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Why Your Boiler Keeps Turning On and Off: Common Causes & Fixes

09 September 2025

A boiler that keeps turning on and off can be frustrating, confusing, and even costly. Whether it’s your combi boiler constantly running, randomly firing up for hot water, or needing frequent resets, understanding the reasons behind this behaviour can help you take the right steps to fix it.

At Shropshire Gas, we’ve put together a guide to explain why your boiler keeps turning on, why it may constantly run, and what you can do to resolve the issue.

🔗 Quick Links:

Short-Cycling

What it means:

Short-cycling occurs when your boiler switches on and off too frequently. Common causes include:

  • A boiler that is oversized for your home
  • Low water pressure or flow issues
  • Faulty thermostats or room stats

How to fix it:

  • Check the system pressure and ensure it’s within the manufacturer’s recommended range.
  • Bleed radiators to remove trapped air and improve flow.
  • If issues persist, call a Gas Safe engineer for a full inspection.

Faulty Thermostat or Programmer

What it means:

If your thermostat is giving incorrect readings, the boiler may fire up unnecessarily. This can cause your boiler to run constantly or turn on at odd times.

How to fix it:

  • Ensure the thermostat is located away from heat sources.
  • Check that your timer and programmer settings are correct.
  • Replace a faulty thermostat if needed, with the help of a qualified engineer.

Pressure or Flow Issues

What it means:

Incorrect pressure or flow can cause your boiler to keep turning on and off or need frequent resets. Signs include unusual noises and fluctuating pressure readings.

How to fix it:

  • Check the pressure gauge – it should typically be between 1–1.5 bar when the system is cold.
  • Bleed radiators to improve flow.
  • Consult a Gas Safe engineer if the problem persists.

Diverter Valve Problems

What it means:

In combi boilers, the diverter valve directs hot water or heating. A stuck or faulty valve can cause the boiler to turn on randomly or stop unexpectedly.

How to fix it:

DIY Checks Before Calling an Engineer

  • Check boiler pressure and ensure it’s within recommended levels.
  • Bleed radiators to remove trapped air.
  • Review thermostat and timer settings to ensure they are correct.
  • Reset the boiler only according to manufacturer instructions.

FAQs

Q: Why does my boiler keep turning on?

A: It could be caused by short-cycling, a faulty thermostat, pressure problems, or a stuck diverter valve.


Q: Can leaving the boiler on damage it?

A: Modern boilers are safe to leave on, but constant running or short-cycling can cause wear, reduce efficiency, and increase energy costs.


Q: Why does my combi boiler constantly run?

A: This is often due to a stuck diverter valve, thermostat issues, or incorrect sensor readings for hot water demand.


Q: Why does my boiler keep needing to be reset?

A: Frequent resets may indicate pressure problems, overheating, or a failing component that requires professional attention.

Need Help With a Boiler That Keeps Turning On and Off?

At Shropshire Gas, our Gas Safe registered engineers can quickly diagnose and repair boilers that randomly fire up, constantly run, or need frequent resets. We ensure your heating system works safely, efficiently, and reliably.

📞 Call us today on 01952 259 019 or book your repair online.

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