Heating controls parts, terminal by terminal

A fused spur is a switch and a fuse in one small box on the wall. Its proper name is a fused connection unit (FCU).

The same fused connection unit (FCU) as the Fused Spur, on a silver plate marked 3 kW.

A programmer is the clock of a heating system: it decides when heat is allowed.

A single channel programmer is a clock with one channel, also called a time switch. Inside is one changeover switch, fed from the live (L).

A room thermostat (stat) is a switch worked by the temperature of the room. It tells the heating when the room is cold.

A digital room thermostat (stat) runs on batteries, so it needs no live (L) or neutral (N) of its own.

Room thermostat C is a mains-powered room thermostat (stat) with a volt-free changeover switch.

A cylinder thermostat (stat) is strapped to the side of the hot water cylinder.

The same cylinder thermostat (stat) switch as A. Beside its terminals it prints NO, normally open, and NC, normally closed, as some makers do.

A frost thermostat (stat) is a switch worked by the air where it is fitted. It is never fitted outdoors.

A pipe thermostat (pipe stat) is a thermostat strapped to a pipe. It feels the water in the pipe through the copper.

An outside sensor measures the outside air. A boiler with an outside sensor input reads it, and sets the heating's flow temperature by the weather.

An OpenTherm thermostat is a room unit that talks to the boiler on two extra-low-voltage wires.

A wireless thermostat (stat) runs on batteries and has no terminals. It tells its receiver by radio whether it is calling.

A wireless receiver is the switch its thermostat (stat) works by radio.

A two-channel wireless thermostat (stat) is a battery programmer and thermostat in one.

A two-channel wireless receiver has the same terminals as a dual channel programmer, but its thermostat (stat) works it by radio.

The wireless zone stats are the two wireless room units of an OpenTherm two-zone set, side by side: zone 1 and zone 2.

The OpenTherm receiver is the wall box of an OpenTherm two-zone set.

A 2-port zone valve opens and shuts one pipe, such as the pipe to the radiators. The zone is the part of the system that pipe feeds.

A 3-port mid-position valve is one valve with three pipe ports. The water comes in at AB, from the boiler.

A diverter valve is a three-port valve with only two positions. Water comes in at one inlet, AB, from the boiler.

A heat-only boiler heats the water for the radiators and the hot water cylinder. Its pump and its controls are separate parts, outside it.

A combi boiler (combination boiler) gives heating and hot water from one appliance. Its pump and its valve are inside it.

A circulating pump is the motor that moves the water round the system: through the boiler, the radiators and the cylinder.

Underfloor heating (UFH) warms a room through pipes laid in its floor.

A condensing boiler makes water from its flue gases: condensate.

An immersion heater is an electric element that screws into the hot water cylinder.

A wiring centre is the box where the wires of a heating system meet and are joined.

A DrawWarm wiring centre with 25 lever-connector blocks round three sides.

A DrawWarm wiring centre with 16 lever-connector blocks round three sides.

Twin and earth is a flat, grey cable with solid cores: each core is one solid copper wire.

Flex is a round, flexible cable: stranded cores in one sheath. Each core is made of many thin wires, so it bends easily.

Heat-resistant flex is a flexible cable whose insulation stands more heat than ordinary flex.

An indicator lamp is the simplest load: a part that uses electricity.

A changeover switch has three terminals: the common (COM), normally open (NO) and normally closed (NC).

A proving unit is a small box that gives a known test voltage.

A two-pole tester, a voltage indicator, is the instrument safe isolation is done with. It has two probes and a ladder of lights (LEDs).

A digital multimeter measures volts, resistance and continuity, one at a time, on the setting you choose.