Hot water cylinder: what is it & how does it work?

The hot water cylinder or hot water tank is a vessel for heating and storing hot water for taps 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, is usually bell-shaped in order to prevent air locks.

The hot water cylinder is usually situated in an airing cupboard. It’s usually fed from the cold water storage cistern in the loft. However, in some installations, the cold water cistern may be on a platform directly above it, or may even be part of the cylinder itself. A hot water cylinder with its own integrated storage cistern is known as a Fortic cylinder.

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. This typically 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 never 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. These are known as direct hot water cylinders.

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


A typical hot water cylinder gets its supply of fresh 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. This is because shutting off the cold feed will not empty the cylinder.

On an indirect hot water 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 this point it is then tee’d off, with the lower aspect supplying all of the domestic hot water services,

The expansion pipe

The other aspect of the draw-off pipe rises up into the loft and bends over into the cold water tank, but without dipping into the water. This pipe is known as the expansion pipe, and it has two purposes:

  • To allow for the water to expand when it is heated, and discharge into the cold cistern if necessary.
  • To allow for air bubbles which form to escape the cylinder.

It is useful that the draw-off pipe rises slightly as it runs away from the cylinder. This will prevent air from becoming trapped inside it and forming an air lock. Indeed the purpose of the bell-shaped crown of the cylinder is to allow air bubbles to easily escape.

The expansion pipe and cold water cistern ultimately form as a kind of safety valve for the cylinder, as it would be extremely dangerous to contain hot water without allowing for the venting of gases or changes in its volume. For this reason, there should never be a gate valve on the draw-off pipe.

Immersion heater

On an indirect hot water 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. It 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 is situated 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 hot water cylinder via 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. Its base must be flat, stable and continuous. An ideal base is 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 possible. The mass of water will support the cylinder wall when unscrewing the immersion heater, and help to prevent it from warping.

Never fail to bring an old hot water cylinder to a scrap dealer. Due to their size and the value of copper, they can fetch anywhere up to £100.


Preventing heat loss is one of the most effective ways of saving energy in the home. Applying effective insulation to the hot water cylinder is no exception to this rule. A specially made, glass fibre insulation jacket which is strapped around an exposed copper cylinder only costs a few pounds. Plus, it 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. This 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 one or more motorised valves. This will prevent hot water from the boiler from flowing through the coil. If the central heating is on, water will resume flowing through the radiators.

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. This is typically used as a dedicated supply for a shower pump. A Surrey flange 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. They are then expelled out of the system via the expansion pipe as normal.

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, around a circuit, and then back 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 also provides the water for the central heating circuit. This prevents 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 inside the heat exchanger. These are formed as the cylinder fills with water from the cold water cistern.

While systems using a primatic hot water cylinder may be cheaper to install, they have significant long-term disadvantages:

  • You can’t use a shower pump as it may disrupt the airlock.
  • You can’t add inhibitor chemicals or other additives to the central heating circuit. This is because, if the airlock is lost, the domestic hot water and the central heating water will mix, contaminating the hot water for the taps.
  • Since you can’t use a chemical inhibitor, primatic central heating systems are massively more susceptible to corrosion.

Fortic hot water cylinder

A fortic hot water 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 suggests, unvented cylinders don’t contain water at atmospheric pressure. This is because they are fed directly by the mains, eliminating the need for a cold water storage cistern. They are usually made out of stainless steel instead of copper. You can get them in both direct and indirect versions.

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

  • Hot water at mains pressure. This means 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, only G3 certified engineers are legally allowed to install unvented cylinders. Following installation, the cylinder must have a label which clearly shows the installer’s name.