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This article is provided for information purposes only. You should only attempt any procedures explained here if you are competent to perform the task and have the appropriate tools and materials, including any relevant safety equipment.

Readers acting upon this article do so at their own risk and no responsibility is accepted by the author or publisher for any loss or damage incurred by following the article or failing to follow it.

If in doubt always consult a properly qualified tradesperson or other competent person.


An unvented hot water storage cylinder is a way of keeping hot water at mains water pressure to improve the water flow around a home.

In the UK, hot water has traditionally be kept in open vent storage cylinders. 'Open vent' simply means that there is an permanently open overflow pipe which allows water to discharge into the cold water storage tank if it becomes too hot.

Water kept at mains pressure can't be kept in open vent cylinders as the water would simply keep flowing out of the open vent at mains pressure.

Instead, hot water in an unvented mains pressure system is kept in a storage cylinder without a permanently open vent, hence the term 'unvented'.


An unvented hot water cylinder works by storing hot water at mains pressure in a reinforced cylinder so that when a hot water tap is opened the hot water is delivered at mains pressure.

This means that an unvented hot water storage cylinder can usually supply more than one outlet at once and can supply outlets on floors above the one where the cylinder is positioned by using mains water pressure to push the hot water up to the higher floors.

To cope with the mains water pressure unvented cylinders have to be built much stronger than standard open vent cylinders.

Also, because hot water is locked in at mains pressure there are safety devices fitted to prevent the cylinder exploding should the pressure get too high, either because the mains pressure itself is too high or because the water gets too hot, expands and turns to steam.


An unvented hot water storage cylinder has a series of safety devices to protect the user and the household, principally:
  • A line strainer to filter the water entering the cylinder so debris and grit in the water supply doesn't damage any of the safety controls
  • A pressure reducing valve to bring the mains pressure down to a maximum of 3bar.
  • A check valve or non-return valve to stop water stored in the cylinder from flowing back down the supply pipe and into the mains drinking water supply.
  • An expansion vessel, basically a container to hold water when it expands under heat.
  • A thermostat to control the temperature of the water in the cylinder.
  • A temperature/pressure relief valve, typically a combination valve that will allow water to escape to a special discharge pipe if the water gets too hot or the pressure in the unvented hot water cylinder gets too high.


Hot water in traditional open vented cylinders is stored at low pressure with gravity providing the force to distribute the water around the home.

This works well in a home where the hot water cylinder and the cold water storage tank that feed it are high up in the home (typically the cold tank in the loft and the hot water cylinder on the first floor) and where the distribution pipework is of a wider bore (3/4" or 22mm or more) to ease the water flow.

However, gravity based systems may struggle to provide enough hot water simultaneously to more than one outlet, such as a shower outlet when a tap is being opened elsewhere on the home, or where there are outlets above the height of the storage cylinder, such as on a second floor or in a loft conversion.

An unvented cylinder can overcome this problem by storing the hot water at mains pressure (typically limited at around 3bar) so that the pressure is much greater and better able to cope with more than one outlet open at once.

Multiple unvented hot water cylinders can also be linked together on the hot water system to increase the storage capacity provided that the mains water pressure and flow rate is sufficently high.


Because unvented hot water storage cylinders are built to very high strengths and have a number of sophisticated safety controls they are both more expensive to buy and more expensive to fit than traditional open vented cylinders.

They should only be installed by qualified persons holding a current unvented cylinder installation certificate. Each installation must be registered with the Building Control Department of the local government authority for the property.

Building Control will usually want to inspect the installation unless it has been self-certified by a qualified installer who is a member of a 'competent persons' scheme such as 'WIAPS'.  Building Control Departments usually charge a fee for the inspection

Because of the number of safety devices on an unvented hot water storage cylinder it is important that they are serviced every year by a competent person.

When purchasing a property where an unvented hot water storage cylinder is installed you should insist on seeing the service history of the cylinder as an unserviced cylinder can develope faults that are expensive to repair.


line strainer before cleaning

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