How to size a hot water cylinder



Typical useage of hot water

  • 70-80 litres of hot water to run a bath

  • 45 litres of hot water for a 5 minute shower

  • 50-100 litres of hot water for a full load in the washing machine

  • 12-20 litres of hot water per cycle in the dishwasher

  • 0.2 litres of hot water per second when you run the kitchen taps


Indirect hot water cylinders powered by central heating boilers tend to require a smaller capacity due to the fact that a boiler has more power available than a standard 3kW immersion heater. The larger the boiler the quicker the reheat time.

Our general guideline for domestic hot water cylinder sizes is therefore as follows:


Indirect Hot Water Cylinders

Direct Hot Water Cylinders

1 Bedroom + Bath/Shower



2 Bedroom + Bath/Shower



3 Bedroom + 2 Baths/Shower



4 Bedroom + 2 Baths/Shower



Larger Properties




Whats the difference between direct and inderect water tanks?

What are Direct Hot Water Cylinders

Direct hot water cylinders use heat sources  located in the cylinder, like an immersion heater to heat the water. These systems tend to be located in premises where there is no access to a heat source

On occasion these systems will feature two immersion heaters which will enable you to take advantage of off-peak energy tariffs and therefore minimise the overall running costs.


What are Indirect Hot Water Cylinders

Indirect hot water cylinders typically use an external energy source, such as a traditional central heating boiler, solar thermal system or biomass, which is fed into the cylinder via a coil or heat exchanger which then ‘indirectly’ heats the hot water.

Many indirect hot water cylinders also come with an immersion heater to act as a back-up should the primary energy source fail or be unavailable. 

Both systems can be incorporated into vented and unvented cylinders depending on the specific nature of the project.


What is a Vented Hot Water Cylinder?

A vented hot water cylinder is the more traditional approach where there is a cold water storage tank, often in the loft, which supplies water to the hot water cylinder on a gravity system.

The cold water is heated either directly from an immersion heater within the hot water cylinder or indirectly via the central heating boiler or solid fuel appliance.

The cold water tank is used to enable trapped air to escape as well as acting as an expansion tank to accommodate the expansion of the hot water during the heating process and the occasional instance of an overheat situation.

As they are relatively simple in their construction and operate at lower pressure, vented hot water cylinders tend to be cheaper than unvented.

The biggest challenge is they may deliver a lower pressure of water.

The system also takes up more space in the home with the combination of the cold water tank and the hot water cylinder.

However, on the plus side, Vented Hot Water Cylinders tend to be cheaper and require almost no maintenance.

What is an Unvented Hot Water Cylinder?

An unvented hot water cylinder is a pressurised system which is fed directly from the cold mains water feed and therefore can deliver hot water at mains pressure, which is great for improving the performance of your shower and bath.

An unvented cylinder doesn’t require a cold water storage tank, which is ideal if you are pushed for space as well as offering more flexibility when siting the cylinder.

The water can either be heated directly via an immersion heater or indirectly via the central heating system, solar thermal technology or other renewal energy such as heat pumps and biomass.

The unvented hot water cylinder will require an expansion vessel to be fitted to allow the expansion of water in the system as it heats up. The Unvented cylinder will also comes with other safety mechanisms, such as pressure relief valves and twin thermostats.