Hot water cylinders

Both System and Regular boilers rely on cylinders to store hot water, but the type of cylinder you use — vented or unvented — is up to you.

The main difference between them is the way each cylinder is supplied with water. An unvented hot water cylinder takes its water supply directly from the mains, whereas a vented cylinder is connected to a water tank (by a vent pipe, hence the name.) This water tank is stored in the loft as it relies on gravity to pull the water down into the cylinder and through the pipes of the house. Get quotes for unvented cylinders now.

Vented Cylinders

In a vented cylinder, a store of water from the mains is kept in a tank in the loft. The natural pull of gravity carries this water, via a vent pipe, down to the hot water cylinder which is usually stashed away in an airing cupboard.

Once in the cylinder the water is heated and either stored for domestic use in your taps, shower or bath, or used to heat your radiators. As water is heated it expands but the presence of the vent pipe and tank in the loft provides an escape route for the excess.


  • A vented cylinder is the simplest and cheapest option to both install and maintain.
  • If, for some reason, your water supply is turned off at the mains you will still have access to a store of water.


  • The main drawback of a vented cylinder is that it relies on gravity. The height of the cold water tank dictates the pressure, i.e. the more height you can create between the tank and your taps/shower the more gravity will drive the flow of water.
  • This can mean that taps upstairs are weaker than downstairs, or that you need to install an additional pump to get things moving.

Unvented Cylinders

In an unvented cylinder there is no need for a cold water tank in the loft. The cylinder is connected directly to the mains so you are essentially cutting out the middleman and the pressure of your water should be stronger.

The water is then heated within the cylinder but, unlike vented cylinders, there isn’t a vent pipe or tank to cope with excess water as it heats and expands. To remedy this unvented cylinders have inbuilt safety features like an expansion unit at the top or an air bubble that is inserted at installation. Get quotes for unvented cylinders now.


  • They don’t need a water tank which means the system takes less space in your home and you can be more flexible about where you install it.
  • Unvented cylinders tend to provide stronger water pressure as they are directly supplied by the mains, but of course this depends on the strength of the mains in the first place.
  • No need for a tank of water that could freeze in winter or be contaminated, it is a completely sealed system.


  • They have more complicated technology so are more expensive to install and maintain.
  • An unvented cylinder relies on a supply of water from the mains, so if this is turned off for any reason you won’t have any hot water.
  • Unvented cylinders aren’t always compatible with modern power showers and mixers.