A boiler that continues to fire when your heating is off can be more than just annoying—it might be a sign of an underlying issue. At A&C Heating and Plumbing Ltd, we help homeowners and landlords across North and West London identify and resolve these common boiler problems with speed and efficiency.
💡 Common Reasons Why Your Boiler Keeps Firing
Even when the timer or thermostat is turned off, your boiler may still ignite due to several reasons:
1. Preheat Function (Combi Boilers)
Many modern combi boilers include a preheat feature that keeps hot water instantly available. This function may cause the boiler to fire briefly, even if heating is off.
2. Faulty Thermostat or Timer
A malfunctioning thermostat or electronic timer can send false signals to the boiler, causing it to ignite even when it’s supposed to be idle.
3. Stuck Diverter Valve
If the diverter valve is stuck in the ‘heating’ position, your boiler might activate when you run hot water or due to residual signals in the system.
4. PCB or Sensor Malfunctions
Internal control boards (PCB) or faulty temperature sensors can trick the boiler into activating.
5. Sludge or Airlocks
System sludge or trapped air can cause inconsistent water flow, which may falsely trigger the boiler.
⚠️ Should You Be Worried?
Not all firing is dangerous, but repeated unexpected ignition should never be ignored. It can:
- Waste energy and increase bills
- Shorten the lifespan of components
- Indicate deeper mechanical or electrical faults
⚖️ Diagnostics & Repairs from A&C Heating
Our qualified Gas Safe engineers can help with:
- Full fault diagnosis
- Thermostat and timer testing
- Valve and sensor replacements
- Disabling unwanted preheat functions
- Same-day emergency boiler repairs
We bring over 15 years of experience in repairing combi, system, and regular boilers across all major brands.
🚪 Available Across North and West London
We cover all areas including:
- Islington, Camden, Barnet, Brent
- Hounslow, Shepherd’s Bush, Acton, Ealing
- Chiswick, Pimlico, Kensington, Fulham
✅ Why Choose A&C Heating and Plumbing?
- Certified Gas Safe engineers
- No call-out fees
- Transparent pricing
- Emergency support 7 days a week
- Trusted by hundreds of local clients
✉️ Contact Us for Immediate Assistance
Still hearing your boiler fire when it shouldn’t? Book a same-day diagnostic with A&C Heating today.
📞 Phone: 0779 559 5757
📧 Email: info@ac-heating-plumbing.co.uk
🔗 Request a Quote
❓ FAQs – Boiler Firing Unexpectedly
Why is my boiler turning on when the timer is off?
Likely due to the preheat function, faulty timer, or a stuck valve.
Is it dangerous if my boiler fires up randomly?
Not usually, but it can waste energy and signal faults—get it checked.
Can I disable the preheat function?
Yes, most boilers allow you to turn this off in the settings menu.
Should I call an engineer if this happens?
Yes—especially if the issue persists or the boiler is old.
Does this affect combi or regular boilers more?
Combi boilers with preheat functions are more likely to exhibit this behaviour.