Unvented Cylinders

1st May, 2021 | Boiler, Gas Safety, Heating, Plumbing, Space Saving

Connected directly to your mains water system, unvented cylinders ensure ample amounts of stored hot water is available on demand and they operate at mains pressure to offer much higher flow rates. This makes them a great option if you want to improve the performance of your bath or shower.

They do not require a cold water storage tank, which is helpful if you do not have a lot of free space, as well as lowering the risk of any breakdowns or issues.

In an unvented system there is no cold water tank – instead a sealed hot water cylinder is fed directly by the cold water mains. An unvented cylinder is kept under pressure from incoming water supply and is heated by either immersion heaters or by an external heat source, such as a boiler or wet solar system. If you have solar panels or other renewable energy sources, then these will be compatible and a great way to lower your energy bills.


Unvented cylinders can free up valuable space in your home and, depending on the strength of the mains system, tend to provide stronger water pressure than an open vented system.

They are also perfect space-saving options for modern homes or those with limited storage, as they don’t require a feeder tank in the loft. Unlike vented cylinders, the unvented cylinder doesn’t rely on gravity to function, so it can be installed almost anywhere in your home, which makes it easier to store in a place more suitable for you.

Another advantage of installing an unvented system is the reduced noise in the system, due to the fact that there is no cold water filling up the water storage cistern. The unvented system is completely sealed too, so the water is not at risk of contamination and you don’t have to worry about the water tank freezing throughout winter.

How it Works

An unvented cylinder will have outlet pipe work connected to it that distributes the hot water to multiple outlets around the house. As soon as an outlet (i.e. tap, shower or washing machine) 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’, so they require 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.

Unvented cylinders are designed to withstand considerable internal forces and are extremely safe to use when fitted with all the correct pressure equipment. They are usually made from stainless steel due to the high tensile strength that can be achieved from a fairly thin sheet.

An unvented cylinder can last in excess of 30 years when fitted to the highest standard; they must be fitted by a qualified team. If your cylinder is beginning to leak water, then 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.