A pressurised water tank system is a cylinder that provides the home with pressurised hot water.

Most traditional water tanks are gravity fed, meaning that the water that goes into the cylinder tank is fed from another tank that is located above the cylinder – usually in the loft. Pressurised system tanks, on the other hand, do not require gravity and can produce hot water for all taps around the home without the need of a secondary tank in a higher position.

There are typically two types of pressurised water tank systems that are installed into UK homes, unvented cylinders and thermal store cylinder systems.


An unvented cylinder is a singular tank (typically manufactured out of copper or stainless steel) that stores hot water that can then be transferred to your hot taps. The unvented cylinder will store cold water which is then heated ready to be distributed into the home. There are two types of unvented cylinder, indirect and direct.

What is an Indirect Unvented Cylinder?

An indirect unvented cylinder is a water tank that has a connection from your boiler to the tank. The boiler will heat the water which is in the tank. The tank is then able to distribute pressurised hot water. Most unvented systems will have an inlet pipe that sends the hot water through a spiral coil. The coil will transfer the hot water that runs through the pipe to the cold water that is stored within the tank, and thus heats the water in the tank. Indirect systems still have immersion heaters in case your boiler breaks down, but the immersion heaters can be switched off whilst the boiler heats the water.

What is a Direct Unvented Cylinder?

A direct unvented cylinder is a tank that uses immersion heaters to heat the hot water. This is a more expensive method of heating your water. It should only be considered if you use night storage heaters, or if there is no gas available in your area.

Both of these types of cylinders will have an expansion vessels or air gaps to store hot water when there is too much hot water in the tank. This is needed because hot water expands and as it rises the amount of space that is required for 50L of water, for example, is not enough to store the water once it is heated. Traditional gravity-fed systems do not require expansion vessels because the tank located in an area above the storage tank acts as an expansion vessel when the water expands.

What is an Indirect Thermal Store?

An indirect thermal store is a pressurised water tank system that heats the stored water using the heat produced from other heat sources including heat pumps, boilers and solar panels. This is the best option for homeowners that heat the house using gas. This system does come with immersion rods in case your other heat sources breakdown.

What is a Direct Thermal Store?

A direct thermal store uses immersion heaters to heat the stored water. It uses the exact same transfer method as the indirect – by transferring the heat from the stored water to the incoming cold water. Direct thermal stores should only be installed into homes that are not heated by gas. They are best suited for homeowners with night storage.

Benefits of installing pressurised water systems:

  • Install a power shower
  • Less chance of tank freezing
  • Quicker to fill a bath or basin
  • Save space compared to a traditional system
  • No noisy pumps
  • Less Noisy Compared to Gravity Fed Systems

Should I install a pressurised system?

There is no simple answer to this question and it does depend on a number of factors including:

  • Type of heating system you have
  • Number of people living in your property
  • Number of bathrooms in your property
  • Water flow rate and pressure from your mains water supply