How to turn on your central heating and boiler

The three switches that control your heating

Most heating systems in London homes have the same three layers of control, even when they look different. Knowing which is which saves a lot of guessing when the heating goes on for the first time after summer.

The boiler itself usually has a heating control, either a dial with a radiator symbol or a button that cycles between heating and hot water, hot water only, and off. Over summer many people turn the heating side down or off and leave hot water running, so this is the first thing to check.

The timer or programmer decides when heating is allowed. It may be built into the front of the boiler, mounted on the wall near it, or combined with a wireless thermostat. On a system with a hot water cylinder it has two channels, one marked CH or heating and one marked HW or hot water.

The room thermostat decides how warm the house gets. When the room is colder than the setting, it tells the boiler to fire. When the room reaches the setting, it stops calling for heat, even if the timer is still on.

Turning on a combi boiler, step by step

A combi boiler heats water on demand for taps and heats the radiators, with no cylinder in an airing cupboard. Its controls are normally all on the front panel, sometimes behind a flap.

• Check the boiler has power. There should be a display, a light or a clock. If it is blank, check the fused spur switch next to the boiler and the consumer unit.
• Look at the pressure gauge. On most sealed systems it should read about 1 to 1.5 bar when the heating is cold. If it is below 1, top it up before going further, or the boiler may refuse to fire.
• Set the boiler to heating. Turn the radiator dial up from off, or press the mode button until the radiator symbol shows alongside the tap symbol.
• Set the timer to on. On a mechanical timer with segments, push the segments in (or out, depending on make) for the hours you want, or move the switch from timed to constant to test. On a digital programmer choose on, constant or override.
• Turn the room thermostat up a few degrees above the current room temperature. You should hear a click, and many boilers show a flame symbol once they fire.
• Open the radiator valves and wait 10 to 20 minutes, then feel each radiator from top to bottom.

If the display shows a fault code or the boiler locks out, write the code down before pressing reset. Our guide to resetting a boiler explains when a reset is sensible and when it is not.

Systems with a hot water cylinder

System and regular (heat-only) boilers work with a hot water cylinder, usually in an airing cupboard. The boiler often has only an on/off switch and a temperature dial, and the heating is controlled from a separate programmer on the wall.

The mistake we see most often is the heating channel still being set to off from the summer while the hot water channel runs normally, so the boiler fires for the cylinder but never for the radiators. Set the CH channel to on or timed, check that the HW channel is set how you want it, then turn the room thermostat up.

These systems use motorised zone valves to send hot water to the radiators, the cylinder or both. If the boiler fires and the cylinder gets hot but the radiators stay cold with the programmer and thermostat both calling for heat, a stuck zone valve is a common cause. That is a job for an engineer, not something to force by hand.

Older boilers with a pilot light

Some older boilers, and a few back boilers behind gas fires, have a permanent pilot light. If you turned the pilot off for the summer, or it has gone out, the boiler will not fire whatever the controls are set to.

Relight it only by following the instructions printed on the boiler or in its manual, which usually involve holding a control knob in, pressing an igniter and keeping the knob held for a set time. The pilot flame should be crisp and blue. If you smell gas at any point, stop, turn the gas off at the meter, open windows and call the National Gas Emergency Service on 0800 111 999. If the flame is yellow or lazy, or the pilot will not stay lit, do not keep trying: book a Gas Safe registered engineer.

Once it is on: the first-run checks

The first time the heating runs in autumn is the best chance to find faults before the cold weather, when engineers are busiest. Let it run for half an hour and then go round the house.

• Radiators cold at the top but warm at the bottom have air in them and need bleeding. Check the pressure again afterwards, because bleeding lets water out.
• Radiators warm at the top and cold at the bottom usually have sludge settled in them.
• One radiator completely cold, with others hot, often has a closed valve or a stuck pin in the TRV.
• Banging, kettling or gurgling from the boiler or pipes is worth reporting early rather than living with.
• Look under the boiler and around radiator valves for drips that were not there before.
• Test the carbon monoxide alarm in any room with a gas appliance.

When the heating still will not come on

If the boiler has power and pressure, the programmer and thermostat are both calling for heat, and the radiators are still cold after 20 minutes, the fault is past the controls. Common causes are a failed pump, a stuck diverter or zone valve, a thermostat that has lost its connection to the boiler, or an ignition fault, which usually shows as a code on the display.

Hot water working but no heating is a separate, common pattern with its own checks, and a boiler that shows nothing at all is another. Both are covered in our guides linked below.

If you would rather have it looked at, Loyal Plumbers covers London and everywhere inside the M25, 24 hours a day, with 10 vans on the road and Gas Safe registered engineers for gas work. Our call-out charge is £60. Call 020 4652 8585 or send an enquiry with the boiler make, what the display shows and what you have already tried.