Combi Boiler No Heating? Causes & Fixes UK 2026
If your combi boiler gives hot water but your radiators stay cold, the boiler can usually fire and the issue is often controls or heating-side circulation. This guide covers the most common UK causes, safe checks you can do, and when to call a Gas Safe engineer.
First: Confirm the Boiler Is Actually Being Asked to Heat
Turn the room thermostat up and check the programmer/schedule is set to heating “on”. After a power cut, clocks and schedules can be wrong. If you have smart controls, try a manual override.
Common Cause 1: Diverter Valve Problems
Combi boilers use a diverter valve to send heat either to hot water or the heating circuit. If the valve sticks on hot water, radiators may not warm up. This usually needs an engineer.
Common Cause 2: Low System Pressure (Sealed Systems)
Many combi boilers are sealed systems. If pressure is too low, the boiler may protect itself and restrict heating. Check the gauge when cold and top up via the filling loop if needed (follow your manual).
Common Cause 3: Pump, Airlocks, or Poor Circulation
If the boiler fires but heat isn’t circulating, the pump could be stuck/failing or there may be air in the system. If radiators are cold at the top, bleeding may help. Persistent circulation problems need diagnosis.
Common Cause 4: TRVs, Balancing, or a Single Cold Radiator
If only some radiators are cold, it may be TRVs stuck closed, poor balancing, or sludge restricting flow. Try opening TRVs and checking the lockshield positions (mark settings before changes).
When to Call a Gas Safe Engineer
If safe checks don’t help, the diverter valve, pump, sensors, or internal controls may need repair. Don’t remove the boiler case or attempt internal fixes.
Upgrading Your Combi? Check the kW Output
If heating problems are frequent and the boiler is aging, replacement might make sense. Use our calculator to estimate the right kW range for your home.
Use the Boiler Size CalculatorFrequently Asked Questions
Why do I have hot water but no heating?
Common causes include heating controls not calling for heat, diverter valve faults, low pressure, or circulation problems.
Can low pressure stop heating but not hot water?
It can. Some boilers behave differently across modes, but low pressure often triggers protective shutdowns that affect heating.
Could it just be the thermostat?
Yes. Incorrect schedules, flat batteries, mis-pairing, or a low setpoint can all prevent heating demand.
Should I bleed radiators if heating isn’t working?
If radiators have cold tops, bleeding may help. Always recheck boiler pressure afterwards on sealed systems.
When should I call an engineer?
If safe checks don’t restore heating, or the boiler shows recurring error codes/lockouts, book a Gas Safe engineer.