Jargon Buster - C

Confused by DIY terms? We’ve got you covered from A-Z with our Jargon Buster!

Cap Nut

A slotted screw or hexagonal nut with a plastic cap in the centre of a water circulating pump. Used to position the pump rota in place and can be used to vent the pump of air or free the rotor when stuck.

Central heating pressure

A constant flow of water pressure in your central heating system is an essential factor to a fully functional boiler.

Check valve

Also known as a non-return valve, a device that normally allows a fluid to flow in only one direction.

Circuit board electronics

Electrical circuitry, often of low voltage and non-maintainable, found in most modern appliances.

Circuit pressure

The pressure at which a central heating system operates. Combination boilers and refrigeration systems have a critical circuit pressure at which they operate most efficiently and safely.

Circulation pipe

The main water flow and return pipes to and from the central heating boiler for the radiator circuit.

Circulation pump

A device used to pump water within the central heating system. A non-ferrous type should be used for the hot water (where a standard cast steel pump would quickly rust away).


Fixed, vented container for holding water at atmospheric pressure.


Suitable for all central heating systems to protect against mineral oil contaminant.


The heating element in a domestic hot water tank. Known as the 'primary circuit', it circulates water from the boiler to indirectly heat hot water.

Cold mains supply

The potable (drinkable) water piped to the premises by the utilities or water company.

Cold water storage tank

A container for holding water, usually used for providing a feed to domestic cold water and/or to a vented domestic hot water system.

Combination boiler

Boiler designed to provide domestic hot water and hot water for central heating without the need for additional secondary storage.

Combustion chamber

The area within the boiler where the main combustion of gas takes place.

Combustion gases

Produced when fuel is burnt. They consist mainly of CO2 and water vapour.

Compression fitting

Formed by using a tube nut being tightened on a thread to compress a compression ring (or olive) onto the outer face of the tube.

Condensate pipe

A pipe, usually plastic used to allow the safe removal of condense from an appliance.

Condensing boiler

Appliance designed to make use of the latent heat from water vapour in the combustion products by condensing the water vapour within the appliance

Consumer unit

A component of an electrical power system containing fuses or breakers that splits the incoming electrical supply into current-protected circuits.

Convector heater

Appliance that is designed to heat a room mainly by the emission of air heated by convection

Conventional boiler

Heat your central heating system directly and produce hot water for your cylinder.

Conventional open flue

Chimney that evacuates the products of combustion to the outside air, the combustion air being drawn from the room or space containing the appliance.

Copper-lined waterway

An additional protective, non-rusting skin added to the primary metals of waterways to help them resist corrosion.


Central heating system corrosion results from hard-water deposits or from oxygenated water reacting with the system's metal components. Boiler corrosion is the deterioration of boiler components due to reactions with its surroundings.

Corrosion inhibitor

A corrosion inhibitor is a chemical compound that when added in small concentration stops or slows down corrosion in your boiler.


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