If you smell gas or suspect a gas escape, do not operate electrical switches, light naked flames, or smoke. Open doors and windows, turn off the gas at the meter only if it is safe to access, leave the property, and immediately call the National Gas Emergency Service on 0800 111 999. If there is no smell of gas, a sudden loss of heating or hot water is often caused by something simple. A tripped circuit breaker, a depleted prepay gas meter, flat thermostat batteries, low boiler pressure below 1.0 bar, or a frozen condensate pipe in winter. If your combi boiler delivers boiling hot water but radiators stay cold, the boiler’s burner is working fine. The fault is isolated to your room thermostat, programmer, or a stuck motorised diverter valve. The Happy Heating Co’s Gas Safe registered engineers (657972) provide diagnostic boiler repairs and emergency boiler repair across West and North West London.
A boiler that suddenly stops working feels like an emergency. Often it is not. The cause is frequently a simple external issue you can put right in five minutes without opening the appliance casing. Work down this checklist in order. It is the exact diagnostic sequence our engineers use when triaging calls over the phone.
Key Takeaways
- Rule out gas leaks first. If you smell gas, leave the property immediately and call 0800 111 999.
- The Combi Priority Rule. If hot water works at your taps, the boiler’s burner, gas valve, and PCB are healthy. The issue is external controls or a stuck diverter valve.
- Check cold boiler pressure. A reading below 1.0 bar causes modern boilers to lock out as a dry-fire safety measure.
- Wireless thermostats are the most common culprit. Flat batteries or loss of RF signal to the receiver account for dozens of breakdown calls each autumn.
- Never attempt DIY internal gas work. Opening the sealed boiler casing or tampering with gas valves is illegal and dangerous. Always use a Gas Safe registered engineer.
Symptom Diagnostic Triage
Use this matrix to understand what your symptoms mean before booking a call-out:
Completely blank boiler display
No electrical power or blown PCB fuse
Check fused spur switch and consumer unit breaker
Call engineer if electrical power is live but display remains dark
Hot water works but radiators stay cold (Combi)
Stuck diverter valve, flat stat batteries, or summer mode
Check room stat batteries and programmer schedule
Call engineer if stat calls for heat but diverter valve fails to move
Radiators warm but no hot water (Combi)
Faulty domestic hot water flow sensor or plate heat exchanger
Verify hot water tap flow rate
Call engineer to test flow turbine or descale and replace plate exchanger
Boiler firing for 30 seconds then shutting down
Low system pressure or seized circulation pump
Check pressure gauge and repressurise to 1.0 to 1.5 bar
Call engineer if pressure is correct but boiler overheats and locks out
Boiler humming or gurgling in freezing weather
Frozen external condensate drain pipe
Thaw pipe with warm water, never boiling
Call engineer if pipe is clear but boiler still locks out on ignition
Metallic bang on ignition
Delayed gas ignition or faulty spark electrode
Do not attempt
Immediate Gas Safe attendance required. Shut down boiler
The 7-Point Safe Diagnostic Checklist
Work through these seven external checks before calling out an engineer. Most take under two minutes.
1. Power Supply and Fused Spur
Is the digital display on the front of the boiler lit? If it is completely blank, check the 3-amp fused spur switch next to the boiler. Make sure it has not been switched off accidentally. Then check your main electrical consumer unit for a tripped circuit breaker or tripped RCD.
2. Gas Supply and Prepay Meters
Ensure the emergency control valve at the gas meter is open. If you have a prepay gas card or key meter, verify that it has not run out of credit or switched into emergency reserve mode. It sounds obvious. It catches people out more than you would think.
3. Thermostat, Programmer and Summer Mode
Replace the AA or AAA batteries in your wireless room unit. A fading battery may light the LCD screen but lack the RF signal strength to trigger the boiler receiver. That is a very common miss. Also check that the thermostat is set at least 2°C higher than the current room temperature, and that the boiler or programmer is not set to hot water only mode.
4. Boiler Water Pressure
Look at the pressure gauge on the front of the boiler. Cold, it should sit between 1.0 and 1.5 bar. If the needle is below 1.0 bar, often deep into the red zone below 0.5 bar, the boiler will lock out to protect the heat exchanger from dry-firing.
Locate the external braided filling loop underneath the boiler. Open the valve slowly until the pressure reaches 1.2 to 1.5 bar, then close it tightly. See our full guide on why boilers lose pressure.
