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Back to basics – how does an unvented cylinder work?

Unvented hot water cylinders were made legal in the UK in 1986, and have grown in popularity ever since. George Linder, product manager, at Heatrae Sadia explores the advantages they can offer, how they work, and what this means for the heating engineers who fit them:

George Linder, product manager, at Heatrae Sadia.

What are the advantages of unvented hot water cylinders?

“The pressure of the hot water coming out of taps and showers is greater in an unvented system. This is because water is drawn from the cold-water mains supply, as opposed to from a water tank (which is often found in the loft). Higher pressure becomes especially important when there are multiple bathrooms which may be running hot water from several taps at once. An effective unvented cylinder could make all the difference when it comes to enjoying nice, hot, and pressurised showers.

“The benefit of using an unvented cylinder is that a cold-water tank in the property is not required, meaning homes with limited space do not need to lose precious storage. Removing the need for a tank also prevents potential freezing issues during long cold snaps.

“There is a reduced risk of contamination because it is a closed system, with less chance for foreign materials to enter the system and ultimately cause potential damage.

“Homeowners can benefit from an unvented system as noise is reduced. This is because there is no cold-water filling of the tank, meaning the sounds that vented appliances make can be avoided.”

How does it work?

“The unvented system draws water from the mains. It then heats the water in one of two ways, depending on whether it is direct or indirect.

“An indirectly heated system (which is the most common) gets heat from an external source, often in the form of a gas boiler or solar panels. The hot water then travels through a copper coil in the hot water cylinder, heating the surrounding water without actually mixing with it.

“A direct cylinder system uses an internal element, often in the form of an immersion heater. This is required for properties which have no access to gas, such as off-grid homes.”

How is pressure eased?

“Since water expands when it gets warm, it’s important that there are mechanisms to ease pressure in the cylinder. There are two main ways of doing this, the first of which is the bubble top unit.

“This type of cylinder uses an internal air bubble that is produced and trapped at the top of the cylinder when it is installed. The other type is an external expansion unit, which utilises an expansion vessel to contain the expanded hot water.

“Since unvented hot water tanks operate at a higher pressure than vented systems, and have many additional safety features installed, they need to be fitted and maintained by specially qualified technicians.

“This means that in order to benefit from the superior performance offered by unvented water cylinders, homeowners must ensure they are installed by G3 certified heating engineers.”

7 October 2019


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