Open vented system

A central heating system with a separate feed and expansion tank that accommodates the expansion of the central heating water volume as its temperature rises. This tank is located at the highest point of the system, usually in the loft space.

Sealed system

A central heating system that incorporates an expansion vessel which replaces the separate feed and expansion tank. Systems of this type remove the need to install pipework in the loft space.

Combination or 'Combi' boiler

A boiler that provides both heating and domestic hot water from one unit. It produces domestic hot water instantaneously on demand, eliminating the need for roof tanks or a hot water cylinder to store domestic hot water.

Condensing boiler

A more efficient, modern boiler incorporating an extra heat exchanger which extracts most of the heat from the exhaust gases. This heat is used in the heating process rather than being vented outside and wasted.

Control valve

A valve that is installed on the 'flow' side of the radiator, to control the flow of water coming from the boiler.

a) Manual – a valve which is turned on and off by hand

b) Thermostatic – a thermostatic radiator valve (or TRV) is used to control the heat output from a radiator by controlling the water flow into it with a self-activating air temperature sensor. TRVs provide individual room control and shut off the radiator when there are other sources of heat such as the sun, people and electrical appliances.

Lockshield valve

A valve that is installed on the 'return' side of the radiator to control the flow of water returning to the boiler and to 'balance' the heating system.

If all the valves in a system are fully open, the temperature of the radiators will vary throughout the system as the hot water will tend to follow the path of least resistance and flow through the radiators closest to the boiler. The radiators farthest from the boiler will not reach the same temperature. Your plumber should 'balance' your system after installing new radiators.

All Bisque valves are sold as sets which include a manual or thermostatic control valve and a lockshield valve.

Air vent (or bleed valve)

This is a small valve which enables air that has accumulated at the top of a radiator to be let out or 'bled' from the radiator using a special air vent key.

Flushing and chemical cleaning

On installation of a new system and after installing a new radiator, the entire system should be flushed and chemically cleansed. A chemical inhibitor such as that supplied by Fernox should then be added to the water in the system by your plumber in order to minimise possible corrosion in the radiators and pipework.