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What is a hot water cylinder?

Read our guide to hot water cylinders and their benefits.
a Vaillant hot water cylinder near a door with a pair of wellies and a shovel. There is a green plant in a white pot to the right

Many households rely on hot water cylinders for their central heating and hot water if they have a system or open vent boiler or a heat pump. However, few know exactly how they work, how to look after them or how to diagnose potential issues until it’s too late.

What is a hot water cylinder?

A hot water cylinder is a well-insulated tank which stores water after it’s been heated. Water is heated through your boiler or heat pump system and is then stored, at temperature, ready for you to use whenever it’s needed.

By storing hot water in this way, there’s no need to run the heating system constantly resulting in lower energy bills, without subjecting household members to cold showers!

Do I need a hot water cylinder?

If you have a system or open vent boiler or heat pump (air source, ground source or water source), you'll need a hot water cylinder as part of your heating and hot water system.

For homes with two or more bathrooms and a high demand for continuous hot water, a heating system with a storage cylinder ensures your needs are always met. If your property has low water pressure and needs a pump for shower flow, a hot water cylinder ensures water is delivered at the right temperature.

Vaillant hot water cylinders are designed to work seamlessly with our domestic heating solutions. Explore our cylinder range or contact your local installer who will be able to advise you on size, type, or any other questions.

How do hot water cylinders work ?

1. Pressurised unvented hot water cylinder

Unvented hot water cylinders get water directly from the mains, which means better pressure and easier water distribution in the home. This is a benefit to its design and a reason behind its growing popularity since it was first introduced to the UK in the 1980s.

Unvented hot water cylinders do not require a separate cold water tank. This makes them more compact than other systems that need additional space in attics and lofts.

2. Vented hot water cylinder

A vented hot water cylinder gets its water from an external cold water tank, typically located in the attic. This setup relies on gravity for pressure, rather than mains pressure. It's called "vented" because it uses a vent pipe to connect the hot water cylinder to the header tank.

Watch the video 'What is an unvented hot water cylinder' for further info.

What is the difference between a direct and indirect hot water cylinder?

As well as choosing between an unvented or vented hot water cylinder, there is the consideration over whether it should be directly or indirectly heated.


A direct hot water cylinder heats water through an immersion heater found inside the cylinder. The cylinder is therefore not connected to a boiler.


An indirect hot water cylinder heats the water using an external heat source such as a boiler or heat pump. The water is heated in the cylinder through a coil that is heated by the appliance.

What are the advantages of a hot water cylinder?

  • Suitable for larger homes

    Hot water cylinders are ideal for big homes with high hot water needs. Unlike combi boilers, which work well for properties with low hot water demand, cylinders handle multiple water outlets at once. They store lots of hot water in advance, ready to distribute throughout the home whenever needed. Boiler systems that use hot water cylinders, however, can store large quantities of hot water in advance. This hot water can then be sent throughout the home as needed.
  • Versatile

    Hot water cylinders are versatile for modern homes. They can heat water directly or indirectly.
  • Backup hot water

    In a vented hot water system with an indirect boiler, a backup heat source can be added to ensure hot water is available in the unlikely possibility that the boiler or heat pump were to fail.

How do I know if my hot water cylinder needs replacing?

Hot water cylinders today typically come with a minimum 25-year guarantee but can last longer if maintained well. Your heating engineer will be able to carry out comprehensive checks for you to see if the hot water tank needs replacing.

However, you should look out for:

  • Cold water

    If you notice cold water in your direct hot water cylinder, it might mean the heating element inside is worn out. Consider contacting a heating engineer to replace it. If your cylinder is 8-12 years old, it's wise to have the whole boiler checked by a an expert to see if it needs replacing. However, if your hot water cylinder is nearing 8-12 years it is advisable to have the whole boiler looked at by a GSR engineer to see whether the entire system needs to be changed.
  • Leakages

    If you spot a leak in your hot water cylinder it is recommended that you arrange for an engineer to inspect your system as soon as possible. Not only can leaks be dangerous, but they also signal that there is an internal issue that cannot be repaired.
  • Strange noises

    Over time, limescale can build up in hot water cylinders, leading to blockages. Without regular maintenance, this buildup can cause strange noises when heating water. An engineer can usually flush it out, but if left untreated, it could cause damage.
  • Discoloured hot water

    If the water that leaves your taps is discoloured or smelly after it is heated, this may be a sign that the anode rod inside has become defective and needs replacing.