If the hot water works but the heating does not, the boiler itself is almost certainly fine, because it is clearly firing and heating water. The fault is in whatever tells the boiler to heat the radiators or sends the hot water to them: the thermostat or programmer, the diverter valve inside a combi, the motorised valves on a system with a cylinder, or the pump. Most of these are common, well-understood faults that an engineer fixes in a single visit, and a couple of them are things you can check yourself in a few minutes.
This guide works through the causes in the order an engineer would, starting with the controls and finishing with the components inside the boiler.
1. The controls are not calling for heat
- Room thermostat. Turn it well above the room temperature. Replace the batteries in a wireless thermostat and check it has reconnected to its receiver.
- Programmer. Make sure it is set to heating and hot water, not hot water only, and that the current time falls inside a heating period. A programmer still on its summer setting is the commonest cause of this fault in autumn.
- Smart thermostat. Check the app for a holiday mode or an off schedule, and that the thermostat is online.
- Radiator valves. If every thermostatic radiator valve is turned to frost or off, the pump has nowhere to send the water and the boiler may shut down. Open a few.
2. Low pressure
Some combis will heat tap water at a low pressure but refuse to run the heating. Check the gauge; if it is below 1 bar, top the system up to about 1.3 bar and try again. Our guide on how to repressurise a boiler shows how.
3. The diverter valve (combi boilers)
A combi has a diverter valve that switches the boiler’s output between the taps and the radiators. Hot water takes priority, and when no tap is running the valve should move to the heating position. If it sticks in the hot water position, you get hot taps and cold radiators. A valve that sticks after summer sometimes frees itself after a few heating cycles; one that has worn needs replacing by an engineer. The diverter is inside the boiler and is one of the most common combi repairs.
4. Motorised valves (system and conventional boilers)
Systems with a hot water cylinder use two-port or three-port motorised valves to direct hot water to the cylinder, the radiators or both. If the heating valve’s motor fails or the valve seizes, the cylinder heats and the radiators do not. The give-away is a valve that is hot on the boiler side and cold on the radiator side, or a valve with a manual lever that will not move. Motorised valve heads are a common, quick replacement.
5. The pump
On a combi, the pump runs for both hot water and heating, so a dead pump usually kills both. On a cylinder system, the pump may only be needed for the heating, so a seized pump gives exactly this symptom. A pump that hums but does not turn, or is hot to the touch, has seized. Tapping the casing occasionally frees it for a while, but it is on its way out.
6. Air locks and blockages
A large air lock in the heating circuit, or a sludge blockage at the pump or a valve, can stop the radiators heating while the shorter hot water circuit works. Bleeding the radiators and checking for the sludge pattern, cold at the bottom and hot at the top, points the way. Our guide to radiators cold at the bottom explains the sludge case.
7. Boiler controls
A fault on the printed circuit board or a failed thermistor can leave the boiler responding to hot water demand but not to the heating call. These are diagnosed with the boiler casing off and are engineer-only.
What you can check yourself
- Thermostat up, batteries fresh, programmer on heating and in a heating period.
- A few radiator valves open.
- Pressure above 1 bar.
- Bleed a couple of radiators to rule out air.
- Run the heating for twenty minutes and feel the pipes at the boiler and at any motorised valves.
If the boiler fires on a heating call but the radiators stay cold, the fault is a valve, pump or blockage. If the boiler does nothing at all on a heating call, it is the controls or the boiler’s board.
How DCI Boilers can help
DCI Boilers repairs diverter valves, motorised valves, pumps and controls across Cornwall and Devon, with Gas Safe and OFTEC registered engineers who carry the common parts and quote a fixed price before starting. Book boiler repairs, or if the house is cold, our emergency boiler call-out runs around the clock.
Need help now? Call DCI Boilers on 07888 862 603 or get a free quote.
Frequently asked questions
Why do I have hot water but no heating on my combi?
Most often a stuck diverter valve, a control that is not calling for heat or low pressure. The boiler itself is working.
Can a diverter valve be repaired?
Sometimes the diaphragm or motor can be replaced; often the whole valve is swapped. Either way it is a routine engineer’s job.
Is this an emergency?
Not usually, since you have hot water. In a cold snap with vulnerable people in the house, treat it as urgent.
Why did this start after summer?
Valves and pumps that have not moved for months stick. Running the heating briefly once a month through summer prevents it.