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What Is a Hot Water Cylinder?

Many households rely on hot water cylinders for their central heating and hot water, but few know exactly how they work or how to diagnose potential issues until it’s too late. Below is our comprehensive guide to hot water cylinders.

How do hot water cylinders work?

A hot water cylinder is a tank used for storing hot water for later use. There are two main types of hot water cylinders available in the UK.

  • Pressurised unvented hot water cylinders
  • Vented hot water cylinders

Pressurised unvented hot water cylinders

Unvented hot water cylinders receive water directly from the mains, therefore benefiting from the greater levels of pressure than a gravity-based system and distributing water more easily throughout 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.

Vented hot water cylinders

A vented hot water cylinder receives its water from an external cold water tank (also known as a header tank) often stored in the attic of a home. This system is usually found in the attic because it uses gravity to build pressure rather than pressure generated from the mains. It is called a vented hot water cylinder because a vent pipe is used to connect the hot water tank to the header tank.

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. 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 a great choice for larger homes with greater demands for hot water. While combi boilers are able to heat water on demand in smaller properties, they cannot cope with several taps, showers and radiators being turned on simultaneously.

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.


A hot water cylinder is also a versatile option for modern homes. Water can be heated either directly or indirectly or through sustainable means like solar power and biomass fuels.

Backup hot water

A vented hot water system backup heat source can also be added to an indirect boiler system to ensure that hot water is available should the boiler fail.

Points to bear in mind

Less efficient

One disadvantage of a hot water cylinder is that it can be less efficient than a combi boiler system. The hot water is heated in advance and is often heated more than is necessary. This results in slightly higher energy and water bills than a more efficient system.

Limited hot water supply

Another disadvantage is that a hot water cylinder can only hold so much water before it runs out. Although the tank is large enough to deal with normal amounts of water use on a given day, it will run out of hot water if the demand is too high. The tank can be refilled but this can take several hours.

Less pressure

Vented hot water cylinders use gravity to build pressure for the water, which means that certain parts of the home do not receive equal levels of pressure. Upper floor bathrooms, for example, may experience lower levels of water pressure than kitchens on the ground floor.

Difficult to install

Unvented hot water cylinders are complicated to install because they operate at a higher pressure than vented systems and require additional safety features. Hot water expands as it heats and therefore needs mechanisms to keep pressure within the system at a safe level. The two solutions for this are:

  • Having a bubble top unit - a unit with an internal air bubble that is trapped in the cylinder upon instalment.
  • Installing an external expansion vessel

These two mechanisms are difficult to install and can make the system more expensive upfront than a vented hot water tank.

How long does a hot water cylinder usually last?

A hot water cylinder will usually need to be replaced after 8-12 years, but can last longer if maintained well. Your boiler engineer will be able to carry out comprehensive checks for you to see if the hot water tank needs replacing.

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

Hot water cylinders are usually reliable but can experience irreparable problems over time. The signs you can look out for are:

Cold water

If your home has a direct hot water tank, cold water could be a sign of an issue with the internal heating element. This element wears down over time and may need replacing by a qualified Gas Safe Registered (GSR) engineer.

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.


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

Strange noises

Limescale builds up in cylinders over time and causes blockages. If your system is not regularly maintained you may experience a significant build up that creates strange noises when water is heated in the system. This can usually be flushed out by an engineer but it could cause damage if left untreated.

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 to be replaced.


  • A hot water cylinder is a tank used for storing hot water for later use. The two types of hot water cylinders available in the UK are pressurised unvented hot water cylinders and vented hot water cylinders.
  • Unvented hot water cylinders receive cold water from the mains, while vented hot water cylinders use an external cold water tank often stored in the attic.
  • The advantages of a hot water cylinder include its suitability for larger homes and its ability to store hot water. Its disadvantages include being less efficient and being more difficult to install.
  • Cold water, leakages and strange noises are all signs that it needs replacing.