Boiler Not Working? 8 Common Causes and Fixes for London Homes

Before You Start: Safety First

Most boiler faults are entirely safe to investigate yourself. A few are not. Before you touch anything, run through this check:

If none of the above apply, carry on. The eight causes below cover the vast majority of boiler breakdowns across London homes — from Victorian terraces in Hackney to modern converted flats in Canary Wharf — and most can be resolved without a call-out.

1. Low Boiler Pressure

Low pressure is the single most commonly diagnosed boiler fault in London. Modern condensing boilers (fitted as standard since 2005) require the system to hold between 1 and 1.5 bar of water pressure to operate correctly. When pressure drops below 1 bar, most boilers will lock out or refuse to fire.

How to check

Find the pressure gauge on your boiler — it's usually a circular dial on the front panel, or a digital reading on the display. On cold systems, anything between 1 and 1.5 bar is healthy. Below 1 bar means you need to repressurise.

What causes low pressure?

• A small water leak anywhere in the heating circuit (radiators, pipework under floors, at joints)
• A radiator bleed valve that wasn't fully closed after bleeding
• Normal system losses over time — a small drop each year is expected
• A faulty or weeping pressure relief valve

The fix

If you have a filling loop (a flexible silver hose or fixed valve usually located beneath the boiler), you can repressurise the system yourself. Open the valve slowly, watch the gauge rise to around 1.2–1.3 bar, then close it firmly. For a step-by-step walkthrough, see our how to repressurise a boiler guide.

2. Pilot Light Out or Ignition Fault

Older boilers use a permanent pilot light. Newer boilers (any boiler installed since roughly 2004) use an electronic ignition that fires on demand. Either can fail, leaving the boiler unable to light the main burner.

Symptoms

• Boiler attempts to fire (you hear clicking) but shuts down after a few seconds
• Display shows an ignition fault code (varies by manufacturer — common codes are E1, F1, or similar)
• No flame visible through the inspection window

What to try

Press the boiler's reset button and hold it for 3–5 seconds. The boiler should attempt to ignite again. If it fires and stays on, the issue may have been a one-off gas supply fluctuation or a temporary lockout. If it fails to ignite after three consecutive reset attempts, stop — further resets won't help and may mask the real fault.

3. Frozen Condensate Pipe

This is the most common winter boiler fault across London, and the easiest to fix yourself. All modern condensing boilers produce acidic condensate water as a by-product of their high efficiency. This water drains via a white plastic pipe — typically 22 mm or 32 mm diameter — that exits through an exterior wall or runs to an external drain.

When temperatures drop below freezing, this pipe can freeze solid. The boiler detects a blockage, can no longer drain condensate, and locks out. Most Worcester Bosch, Vaillant, Ideal, and Baxi boilers will display a specific fault code for this — often EA229, F28, or a snowflake symbol. In winter, the external boiler flue can also become blocked or frozen — see our guide to a blocked or frozen boiler flue if a condensate thaw doesn't restore operation.

How to fix a frozen condensate pipe

1. Locate the condensate pipe. It's the small white plastic pipe (usually on the outside wall near where the boiler is located). It may run vertically down the wall or exit horizontally.
2. Boil a kettle, then let it cool for 10 minutes. You want warm water, not boiling. Boiling water can crack plastic pipes, split joints, or cause thermal shock.
3. Pour the warm water along the length of the pipe, particularly at bends, joints, and the lowest point where ice tends to accumulate.
4. Listen for the blockage clearing — you may hear a gurgling sound as the condensate drains.
5. Return inside and reset the boiler. Hold the reset button for 3–5 seconds. The boiler should fire and return to normal operation within a minute or two.

4. Thermostat or Timer Issue

Before assuming a boiler fault, check whether the boiler has simply been switched off by your controls. This is surprisingly common — particularly after a power cut resets a digital programmer, or when clocks change in March and October.

Things to check

• Room thermostat: Is the target temperature set above the current room temperature? If the thermostat is set to 15°C and the room is 16°C, the boiler correctly won't fire.
• Programmer / timer: Is the heating scheduled to be on at the current time? Check whether a recent power cut has reset the clock to 00:00, disabling all scheduled periods.
• Thermostatic Radiator Valves (TRVs): If all TRVs are turned to minimum, some boilers won't fire even if the programmer calls for heat. Open at least two or three TRVs fully.
• Smart thermostat (Nest, Hive, tado): Check the app — the system may be in Away mode, holiday mode, or may have lost its Wi-Fi connection and defaulted to off.

If the controls look correct and the boiler still won't fire, a faulty thermostat wiring connection or a failed wireless receiver (for smart thermostats) can prevent the boiler from receiving a heat demand signal. This is a straightforward fix for a Gas Safe engineer.

5. Diverter Valve Stuck (Combi Boilers)

The diverter valve is found only in combination (combi) boilers — the most common boiler type in London flats and terraced houses. Its job is to switch the boiler's heat output between the central heating circuit and the domestic hot water (DHW) circuit when you open a tap.

