Unvented Hot Water Systems

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There are three types of domestic hot water system. This first is a combi boiler system where the boiler directly heats the water on demand. The second is a gravity fed (open vent) hot water tank which is fed from a tank in the loft. The third is a pressurised hot water system.


We are going to look at the pressurised hot water system or unvented hot water system. Please watch the video below for more information on Hot Water Systems:

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What is an Unvented hot water system?

An unvented hot water system takes it’s cold water supply directly from the incoming water main. It is not gravity fed by a tank in the loft. They can be direct or indirect.

How an Unvented Cylinder Works:

As we have said an unvented cylinder is supplied by the incoming main. When you open your tap, that high pressure cold water enters into the unvented hot water tank at the bottom, forcing hot water out of the top outlet and to your taps.

If you’re interested in buying an unvented cylinder, click here.


1. Pressure is much higher on all outlets.

2. No loft tanks which reduce risk of contamination and legionnaires

3. No pipes in the loft so no frozen pipe risk.

4. Very high heat recovery rate after use. Typical warm up time 15-20 minutes.


1. The hot water pressure will only be as good as your incoming main, however this is fine for most people.

2. You cannot fit shower pumps on mains water supplies. However you shouldn’t really need to.

3. When feeding multiple outlets the pressure can drop. This also applies to vented systems. On unvented systems it can be due to a small incoming main pipe (15mm for example). To overcome this on an unvented system, you need to install an ‘accumulator’.

Cylinder Sizes:

It is important when buying any hot water cylinder, unvented or not, to size it correctly for the demand it will supply. Most families of 2-4 people will only require a 150 litre tank. This will accomodate a house with one bathroom with a power shower and ensuite. Any household that is larger and has more bathrooms will be looking at 200 litre+ tank.

Why buy your own unvented hot water cylinder?

If you buy your own pressurised hot water cylinder you will save money by avoiding your engineer’s mark-up. Also you will know what you are getting. If you aren’t sure which cylinder to get and need some advice, ask your engineer. They should provide you with all the information you need to make the right choice and purchase accordingly.

If you are not fully qualified then you cannot legally install an unvented system. Email us if you have any questions.

Remember to buy the male iron connections. Most cylinders are 3/4″ to whatever pipe you are fitting (generally 22mm copper compression)

Where to buy unvented hot water cylinders:

There are thousands of retailers online selling water cylinders. We advise you use an established plumbing merchant with a good track record and proven history.

Other things to consider:

Because of the high pressures involved high pressure relief valves need to be installed. High pressure relief pipes need to be installed according to manufacturer’s instructions. These are usually supplied by the manufacturer with the cylinder. They also come supplied with a small expansion vessel to absorb any expansion when the cold water is heated up to temperature.