PRACTICAL GUIDE · UPDATED 2026-10-30

Heating switched off but the boiler is running: what to check

A room thermostat is one control in a larger system. Understand normal requests, note the symptoms and know when to arrange a repair.

Diagram showing separate room-heating, hot-water, preheat and automatic-protection requests to a boiler — Our original concept diagram: possible requests depend on the appliance and installation. Noise alone does not establish that the burner is firing.

Watch the 45-second explainer. Original animation with on-screen captions, no sound. General information, not a diagnosis. The guide below is the written explanation and lists the independent sources.

Start with what you can observe safely: is there a flame indicator, a pump sound, hot tap water or radiators warming? Write down the display, time and settings without opening the appliance. Its user manual explains the indicators. A noise on its own is not a diagnosis.

Turning down the room thermostat does not switch off every hot-water function. A combi can respond when you open a hot tap; some models also keep part of the appliance warm in a preheat or comfort mode. Worcester Bosch explains that its PreHeat mode can cause firing without a current tap or heating request. The setting, name and operation depend on the model, so use its own instructions rather than another boiler’s button sequence.

Where a system or regular boiler heats a cylinder, separate hot-water controls may still be requesting heat. Check the visible programmer, cylinder programme and any additional heating zones. A smart-control app may have an override or schedule that differs from the wall display. Energy Saving Trust explains the different control arrangements; ask your installer to demonstrate yours.

Some models run a pump after a demand ends, and frost protection can operate automatically in low temperatures. These features are model-specific. Worcester Bosch user literature describes frost functions for particular appliances; those settings should not be transferred to another model. Do not disable a safety function or cut power merely to silence activity. Follow your own manual and seek advice if the pattern is unfamiliar.

Arrange a suitably qualified engineer if radiators keep heating despite the relevant heating controls being off, the home overheats, error messages recur or the behaviour has changed unexpectedly. Explain the symptom and ask whether the visit is fault diagnosis or routine servicing, what diagnosis costs if no repair is made, and how further work will be authorised. Unexpected operation alone does not establish that replacement is needed.

For gas work, HSE advises using a Gas Safe registered engineer qualified for the specific work. Do not open the casing, alter wiring, adjust internal valves or repeatedly reset a recurring fault. Gas-smell or carbon-monoxide concerns require the separate emergency guidance immediately; do not wait for directory advice or a routine appointment.

Your checklist

Record the display or error code and when it happens.
Check visible schedules, hot-water demand and other heating zones against the user manual.
Confirm whether the sound is the burner, pump or another component using the manual; do not open the casing.
Ask for a diagnosis and clear charges when heat is unwanted or faults recur.
Check the attending engineer’s current qualifications for the work.

Sources & further reading

General information, not a property assessment, booking or assurance about a listed business. Dates indicate content updates; they are not invented publication dates.

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