Boiler Not Firing Up: Causes and Fixes

Listen to what it does when it tries to start

Every modern boiler follows the same start sequence. The board gets a call for heat, the fan spins up to clear the combustion chamber, a pressure switch or sensor confirms the flue is clear, the spark generator clicks, the gas valve opens and the burner lights with a soft whoosh. A flame sensor then has a few seconds to confirm the flame, or the gas shuts off.

Where that sequence stops tells you where the fault is. Turn the heating up or run a hot tap, stand next to the boiler and listen.

• Nothing at all, display dark: no power
• Display lit, but no fan sound: the boiler is not being asked to start, or it is blocked by low pressure
• Fan runs, then stops, no clicking: fan speed, flue or air pressure switch fault
• Clicking, then silence and a fault light: no flame, so gas supply, electrode or gas valve
• Lights, then goes out after a few seconds: the flame is not being detected, often a flame sensor or blocked condensate

Checks you can safely do yourself

These are all outside the casing. Start with the one that matches what you heard.

If it was silent and the boiler has no lights on, check the display, the fused spur switch next to the boiler and the consumer unit. Then check the thermostat is set above room temperature and the programmer is on. After a power cut, programmers often lose their time and switch the heating off without you noticing.

If the fan never started, look at the pressure gauge. Most combis will not fire below about half a bar and show a low-pressure code instead. Topping up to 1 to 1.5 bar cold often brings them straight back.

If it clicked without lighting, light a gas hob. No flame on the hob means no gas to the property, so check prepayment credit and the lever by the meter. If the hob lights, the gas is reaching the house but not the burner.

In freezing weather, check the plastic condensate pipe outside. When it freezes, water backs up inside the boiler and it refuses to fire or lights and cuts out. Warm, not boiling, water poured along the outside section usually clears it. Then reset once.

If you smell gas at any point, stop. Open the windows, turn off the gas at the meter if you can reach it safely, avoid switches, then leave and call 0800 111 999. Do not open the casing, and do not keep pressing reset. One reset is the limit.

Why the boiler fires for hot water but not the heating

On a combi, running a hot tap makes the boiler light, but turning the heating up does nothing. That tells you the burner, gas and ignition all work. The fault is in how the heating asks for heat.

Start with the controls. A wireless thermostat with flat batteries, a receiver that has lost its pairing, or a smart thermostat in away mode will all leave the boiler idle. Next is the diverter valve, which switches hot water between the taps and the radiators. When it sticks in the hot water position, the heating side never gets a call. On system and regular boilers, the same job is done by motorised zone valves, and a failed valve head has the same effect.

Our page on hot water but no heating walks through the controls side in more detail.

The parts that stop a boiler igniting

If the supply checks pass and it still will not light, the fault is inside, and these are the usual suspects.

The ignition and flame-sensing electrodes wear, get coated and crack. A dirty sensor can let the boiler light but then shut the gas off because it cannot prove the flame is there. The spark generator that powers the ignition can fail on its own. The fan bearings wear and the fan runs too slowly, so the board never gets confirmation the flue is clear. The gas valve can stick shut or lose its coil. And the main circuit board can stop sending the signal at any stage.

Most of these produce a code, and on a Baxi with a small display it can appear as E1 followed by two more digits. On a Vaillant, F28 means it failed to light at start-up. Worcester Bosch shows EA 227 when no flame is detected. If you see a lockout code for the first time, read our guide to boiler ignition lockout before calling.

What the engineer does, and typical repair costs

An engineer on an emergency boiler repair call confirms the gas pressure at the meter and at the boiler’s gas valve, reads the stored fault history and runs the boiler to watch each stage of the start sequence. They measure the flame signal, check the fan speed and flue, and test the electrodes before replacing anything. Gas work is carried out by Gas Safe registered engineers.

As typical UK market ranges, an ignition electrode costs about £75 to £200 fitted, a gas valve about £200 to £500, and a main circuit board roughly £350 to £700 depending on the make and whether the board is still made. Fans and diverter valves usually land between those. On a boiler over ten years old, a board failure is the point to weigh repair against replacement.

The £60 call-out brings an engineer to you at any hour, and the repair price is agreed with you on site before work starts. See our prices for details.