Unvented Cylinder

What is an unvented cylinder?

An unvented cylinder stores hot water that is supplied to your taps, shower and any other hot water outlets. Unvented cylinders are sealed to the atmosphere whereas conventional cylinders have a vent pipe that vents over to a tank normally located in the loft area.

The water within the tank is stored at high pressure which is normally feed by the incoming water mains, it could also be supplied by a break tank that increases the water pressure via a booster pump.

Most unvented cylinder makers recommend an incoming water pressure of around 2-3 bar, typically this should give around 20 litres of water per minute, this will also depend on pipe size.

Normally the pipe supplying the tank should be 22 m, but I have seen them fitted in 15 m pipe and they seem to work OK, It’s alway best though to follow to the makers recommendations.

The water within the unvented cylinder is typically heated up by the boiler that also heats the radiators within the property, they can also be heated by electric immersion heaters should a boiler not be present. Heating the water by a boiler is the cheapest option.

Unvented cylinders have to be installed by a person or company who are registered to install unvented cylinders.

A normal hot water cylinder can be installed by a normal plumber.

Today most Gas Safe Registered Engineers are also registered to installed unvented cylinders, this is because many boilers today are classed as unvented, so the boiler engineer needs to be registered.

There are many types on unvented cylinders to choose from these days so what one is best for you?

My advise would always be don’t go for the cheapest one, you only get what you pay for.

One of the most expensive is Megaflo, they have been around a long time and the build quality is second to none. With Megaflo they have a built in expansion gap whereas most other makes come with a separate expansion vessel that has to be connected into the pipe work.

When you buy an unvented cylinder you will be supplied with the tank and expansion vessel if not Megaflo, and all fittings required. This normally consists of a pressure reducing valve sometimes called a combination valve, a tundish and a motorised valve, the motorised valve is supplied if the water is to be heated via the boiler, if heated via immersion heaters then the valve is not required.

When fitting the supplied combination valve be careful to fit the right way round, they normally have an arrow indicating the flow of water, if fitted the wrong way you will not have any water at the outlets. The combination valve also has a connection for the cold supply going to the taps so you will have equal pressure all round.


The tundish is normally a black bottle shape fitting that connects to the temperature and pressure relief valve discharge pipe, you will notice that it has parts cut out so you can see if any drips of water are present. If you see any water dripping within the tundish this is an indication of a problem with the tank. The outlet pipe of the tundish should be run to ground level and into a drain if possible.

Expansion Vessel

The expansion vessel, if supplied needs to be connected into the pipe work in the correct position, it should be connected after the combination valve and not before, if fitted before the valve it will have no effect and you could have a big problem on your hands. The expansion vessel is a very important part of the unvented cylinder and needs to be fitted correctly. Always follow the installation instructions.


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