Boiler on but no heating — what's going on
This is the classic 'summer break, winter fail' fault. Nine times out of ten it's a stuck diverter valve or a seized pump — both very fixable — but a few quick checks at home first will save you a call-out.
Make it safe — before anything else
Only Gas Safe registered engineers work on gas
Anything behind the boiler casing is Gas Safe only. Externally you can safely check pressure, thermostat batteries and radiator valves.
Don't run the boiler dry
If pressure has fallen to zero, don't keep firing it — the pump will overheat. Top up carefully to 1.2 bar via the filling loop and no more.
What Kevin (our AI) will check first
Message Kevin and he'll ask exactly these questions before we send an engineer, so we bring the right parts on the first van.
- Do you have hot water at the kitchen tap? Yes / no changes the diagnosis completely.
- Boiler make, model, and pressure gauge reading.
- Are ALL radiators cold, or just some?
- Any error code flashing on the boiler display?
- Smart thermostat brand and current schedule.
Common causes of a boiler on but no heating
Stuck diverter valve
The diverter routes hot water to either the taps or the radiators. When it seizes in the 'hot water' position after summer, you get taps but no heating. Replacement £180 – £320 fitted.
Seized central-heating pump
The pump circulates hot water through the radiators. Common failure after long idle periods. A Gas Safe engineer can sometimes free it, otherwise £220 – £340 fitted.
Motorised zone valve stuck (system / regular boilers)
S-plan and Y-plan systems use motorised valves to open the heating circuit. When the actuator fails, the boiler runs but no flow reaches the radiators. £150 – £240 fitted.
Low system pressure lockout
If pressure drops below ~0.5 bar, most combis lock the heating circuit out as a safety measure. Top up to 1.2 bar cold. If pressure keeps falling, you have a leak — book a diagnostic.
Thermostat or programmer failure
Flat batteries, wrong schedule, or a dead room stat. Free to check yourself before booking a call-out.
Honest cost band — why we won't quote blind
Typical
£90 – £180 for pressure top-up, thermostat swap or programmer reset
Complex
Diverter valve £180 – £320. Pump £220 – £340. Motorised valve £150 – £240.
Every boiler visit starts with a £50 diagnostic and combustion check — you get one fixed written price, not a running meter.
Fixed £50 diagnostic
Flat fee. No hourly meter running.
Fair quote or refund
Transparent pricing — refunded if we don't fix it.
Vetted UK engineers
DBS-checked, insured, real reviews.
AI-triaged in seconds
KJ chats first so the right engineer arrives.
Related emergencies
See all in the emergency plumbing hub — or the leaks & water damage hub.
Areas we cover for boiler on but no heating
Same-day across Greater Manchester and Rochdale.
Boiler on but no heating — FAQs
Dealing with a boiler on but no heating? Chat with Kevin now
Real triage in seconds. £50 diagnostic visit — refunded if we don't fix it.
Related hubs: emergency plumbing, leaks & water damage, all services.
Chat with Kevin about Boiler on but no heating