Understanding Vented & Unvented Hot Water Cylinders

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Last updated: October 31, 2023

The domestic hot water cylinder is often seen as a permanent fixture in the home. However that is not always entirely true. It may be that your current hot water system is in need of an overhaul or perhaps an upgrade. Perhaps you are embarking on a new build or house renovation project. In any of these scenarios, you will need to make choices about the design of the hot water system and one decision will be whether to choose between a vented and unvented heating system. In this article we will explain the difference between the two types of hot water systems..

What is an Unvented Hot Water Cylinder

As Mr Scot, the chief engineer from Star Trek taught us, “you can’t defy the laws of physics”. The fact is that when various materials are exposed to heat, they have a tendency to expand. You may wish to read up on Charles’ law which explains the principle.

This is especially the case when we consider a hot water cylinder, where the water itself as well as the air on top of it, needs to have a potential escape route to prevent too much pressure building up and causing damage.

There are a number of ways of achieving this safe reduction of pressure. As its name suggests, an unvented hot water cylinder is a sealed heating system. There is no water tank in the loft. Instead, water to be heated is fed into the water tank from the mains supply and heated by the electric element or a heat exchanger coil from the boiler.

Since the system is effectively sealed, there has to be a mechanism which allows the pressure to be safely relieved as the water in the cylinder is being heated. This is achieved by using a device called an expansion vessel. An expansion vessel is a sealed chamber fitted with an internal flexible membrane which stretches to allow for the increase in pressure by up to 4.5%, thereby reducing the risk of damage to the heating system.

Pros and Cons of Unvented Hot Water Cylinders

Here are some facts which you should be aware of before you choose your domestic hot water system.


  • No water storage tank in the loft is required as the water supply is directly from the mains.
  • This gives a much wider choice when it comes to the location of the hot water cylinder.
  • With no water storage tank in an unheated roof space, this also means that there is very little risk of the water freezing or becoming contaminated.
  • With no tank of water feeding the cylinder, the unvented cylinder is much quieter in operation than in a vented system.


  • Unvented hot water systems are relatively complicated so maintenance and installation can be costly.
  • Expansion vessels have a limited lifespan, usually around 10 years, before they need to be replaced.
  • There is no cold water tank in the loft which means that there is no reservoir of water if the mains supply is turned off.
  • Unvented heating systems may not supply enough water pressure for modern power showers, or sufficient flow for some types of mixer tap..

What is a Vented Hot Water Cylinder?

Unlike in the unvented system, a vented hot water cylinder is not entirely sealed. They are also known as ‘gravity fed’ cylinders as the water supply comes from the cold water storage tank in the loft rather than from the mains. Instead of the excess pressure being relieved by an expansion vessel, the water is allowed to flow up an open vent pipe, back into the storage tank. This can lead to a certain amount of heat loss, although good insulation around the tank and pipes helps to reduce this.

Pros and Cons of Vented Hot Water Cylinders

As with the unvented cylinder, vented cylinders also have advantages and disadvantages.


  • The vented system is considerably simpler than the unvented system, so there is less to go wrong, particularly as there is no expansion vessel in place.
  • The vented system is therefore cheaper to install and to maintain.
  • There is a ready source of cold water to the system even if the mains supply is disrupted, in the form of the cold water tank in the loft.


  • The water pressure is dependant on the height of the water tank above the taps. So it follows that in any building with more than one storey, water flow will be more powerful from taps in the lower floor than those at a higher level.
  • Because of this variation in water pressure, it may be necessary to fit a water pump in order to boost water flow, which adds to fitting and running costs and potential maintenance issues.
  • The requirement to have a water tank in the loft means that more space is required for a vented hot water cylinder and as a result, the options for locating it are also reduced.

Should I buy a Vented or Unvented Hot Water Cylinder?

As we have already seen, there are both advantages and disadvantages with both types of hot water cylinder. It should be emphasised that if your home is already fitted with one type of cylinder and it is performing its job of supplying hot water adequately, there is probably no need to swap it for a different model. Hot water cylinders generally have a life expectancy of about 30 years, so it is sensible that older models are regularly checked for water leaks.

If however, you do need to replace your existing hot water tank, or are installing one in a new build project, an unvented hot water cylinder is the preferred choice. Whilst it is a more complicated system than a vented cylinder, it has distinct advantages.

There is no need for a tank of water in the loft, which allows much more freedom of choice when it comes to the location of the cylinder. While the expansion vessel is likely to need replacement before the main cylinder, this is a routine job that any qualified plumber can undertake with no difficulty. Of the two heating systems we have looked at here, the unvented hot water cylinder is quieter and more efficient in terms of water usage than the vented system, so this is the heating system we would recommend.