Symptoms of diverter valve failure

• Heating works but there is no hot water from taps or showers
• Hot water works but radiators are cold
• Lukewarm water from taps even when the boiler is firing
• Heating comes on when you open a hot tap

A stuck diverter valve cannot be fixed by resetting the boiler. The valve itself is a mechanical part that wears over time — particularly in heavily used combi boilers in London flats where the DHW demand is high. Repair typically involves either freeing a jammed actuator or replacing the valve cartridge entirely.

6. No Gas Supply

Occasionally the problem isn't the boiler at all — it's the gas supply to the property. This can happen due to works on the gas mains (National Grid Gas carry out planned and emergency works across all 33 London boroughs), a meter fault, or a gas isolation valve being closed accidentally.

How to check

• Try a gas hob burner or gas oven. If these won't light either, the problem is almost certainly supply, not the boiler.
• Check that the emergency control valve (ECV) at the meter is open — the handle should be parallel to the pipe, not perpendicular (off).
• Check whether your neighbours have gas — if there's a wider street outage, call Cadent Gas (National Gas Emergency Service: 0800 111 999).
• If your meter is a prepayment meter, check your credit balance.

7. Boiler Lockout (Fault Code)

Modern boilers have onboard diagnostic systems. When something goes wrong, the boiler locks out for safety — preventing it from firing — and displays a fault code on the screen. This is the boiler protecting itself (and you) from running in an unsafe or damaged state.

What to do when your boiler shows a fault code

1. Write down the fault code before doing anything else. Take a photo of the display. This code tells a Gas Safe engineer exactly what the boiler detected, saving diagnostic time (and cost).
2. Look up the code in your boiler's manual or search "[your boiler model] + [fault code]" — manufacturer websites publish full fault code lists.
3. Press reset once and wait to see if the boiler recovers. Some codes (e.g. condensate blockage, temporary overheat) clear on a single reset.
4. If the same code returns within an hour, do not keep resetting. Call a Gas Safe engineer and give them the code — they may be able to diagnose over the phone and arrive with the correct parts.

8. PCB or Heat Exchanger Fault

The printed circuit board (PCB) is the electronic brain of the boiler — it controls ignition timing, fan speed, gas valve operation, and safety shutdowns. The heat exchanger is the component through which the flame heats the water in the system. Both are major components and both fail over time.

PCB failure symptoms

• Boiler behaves erratically — fires intermittently with no pattern
• Multiple different fault codes appearing in sequence
• Controls unresponsive or display behaving strangely
• Boiler fires but shuts down almost immediately without a clear fault code

Heat exchanger failure symptoms

• Boiler repeatedly overheats and locks out (overheat fault code)
• Noisy operation — kettling, banging, or rumbling sounds when firing
• Pressure drops frequently despite no visible leaks (internal crack in the heat exchanger)
• Flue gases smell unusual (combustion products leaking into the water circuit)

Neither of these faults has a DIY fix. PCB replacement costs £300–£600 depending on the boiler model (some boards are difficult to source for older units). Heat exchanger replacement typically costs £400–£700 — at which point the age and condition of the boiler should be carefully considered before committing to repair.

London Hard Water and Your Boiler: The Hidden Threat

London sits above one of the UK's most mineralised groundwater tables. Thames Water's own figures consistently show London water at 250–320 mg/litre calcium carbonate — classified as very hard. That's roughly three to four times harder than water in Manchester, Glasgow, or Bristol.

Inside your boiler's heat exchanger, this mineral-rich water is repeatedly heated to high temperatures. Calcium carbonate precipitates out of solution and bonds to the inside of the heat exchanger as limescale. Over time, this layer:

• Acts as thermal insulation, forcing the boiler to work harder to reach target temperatures
• Reduces flow rates, causing overheating and repeated lockouts
• Creates hot spots that can crack the heat exchanger — an expensive failure
• Reduces boiler efficiency by up to 15%, increasing gas bills noticeably

This is the primary reason heat exchanger failure rates in London are significantly higher than national averages. Engineers working across boroughs like Kensington, Lambeth, and Islington — where Victorian and Edwardian pipework is common — regularly find systems that have never had an inhibitor added or a flush carried out.

Power flushing and inhibitor: the prevention

A power flush in London involves connecting a high-velocity pump to your central heating system and circulating a chemical cleaner at low pressure to dislodge and remove magnetite sludge, limescale, and corrosion debris from the circuit. Following the flush, a corrosion inhibitor (such as Fernox F1 or Sentinel X100) is added to protect the system long-term.

For London properties, we recommend a power flush every 5–7 years, or immediately if your boiler is showing signs of heat exchanger stress: kettling noises, slow heat-up times, or repeated overheat lockouts. If you're replacing a boiler, always flush the existing system first — fitting a new boiler onto a contaminated circuit voids most manufacturer warranties.

Boiler Repair Costs in London 2026

London boiler repair costs vary widely depending on the fault, the boiler make and model, and parts availability. The table below shows typical market rates for common repairs carried out by Gas Safe registered engineers in the capital in 2026.

Trusted Plumber London charges a £60 call-out fee, which is waived if the job runs over 2 hours — meaning for any repair that takes more than two hours (most significant boiler repairs), there is no call-out charge on top of the labour rate. For planned maintenance, see our boiler service cost guide.

Frequently Asked Questions