Vented vs Unvented Hot Water Cylinders

To vent or not to vent? That is the question!

Let's assess which cylinder is better in the Vented Vs Unvented cylinder battle.

With the advancement of materials and components, the choice of quality cylinders is greater than ever. Choosing between the two types of heating systems and cylinders depends on your budget and your type of home.

Click here to find a wide range of discounted cylinders including:

Vented cylinders

Vented systems are known as gravity fed hot water systems.

Vented cylinders (also called open-vented cylinders) need to be supplied by a large tank of cold water which is kept in the loft. The natural pull of gravity carries this water via a vent pipe down to the hot water cylinder which heats the water.

Vented cylinders rely on gravity and the height of the cold water tank dictates the pressure. The greater the height, the more powerful the water pressure. This means that the cold water tank should be placed in the loft.

As the water is heated it expands, but the presence of the vent pipe and tank in the loft provides an escape route for the excess.

Traditionally, vented cylinders were made out of copper as they were deemed the best way to store hot water and get an adequate heat transfer. However, the heat loss from a naked copper cylinder sparked a nationwide campaign to lag and insulate hot water tanks throughout the UK.

Manufacturers now pre-spray insulate the tank to minimize heat loss.


  • A vented system is usually cheaper to install and maintain because they are more basic.
  • A vented installation is the preferred option only if the property owner doesn't want to pay for an upgrade to an unvented system.
  • If the mains water supply is cut off you will still have access to the tank of water in the loft.


  • Your home's water pressure depends on the height of the cold water tank. This means that taps and showers upstairs have much weaker water flow than the ones downstairs.
  • Vented installation rarely delivers hot water at the pressure of an unvented system. You may need an additional pump to boost water flow.
  • Requires storage space in the loft for the cold water tank.

Unvented Cylinders

Unvented hot water systems differ in that they don't require a water storage tank. Instead, a sealed hot water cylinder is fed directly by the cold water mains.

An unvented cylinder is kept under pressure from an incoming water supply and the water is heated directly by immersion heaters (a direct model) or by a heat source located outside the vessel such as a boiler or wet solar system (an indirect model).

Outlet pipework connected to the cylinder distributes the hot water to multiple outlets around the house. As soon as an outlet (tap, shower) is used, the pressure of the incoming cold water displaces the hot water content in the cylinder towards the open outlet at mains pressure.

Unvented systems are sealed, requiring an expansion vessel either next to or incorporated within the boiler or cylinder. An expansion vessel allows for the expansion of water in the system as it heats up and accommodates the increase in pressure by up to 4.5%.

Unvented cylinders are designed to withstand considerable internal forces and are extremely safe to use when fitted with all the correct pressure equipment.

unvented cylinder diagram


  • Don't require a water tank saving valuable space in your property.
  • Doesn’t rely on gravity to function so can be installed almost anywhere in your home.
  • Quieter operating system. No cold water filling the water storage cistern.
  • Completely sealed system, thus no need for a tank of water that could freeze in winter or be at risk of contamination.
  • Operate at mains pressure to offer much better water flow rates.


  • More complicated technology, so are more expensive to install and maintain.
  • Linked directly to the mains, so will not provide hot water if mains are turned off.
  • Unvented cylinders aren't always compatible with modern power showers and mixers.


Unvented stainless steel hot water cylinders are considered a much better choice for new installations and for the replacement of existing ones. They ensure a copious amount of stored hot water is available on demand for a multitude of domestic uses. Operating at mains pressure to provide often much higher flow rates, you can enjoy improved shower and bath performance all year round.

Do I need a new cylinder?

A hot water cylinder generally lasts 30-plus years when fitted to the highest standard. If your cylinder is beginning to leak water, it is time to replace it. At any point, the leaking water can get worse and cause substantial damage to your property. Always check for any damp patches surrounding the cylinder, and consult your plumber to check for any signs of water damage.