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If you have hot water but no heating, check the simple things first: the room thermostat, the heating timer, the boiler pressure and your radiator valves. If they are all fine, the likely cause is the diverter valve on a combi boiler, the heating zone valve on a system boiler, or the circulation pump. Those need a Gas Safe engineer.
A combi boiler uses a diverter valve to send hot water either to your taps or round the radiators. If it sticks in the hot water position, the taps run hot but the radiators stay cold. It is inside the boiler, so it needs an engineer.
Homes with a hot water cylinder usually have a motorised valve that opens the heating circuit when the thermostat calls for heat. If its motor fails, the cylinder still heats but the radiators do not.
The pump pushes hot water round the radiators. If it has seized or failed, the radiators stay cold. A pump that is silent, or making a grinding noise, is a common sign.
If the heating comes on but some radiators stay cold, air or sludge may be blocking the flow. Bleed the radiators first. Radiators cold at the bottom usually need a power flush.
| What you notice | Most likely cause |
|---|---|
| Combi: hot taps work, all radiators cold | Diverter valve stuck. |
| Cylinder heats, radiators cold | Heating zone valve or its motor. |
| Boiler fires but radiators stay cold, pump silent | Circulation pump. |
| Some radiators warm, some cold | Air, sludge or stuck TRVs. |
| Boiler shows a low-pressure code | Low pressure. Top up. |
Safe for you to try
Gas Safe engineer only
Always check your engineer is on the Gas Safe Register.
From the job: Greenstar 28i Junior, radiators stone cold
Worcester Greenstar 28i Junior, hot water fine but radiators stone cold. The diverter valve had stuck on hot water. Drained the boiler and replaced the valve, about two hours from start to finish.
Expert notes by Jamie Watkins, Gas Safe registered engineer (Gas Safe ID 6115118), Homecure Plumbers – Gas Safe Reg 505421
Repairs from £95 + VAT for the first hour, including diagnosis (or a fix if it can be done within the first hour with no parts needed), no callout charge. Typical costs, including parts and labour, fitted:
| Repair | Typical cost |
|---|---|
| Diverter valve replacement | from £248 + VAT |
| Motorised zone valve replacement | from £275 + VAT |
| Pump replacement | from £289 + VAT |
Subject to diagnosis; prices vary depending on boiler model and job complexity. For motorised zone valves, prices vary depending on the job and access. Prices apply during office hours; parking or the congestion charge may be added.
Book a boiler repair or a central heating repair, or call 0203 504 4000.
Most often the diverter valve has stuck in the hot water position. Check the thermostat, timer and pressure first.
No. It is inside the boiler, so it needs a Gas Safe registered engineer.
Sources: your boiler manual, for example Worcester Bosch manuals · Gas Safe Register