Boiler keeps switching off? Common causes in London homes

A boiler that keeps switching itself off is doing exactly what it was designed to do: protect itself, and you, from a fault it cannot operate around. The question is what fault. There are a handful of common ones, and most of them are fixable in one visit.

Read the fault code first

Modern boilers display a code on the front when they shut down. Write it down before doing anything else. The same fault code on a Worcester boiler means something different to the same code on a Vaillant, but the manufacturer manual (or a quick search) will tell you what it points to. Knowing the code saves an hour of diagnostic time and means whoever you call can bring the right parts.

Common codes I see across London:

Worcester Bosch:

• EA / E9 — flame loss or overheat (most common, often dirty burner or scaled exchanger)
• E1 / E2 — sensor fault (NTC sensor failure)
• A1 — pump fault

Vaillant ecoTEC:

• F22 — low water pressure
• F28 / F29 — ignition fault, gas supply or electrode
• F75 — pump or pressure sensor fault
• F23 — temperature differential too high (often pump or flow issue)

Baxi:

• E110 — overheat
• E125 — circulation fault
• E133 — ignition lockout
• E168 — gas supply fault

Ideal Logic:

• F1 — low pressure
• L2 — ignition lockout
• F2 — flame loss
• L1 — overheat or sensor

Glow-Worm:

• F1 — low pressure
• F22 — low pressure (different scale)
• F75 — pump fault

If the code is not in the boiler's quick-reference, the full manual is usually a free PDF on the manufacturer's website.

Five common causes, ranked by frequency

1. Low pressure

If the pressure on the front gauge drops below 0.8 bar, most boilers will refuse to keep running. Top it up to 1.2 bar using the filling loop underneath the boiler.

If you have to do this every few weeks, you have a leak. See the separate post on losing pressure for diagnosis.

2. Frozen condensate pipe (winter only)

Combi and condensing boilers produce a small amount of acidic condensate that drains through a plastic pipe (usually 22mm). If that pipe runs outside the building and freezes in cold weather, the boiler senses the blockage and shuts down. Worcester displays this as EA in some cases; Vaillant as F73 or F75.

How to check: look for a white plastic pipe leaving the boiler and going outside. Often runs down an external wall to a drain. If it is winter and you have had freezing temperatures overnight, this is the most likely cause.

How to fix: pour warm (not boiling) water along the external section of the pipe. The blockage is almost always at a bend or where the pipe drops to a drain. Once cleared, restart the boiler.

Long-term fix: insulate the external pipe, or if you are replacing the boiler, route the condensate internally to a drain. This is now standard practice on new installs in cold areas, but a lot of older London installs still have external runs.

3. Dirty burner or scaled heat exchanger

Most boilers shut down on overheat (variations of E1, E110, F23 codes) when the heat exchanger has scaled up or the burner is dirty. Heat cannot dissipate properly, the temperature sensor reads higher than the safety threshold, and the boiler stops itself.

This is the symptom of an unserviced boiler. A proper service includes cleaning the burner, checking the heat exchanger, and a combustion analysis. Skipping years of service to "save money" almost always costs more in callouts later.

If your boiler is shutting down on overheat and has not been serviced in three or more years, book a service first. In maybe a third of cases the service alone resolves the lockout.

4. Pump failure

If the pump is not pushing water through the system, the boiler heats the same small volume of water to safety thresholds and shuts down. Symptoms include tepid radiators, the boiler cycling on and off rapidly, and pump fault codes.

A pump replacement in London is typically £200 to £350 parts and labour. The pump on most modern boilers is built in. On older systems it is a separate unit, often visible in the airing cupboard or under the boiler.

5. Flame loss / ignition lockout

The boiler is failing to light, or lighting and then losing the flame. Causes:

• Carbon build-up on the spark electrode (cheap to replace)
• Gas supply issue (low pressure at the meter, blocked filter, recent gas works in the street)
• Failed gas valve (less common, more expensive)
• Air in the gas line after meter work

If your boiler is doing repeated ignition attempts (you can hear it clicking three or four times before it locks out) and has been doing so since a recent meter or street works visit, the gas supplier should clear airlocks for free.

How to safely reset and test

After topping up pressure or clearing a frozen condensate, you can reset most boilers with a button on the front panel. The reset symbol is usually a small flame icon or labelled R.

Hold for 3 to 5 seconds, watch the display restart, and turn the heating on. If the boiler runs for more than 30 minutes without locking out again, you have probably solved the underlying issue. If it locks out again within minutes, the cause is something other than what you fixed, and you need a plumber.

Do not reset a boiler more than three times in a row without diagnosis. Repeated lockouts mean a real fault, and bypassing the safety mechanism by force-restarting risks damage to the heat exchanger.

When to call immediately

• Smell of gas, with or without a fault code (call National Gas Emergency on 0800 111 999 first, before any plumber)
• Boiler shutting off and a smell of unburnt fuel (carbon monoxide risk)
• Visible water inside the boiler casing
• Black soot stains around the boiler casing or flue
• Pilot light (older boilers) that will not stay lit at all

When you can wait a day or two

• Single fault that resets and runs fine for hours
• Condensate freeze that clears with warm water and does not recur
• Low pressure that you have topped up and is now stable
• Fault code that points to a specific known repair (e.g. flame sensor)

What I would do at your house

For a repeated lockout call, my standard process:

1. Read the current and historical fault codes from the display
2. Check pressure, water flow, and gas supply
3. Pull the front panel and inspect burner, electrode, heat exchanger
4. Run a combustion test (CO and CO2 readings) to verify safe operation
5. Identify the root cause and quote in writing

If the diagnosis points to a part I can fit the same day, I can usually get the boiler running before I leave. If it needs a part ordered, you get a fixed price and a return date.

WhatsApp me a photo of the fault code and the boiler model and I will tell you whether it is something I can quote on right away or whether it needs eyes-on diagnosis.