be installed on the secondary circuit pipework so that heat loss from the circulating hot water is kept to a minimum.

Primatic hot water cylinder

A primatic or self-priming hot water cylinder is a vented indirect hot water cylinder which provides the water for the central heating circuit, preventing the need for a separate feed and expansion cistern. The two bodies of water – the central heating water and the domestic hot water inside the rest of the cylinder – are separated by one or more airlocks which are formed inside the heat exchanger as the cylinder is filled with water from the cold water storage cistern.

Although a plumbing system involving a primatic cylinder may be cheaper to install, it has significant long-term disadvantages:

  • Shower pumps should not be fitted to primatic cylinders due to the risk of disrupting the air bubble which separates the domestic supply and the central heating circuit.
  • Inhibitor chemicals and other additives cannot be added to the central heating circuit due to the risk of them contaminating the domestic hot water. The absence of a chemical inhibitor will increase the risk of corrosion and damage within the central heating system.

Fortic hot water cylinder

A fortic cylinder or combination cylinder is a vented cylinder with its own self-contained cold water storage cistern. These are ideal for flats, or households where it is impractical or not possible to install a separate cold cistern.

Fortic cylinders are available in both direct and indirect configuration, and are available in a range of capacities. It is also possible to buy primatic fortic cylinders.

Unvented hot water cylinder

As the name implies, unvented cylinders do not contain water at atmospheric pressure. This is because they are fed directly by the mains, eliminating the need for a cold water storage cistern. Unvented cylinders are usually made out of stainless steel instead of copper, and are available in both direct and indirect configurations.

Unvented cylinders are significantly less common than vented cylinders. This is because they were not permitted in the UK until 1986, although they are becoming more and more popular. The main advantages are:

  • Hot water at mains pressure – much better flow rates in comparison to a gravity-fed system
  • No need for a bulky storage cistern and its associated pipework

Due to the risk of danger in association with storing hot, pressurised water, unvented cylinders can only legally be installed by G3 certified engineers, and the cylinder must have a label clearly showing the installer’s name.

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