What Are the Different Types of Hot Water Cylinder?

There are four main types of hot water cylinder in UK homes: vented cylinders fed from a tank in the loft, unvented cylinders fed straight from the mains, thermal stores that hold heating water rather than tap water, and combination units that carry their own small tank on top. Within those, a cylinder is either direct (heated only by an electric immersion heater) or indirect (heated by a boiler or heat pump through a coil). Which one you have decides your hot water pressure, what a replacement involves and who is allowed to fit it.

Vented cylinders: the traditional gravity-fed type

A vented cylinder is an open, low-pressure copper cylinder fed by a cold water storage tank in the loft. Hot water pressure comes from the height of that tank above the tap, so it is gentle downstairs and often weak upstairs.

You can usually recognise one by the vent pipe rising from the top of the cylinder and bending over the loft tank, and by the absence of a small expansion vessel or a cluster of safety valves beside it. Vented cylinders are simple, quiet and cheap to replace like for like, and they carry on working from the tank for a while if the mains is cut off. Their weaknesses are pressure, the loft space the tank takes up, and the risk of an old tank or its pipework freezing or contaminating the water. Most homes with a regular (heat-only) boiler still have one.

Unvented cylinders: the pressurised type

An unvented cylinder, also called a pressurised or mains-pressure cylinder, is sealed and fed directly from the incoming cold main. Hot water comes out at close to mains pressure at every tap and shower, on every floor.

Because the water is stored hot and under pressure, an unvented cylinder carries a set of safety devices: a pressure reducing valve on the cold feed, an expansion vessel or internal air bubble to take up expansion, an expansion relief valve, and a temperature and pressure relief valve, all discharging through a tundish to a safe point. Installing one is notifiable building work and must be done by an installer with the G3 unvented hot water qualification; see how to check your cylinder installer is G3 qualified. Unvented cylinders are usually stainless steel. They are the normal partner for a system boiler in a house with two or more bathrooms, but they only perform as well as the mains supply feeding them, which has to be measured before one is recommended.

Thermal stores: storing heating water instead of tap water

A thermal store holds boiler-heated water from the heating circuit, not the water you wash in. Mains water runs through a coil inside the store and is heated as it passes, so you get mains-pressure hot water without storing drinking water hot.

Thermal stores suit homes that combine several heat sources, such as a boiler with solar thermal or a wood burner with a back boiler, because the store can accept heat from each of them. The store itself is usually open-vented with a small header tank. The trade-offs are that the store has to be kept very hot to deliver good hot water flow rates, which costs heat, and that the hot water flow drops once the store has cooled. For a straightforward gas boiler replacement, a thermal store is rarely the first recommendation; an unvented cylinder or a combi usually does the same job more simply.

Combination cylinders: tank and cylinder in one

A combination cylinder is a vented cylinder with its cold water tank built onto the top, in one unit. It is common in flats and small houses where there was no loft to put a separate tank in.

Because the tank sits only a short distance above the taps, the hot water pressure is very low, often noticeably weaker than a standard vented set-up. Combination units are simple to replace like for like, but when one fails it is worth asking whether a combi boiler or a compact unvented cylinder would serve the flat better. That comparison depends on the mains flow rate and on whether the boiler is also due for replacement.

Direct and indirect: how the water is heated

A direct cylinder is heated only by electric immersion heaters. An indirect cylinder has a coil inside that carries hot water from a boiler or heat pump, and usually has an immersion heater as a backup.

Most homes with a gas boiler have an indirect cylinder. Direct cylinders are found where there is no boiler at all, typically in all-electric flats on an off-peak tariff, where two immersion heaters warm the water overnight. A few variations are worth knowing:

• Twin-coil cylinders have a second coil so that a solar thermal panel can pre-heat the water.
• Heat pump cylinders have a much larger coil surface, because a heat pump delivers water at a lower temperature than a boiler and needs more contact area to heat the stored water in a reasonable time.
• Slimline and horizontal cylinders trade some capacity for fitting into a narrow cupboard or under an eaves space.

If you are thinking of a heat pump in the next few years, that choice matters now: a standard boiler cylinder will not recover quickly enough on a heat pump, while a heat pump cylinder works perfectly well on a boiler in the meantime.

How to tell which type you have

Look at the top of the cylinder and around it. A few features tell you most of what you need to know.

• Copper-coloured cylinder, often with sprayed foam or a red jacket, and a pipe curving up into the loft: vented.
• White or grey metal-cased cylinder with a small expansion vessel, valves and a tundish nearby: unvented. There will usually be a data label with a G3 or installer notice.
• Cylinder with a small tank moulded onto the top: combination.
• Two pipes into a coil connection near the bottom: indirect. No coil connections and one or two immersion heaters only: direct.

If the labels are missing and the cupboard is a tangle of pipes, a surveyor will identify it in a few minutes. Do not open or adjust the safety valves on an unvented cylinder yourself.

Which type should you replace yours with?

It depends on your mains supply, how many bathrooms you use at once, the space you have and what heats the water. There is no single best cylinder, and the right answer is sometimes no cylinder at all.

As a rough guide, a household with good mains flow and two or more bathrooms is usually best served by an unvented cylinder with a system boiler. A household with weak mains flow may be better off keeping a vented cylinder, perhaps with a pump. A one-bathroom flat with good flow is often best with a combi and no cylinder. The comparison is set out in unvented or vented: choosing a replacement cylinder, and the capacity question is answered in what size hot water cylinder you need. Our hot water cylinder replacement survey measures the mains flow rate, checks the space and discharge route, and recommends the type that fits the house, including when a like-for-like swap is the sensible choice.