Glossary of Technical Terms

COMBI
This is the name given to a boiler that heats water instantaneously on opening a hot tap. There are no tanks in the loft and no storage cylinder. The water is fed from the mains so the water pressure is generally stronger.

CONTROLS
All boilers have controls that allow you to adjust the water temperature (thermostat). You may also find a water pressure gauge and a digital or mechanical timer.

DHW
Domestic Hot Water.

EXPANSION VESSEL
As the water in the heating system gets hot it expands; in sealed systems the 'expansion vessel' (a small pressurised container) accommodates the extra volume.

FLOW RATES
Generally the term 'flow rates' is associated with combis. It refers to how much water (usually measured in litres) can be heated as it flows through the pipes in a given time (usually per minute). A powerful boiler will heat more water therefore providing a better flow rate.

FLUE
All boilers have some sort of flue system; there are typically two types as follows:

CONCENTRIC
This system comprises of two pipes one inside the other, the internal pipe takes the exhaust gases from the boiler to the atmosphere and the outer pipe supplies air to the boiler. The majority of concentric flues are a maximum of 4 metres in length and have a wide range of flue bends and extensions. This provides greater flexibility in positioning your boiler.

TWIN PIPE
This system comprises of two pipes, the first pipe takes the exhaust gases from the boiler to the atmosphere and the second pipe supplies air to the boiler. This flue system is useful when the pipes need to be over 4 metres long. It also has a wide range of flue bends and extensions providing greater flexibility in positioning your boiler, however this may be less aesthetically pleasing due to the amount of pipe work on show.

F&E TANK
Usually found in the loft; traditional systems use two. One feeds fresh water to the hot water storage cylinder and the other one (usually smaller) maintains the correct level of water in the heating system. They also allow for expansion of the water in either when it gets hot ­ hence the name 'feed and expansion' tank.

HIGH EFFICENCY (condensing)
All High Efficiency boilers are environmentally friendly and operate at up to 96% efficiency compared to standard boilers at around 80% efficiency. They recover heat that is normally lost into the atmosphere via the flue gases. Installation: Depending on the model, your boiler may be wall mounted, floor standing or hidden away in a fireplace. Many wall-mounted boilers are small enough to be concealed in a kitchen wall cupboard.

OPEN VENTED
If your heating system has a feed and expansion tank (see above) then it's called 'open vented' hot water. A hot water storage cylinder may also be open vented.

OUTPUT
The output ratings of boilers are measured in BTU's (British Thermal Units) or kW (Kilowatts). A low output would be 30/60,000 BTU's (10-17kW). A high output would be 60/100,000 BTU's (17-30kW). Your installer should choose the output best suited to the size of your heating system, the type of property and, in the case of a combination boiler, the amount of hot water you require.

SEALED SYSTEM
If you have a sealed (heating) system there's no feed and expansion tank (see F&E tank) in the loft space. Instead it's filled with water via a temporary connection and expansion is accommodated by a special vessel, which may be inside the boiler. (See also expansion vessel.)

SEDBUK
Products are listed on the SEDBUK Boiler Efficiency Database. This Government database lists the Seasonal Efficiency of all Domestic Boilers in the UK. For full details visit www.sedbuk.com.

STORAGE CYLINDER
The storage cylinder supplies hot water for domestic use and is heated by the boiler. When the hot water in the storage cylinder has been used up it is refilled by the feed and expansion tank. (See F&E tank.)

SYSTEM
A 'system' is the term used to describe the boiler and all of the components required to provide your home with heating and hot water.

UNVENTED
The term 'unvented' most commonly refers to cylinders storing and supplying hot water at mains pressure rather than via an F&E tank and therefore at flow rates which are usually far superior.

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