Boiler Not Igniting: Common Causes and What to Check First in London

Why Boilers Fail to Ignite: The Most Common Causes

A boiler that does not ignite — or that repeatedly attempts to ignite, locks out, and displays a fault code — is one of the most common boiler problems reported in London during the heating season. In most cases the cause is identifiable and, in many cases, the fault is one that does not require an immediate emergency call-out. Understanding the most common causes and how to check for them can save you the cost of an emergency call-out and help you give the engineer useful information when you do call.

Check the Gas Supply First

The most common and easily overlooked cause of a boiler failing to ignite is a problem with the gas supply rather than the boiler itself. Before doing anything else, check whether other gas appliances in the property are working — light a gas hob burner, for example. If no gas appliances are working, check whether your gas meter credit has run out (if you have a prepayment meter) or whether there is a gas supply interruption in your area. You can check for planned or unplanned interruptions at cadentgas.com or by calling the National Gas Emergency Service on 0800 111 999. In London, supply interruptions affecting individual streets or areas are more common than most residents realise.

If the gas supply to other appliances is normal, the issue is specific to the boiler. The emergency control valve — the manual handle on the gas supply pipe entering the boiler — should be in the open (parallel to the pipe) position. If it has been accidentally knocked closed, this alone will prevent the boiler from igniting.

Ignition Electrode and Flame Sensor Faults

The most common component failure causing ignition problems is a worn or contaminated ignition electrode. The electrode generates the spark that lights the gas at the burner; over time, the electrode tip erodes, ceramic insulation cracks, or the electrode becomes coated with combustion residue. A cracked ceramic insulator is the most common single cause of intermittent ignition failure in London boilers — the spark tracks across the crack in the insulator rather than jumping to the burner, so the ignition attempt fails. This fault typically presents as a lockout after three ignition attempts without the flame sensor detecting a flame. It requires a Gas Safe engineer to replace the electrode assembly, typically costing £60 to £120 including parts and labour.

The flame sensor (also called the thermocouple or ionisation probe depending on the boiler type) confirms to the boiler's control board that ignition has been achieved and that the burner flame is present. A contaminated or corroded flame sensor sends a weak or absent signal to the control board, causing the boiler to lock out even when the burner has actually lit. Cleaning or replacing the flame sensor is a straightforward workshop task for an engineer and typically costs £40 to £80.

Gas Valve Faults

The gas valve controls the flow of gas to the burner during ignition and running. A sticking or faulty gas valve can prevent gas from reaching the burner during ignition attempts, causing repeated lockouts. Gas valve faults are more common in older boilers and in boilers that have been subject to irregular maintenance. This is a component replacement that must be carried out by a Gas Safe registered engineer. The cost ranges from £200 to £450 depending on the boiler model and the cost of the replacement valve, and is one of the repair costs that should prompt a repair-or-replace analysis for boilers over 10 years old.

Condensate Pipe Frozen or Blocked

A specific and very common London boiler ignition failure in cold weather is a frozen condensate pipe. The condensate pipe carries the acidic condensate water produced by the boiler during operation to an external drain. In cold weather, condensate can freeze in the external section of the pipe, causing a blockage that prevents normal operation and triggers a fault code. Modern boilers detect the blockage through a pressure or flow sensor and lock out rather than continue running with the condensate backing up into the heat exchanger.

A frozen condensate pipe is one of the few boiler faults a homeowner can safely address without calling an engineer. Pouring warm (not boiling) water over the external section of the pipe, or wrapping it in warm towels, will usually thaw the blockage within 15 to 20 minutes. Once thawed, the boiler should reset and resume normal operation. After thawing, the condensate pipe should be insulated to prevent recurrence — this is a simple and inexpensive job that is best done before the next cold snap.

Low System Pressure

Many modern combi boilers will not attempt to ignite if the system water pressure has fallen below the minimum threshold — typically 0.8 to 1.0 bar. Low pressure prevents proper circulation and could cause overheating in the heat exchanger if the pump runs without adequate water flow. The pressure gauge on the boiler is visible through the front panel on most models. If the needle is in the red zone (typically below 1 bar), the system needs repressurising before the boiler will restart. Repressurising a sealed system combi boiler is a task most homeowners can do safely using the filling loop, bringing pressure up to 1.2 to 1.5 bar. If the system repeatedly loses pressure, this indicates a leak somewhere in the system that requires investigation.

PCB (Printed Circuit Board) Faults

The PCB is the boiler control board that coordinates ignition timing, gas valve operation, flame sensing, and safety lockout. PCB faults can cause a wide range of apparently unrelated symptoms including ignition failure, random lockouts, and intermittent operation. PCB failure is more common in older boilers and in boilers that have experienced repeated overheat events or damp ingress. A PCB diagnosis requires an engineer with the correct diagnostic equipment; the component itself is one of the most expensive replacements on a modern combi boiler, typically costing £350 to £700 including labour. For a boiler over 10 years old, a PCB replacement cost should always trigger a repair-or-replace analysis.

When to Call an Emergency Engineer in London

Call a Gas Safe engineer immediately if: you smell gas anywhere in the property (also call 0800 111 999 and open windows); you see a yellow or orange boiler flame; your carbon monoxide alarm has sounded; or the boiler is making a banging or popping noise. These are safety issues that require immediate professional attention. If the boiler has simply locked out and displays a fault code, and there is no smell of gas and no CO alarm activation, it is generally safe to wait for a same-day or next-day appointment with a Gas Safe engineer rather than paying an emergency call-out rate. Note the fault code displayed — this information is useful to the engineer and may allow a preliminary diagnosis before they arrive.