A hot water cylinder is an insulated tank that stores heated water ready for taps, showers and baths. Water is warmed by a boiler, an immersion heater, or a heat pump or solar coil, then held at temperature until you need it. The two main types are vented cylinders, which are fed from a cold tank in the loft, and unvented cylinders, which connect directly to the mains.
What a hot water cylinder is
A cylinder is simply a store of hot water. Unlike a combi boiler, which heats water on demand as it passes through, a cylinder keeps a volume of water hot so several outlets can draw on it at once.
Inside, the water is heated in one of two ways. A coil — a length of pipe carrying hot water from the boiler — passes through the tank and transfers heat to the stored water. This is called indirect heating. A direct cylinder, by contrast, heats the water with an immersion heater alone.
An immersion heater is an electric element fitted into the cylinder, much like the element in a kettle. Most cylinders heated by a boiler still have one as a backup, useful if the boiler fails or for top-up heating on cheaper overnight electricity tariffs.
Modern cylinders are heavily insulated, usually with a sprayed foam jacket, so heat loss is far lower than in older copper tanks that needed a separate insulating cover. Good insulation means water heated in the morning can still be usefully warm hours later.
Cylinders are commonly found in a heating system known as a regular or "heat-only" setup, where the boiler heats both the radiators and the cylinder. Households with a heat pump almost always need a cylinder too, because heat pumps work best heating a store of water steadily rather than on demand.
Vented versus unvented storage
A hot water cylinder is an insulated tank that stores heated water ready for taps, showers and baths.
The key difference is where the water pressure comes from. This affects performance, the space needed, and the maintenance involved.
A vented cylinder is fed by gravity from a cold water storage tank, usually sited in the loft. The height of that tank above the taps creates the pressure. The system is "open" to the air through a vent pipe, which lets it cope safely with the expansion of heated water.
- Generally simpler and cheaper to install and maintain.
- Needs a cold tank in the loft, plus the space and access for it.
- Water pressure depends on the height of the tank, so upstairs showers can feel weak unless a pump is fitted.
- Less affected by mains pressure problems, since the loft tank acts as a reserve.
An unvented cylinder is sealed and connected straight to the mains cold supply, so the water comes out at mains pressure. There is no loft tank. Because the water is sealed in and heated, it expands with nowhere to overflow, so the cylinder includes safety devices to manage that pressure.
- Delivers strong, balanced pressure to taps and showers across the home.
- Frees up loft space and removes the cold tank entirely.
- Requires safety components such as an expansion vessel, a pressure-reducing valve and a temperature-and-pressure relief valve.
- Performance depends on a good incoming mains flow and pressure; a weak mains supply limits the benefit.
Unvented cylinders must be installed and serviced by someone holding the relevant competence — commonly known in the trade as a "G3" qualification, named after the building regulation that covers them. They need an annual check of the safety valves and expansion vessel. A surveyor or installer should test the mains flow rate before recommending one, because an unvented system on a poor supply can disappoint.
Vented systems are the older, traditional arrangement and remain perfectly serviceable. Many homes keep them simply because they work and a change would mean significant pipework. Unvented systems are now the common choice in new installations where mains pressure is adequate.
How much hot water a home needs
The right cylinder size depends on how many people live in the home, how many bathrooms there are, and when hot water is used. The aim is a tank that meets demand without storing far more than necessary, which wastes energy keeping it hot.
As a rough guide, a single bathroom with one or two occupants is often served by a smaller cylinder, while larger families with two or more bathrooms need more capacity so that a shower and a bath are not competing for the same water. Households where everyone showers in the same morning window need more stored volume than those who spread usage through the day.
A few practical points are worth weighing up:
- Recovery time matters as much as size. This is how quickly the cylinder reheats after a draw-off. A boiler with a fast recovery can use a smaller tank effectively.
- Showers versus baths. A bath uses a large volume in one go, so homes that favour baths benefit from extra capacity.
- High-pressure showers from an unvented system use water faster, which can empty a small cylinder sooner than expected.
- Space. A larger cylinder takes up more room in an airing cupboard or utility space, so physical fit is part of the decision.
Cylinder capacity is measured in litres. As a starting point only, smaller homes are often fitted with cylinders in the region of 120 to 180 litres, while larger households may use 210 litres or more. A heating engineer should size the cylinder to the actual household rather than a rule of thumb, taking the number of outlets and the heat source into account.
It is also worth knowing about legionella, a bacterium that can grow in water held at lukewarm temperatures. Storing hot water at around 60°C and ensuring the whole cylinder reaches that temperature periodically keeps the risk low. This is one reason cylinders are not simply run at the lowest setting that feels comfortable at the tap.
If you are unsure which type suits a property, it helps to start by checking the incoming mains flow and pressure, the space available, and how the household actually uses hot water. Those three factors usually point clearly towards a vented or an unvented cylinder, and to a sensible size.