In This Guide
Why is my boiler on but no heat to the radiators?
When the boiler is clearly running but the radiators stay cold, the heat is being made but not reaching, or not being sent to, the radiators. A handful of causes account for almost all of these, and the pattern of what is and isn’t working usually points to which:
Cause | Sign | Fix |
Stuck diverter valve (combi) | Hot water is fine, but no heating | Engineer frees or replaces the diverter valve |
Failed or seized pump | Boiler hot, radiators cold, no circulation | Pump repair or replacement |
Trapped air | Some radiators cold or gurgling | Bleed the radiators |
Low system pressure | Gauge reads below 1 bar | Top up via the filling loop to 1 to 1.5 bar |
Thermostat or timer not calling | Boiler is not firing for heating | Set the thermostat above room temperature, check the timer |
Sludge build-up | Cold spots, poor circulation | Power flush |
Closed or stuck valves | Individual radiators or a zone stay cold | Free the valve, or an engineer for a zone valve |
The combi clue: hot water but no heating
There is one pattern worth singling out. If you have a combi boiler and your hot water is working perfectly but the heating is dead, the most likely culprit is the diverter valve. This is the internal valve that switches the boiler between heating your radiators and heating your hot water. When it sticks in the hot water position, you get plenty of hot water at the taps but nothing reaching the radiators. It is a common fault, and one 0800 Homefix repairs regularly, but an engineer’s job, as the valve sits inside the sealed boiler casing.
What you can check yourself
Start with the safe checks. Make sure the thermostat is set above the current room temperature and the programmer is set to on for the time of day. Check the pressure gauge and top up via the filling loop if it is below 1 bar. Bleed the radiators to release trapped air, starting with the ones furthest from the boiler. If a single radiator is cold, check its valves are open. These steps clear a good share of no-heat problems on their own.
What you should not do is open the boiler or work on the pump, diverter valve, or zone valves. Those are engineer tasks, and on a combi they are inside the sealed casing.
Do not simply redecorate over water stains or damp patches. Painting over a damp wall without fixing the source will result in the stain reappearing within weeks.
When to call an engineer
Call a Gas Safe engineer if the checks above do not restore heat, if hot water works but the heating does not (a diverter valve pointer), if the boiler is hot but no radiators warm up (a pump pointer), or if the boiler keeps losing pressure. Repeatedly resetting a boiler that fires but sends no heat to the radiators will not fix a valve or pump fault.
How 0800 Homefix can help
0800 Homefix are Gas Safe registered engineers covering London and the South East. When the cause is inside the boiler, a diverter valve or pump, or down in the system as sludge, an engineer will find which it is and fix the actual fault rather than guessing. You can see the full service on our boiler repair page.
0800 Homefix charges as the first hour of labour, paid in advance, with any parts or further work quoted and agreed with you before it begins. For current rates, see our rates page. Our labour is guaranteed for 12 months, and any parts we fit are covered by the manufacturer’s 12-month guarantee.
Frequently Asked Questions
Why is my boiler working but no heat coming through?
My hot water works but the heating does not, why?
On a combi that usually points to the diverter valve sticking in the hot water position. It is a common fault and needs a Gas Safe engineer, as the valve is inside the boiler.
Why is my boiler hot but the radiators are cold?
That points to a circulation problem, most often a failed or seized pump, or sludge restricting flow. Our guide to a central heating pump not working covers this.
Can low pressure stop my radiators heating?
Yes. If the pressure gauge is below about 1 bar, top up via the filling loop to between 1 and 1.5 bar and reset. If it keeps dropping, there is a leak to find.
Why are only some radiators cold?
That usually means trapped air or a valve issue rather than the boiler. Bleed the affected radiators and check their valves are open. Our guide to radiators not heating up fully explains the checks.
Helpful resources
- Gas Safe Register: check an engineer is registered to work on your boiler.
- Energy Saving Trust: independent guidance on running an efficient heating system.
Spotted Any of These Warning Signs?
Boiler running but the radiators stay cold? 0800 Homefix has Gas Safe registered engineers across London and the South East who can find why the heat is not getting through and put it right, seven days a week, 365 days a year.
Editorial note: This article is intended as a general homeowner guide. All plumbing work on supply pipes should be carried out by a qualified, WaterSafe-registered plumber. If you suspect a gas leak, leave the property immediately and call the National Gas Emergency Service on 0800 111 999.