5. Display Fault Code
If the boiler shows a flashing alphanumeric code such as F22, F75, EA, or E119, write it down. Look it up in your manufacturer’s manual. For Worcester Bosch appliances, our Worcester Bosch fault codes guide covers the most common ones.
6. Frozen Condensate Pipe
During winter freezes, the plastic pipe carrying acidic condensate water from the boiler to an outside drain can freeze solid. It backs water up into the burner and causes a lockout. Locate the plastic pipe exiting your external wall. Pour warm water along the length of it to melt the ice plug. Never use boiling water. Reset the boiler once cleared.
7. Reset the Boiler
Press and hold the reset button for 3 to 5 seconds. If the boiler fires and continues running, monitor the pressure gauge. If it locks out a second time, stop. Do not repeatedly reset it. Continuous resetting can damage the ignition PCB or flood the combustion chamber.
The Combi Hot Water First Rule
This is one of the most useful trade diagnostic rules. Inside every combi boiler, domestic hot water takes 100% priority over central heating. When you turn on a hot tap, a flow turbine detects water movement and a motorised diverter valve moves across to direct boiling primary water through the small plate heat exchanger rather than your radiators.
Hot water works but radiators stay cold? The boiler’s gas supply, burner, fan, spark electrode, and main control board are all in full working order. The problem is strictly isolated to the room thermostat, timer, or a diverter valve cartridge stuck in the hot water position.
Radiators warm but no hot water? The diverter valve is stuck in the heating position, or the domestic hot water flow sensor has failed to register tap flow.
Systems with Hot Water Cylinders
If your home has a separate hot water cylinder in the airing cupboard, a conventional regular boiler or unvented system boiler, water is routed using external motorised zone valves.
S-Plan systems use two 2-port valves. One controls central heating and the other controls hot water. If one zone fails, the synchron motor or internal microswitch inside that valve head has likely burnt out.
Y-Plan systems use a single 3-port mid-position valve that shares flow between heating and hot water. When the actuator fails, the manual lever on the side often feels loose and floppy.
Replacing a motorised valve actuator head or valve body requires a qualified heating engineer to test electrical switching and drain the primary circuit.
Is It an Emergency or Can It Wait?
Call 0800 111 999 immediately if you smell gas. Severe water leaks near electrical consumer units, or complete loss of heating during sub-zero temperatures with vulnerable or elderly residents, also warrant urgent attendance. For urgent plumbing leaks, our emergency plumbing team can attend fast.
Standard working-day repair covers no heating in mild weather with hot water still working, intermittent pressure drops requiring weekly top-ups, or a noisy radiator valve. These are not comfortable, but they are not emergencies.
Frequently Asked Questions
Why is my boiler completely dead with no lights on?
A completely unlit boiler display means a loss of incoming electrical power. Check the 3-amp fused spur switch next to the boiler and your main fuse box for a tripped breaker. If power is live to the spur, the boiler’s internal PCB fuse has likely blown.
Why do I have hot water but no heating on a combi boiler?
This almost always means your room thermostat is not calling for heat, usually dead batteries or a summer timer setting, or the boiler’s internal motorised diverter valve is stuck in the domestic hot water position.
What should boiler pressure read when cold?
Between 1.0 and 1.5 bar on the front pressure gauge. If the pressure drops below 0.5 bar, most modern boilers shut down with a low-pressure fault code such as F22 on Vaillant or EA on Worcester Bosch, to protect the heat exchanger.
Can I reset my boiler multiple times if it locks out?
No. Reset once after fixing an obvious fault like low pressure. Repeatedly resetting an appliance that is locking out on ignition or overheating can cause severe damage to the PCB and combustion components.
What should I do if I smell gas?
Do not operate any electrical switches, light matches, or smoke. Open doors and windows to ventilate the area. Turn off the gas supply at the meter only if it is safe to do so. Evacuate the property and call the National Gas Emergency Service immediately on 0800 111 999. It is free and answered 24 hours a day.
Get Local Gas Safe Heating Support in West London
If you have completed the safe external checks and your boiler will not fire, our Gas Safe registered engineers (657972) are here to help across West and North West London.
- WhatsApp us: Book in 60 seconds
- Call 020 8051 2124. Our line is answered 24 hours a day and we will get your details to an engineer.
- Book online: boiler repairs and emergency boiler repair
We cover West and North West London. Heating that just works, no drama.
About the author Adam Said is a Gas Safe registered domestic gas engineer (657972) at The Happy Heating Co, diagnosing and fixing heating and hot water breakdowns in homes across West and North West London.
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