If your hot water is working but your central heating won’t come on, it usually means one part of the system isn’t working properly rather than a complete boiler failure. This is good news because it often points to a specific issue you can identify with a few simple, safe checks at home. By narrowing down the cause, you can decide if it’s something straightforward you can fix yourself or if you need to call a qualified heating engineer. For example, it might be a problem with the thermostat settings, a stuck valve, or an issue with the pump—each of which affects heating but not hot water. Taking these initial steps can save you time and avoid unnecessary repair costs.
Start with the basics: controls and settings
Before assuming there is a fault, check that your boiler and heating controls are actually asking the system to turn on. Many heating callouts in autumn are caused by settings rather than broken parts.
First, look at the boiler display. Make sure it is on and set to provide heating and hot water, not just hot water. Some boilers have separate modes or symbols, so check for a radiator icon as well as a tap icon.
Next, look at your programmer or timer. Confirm that:
The central heating channel is set to "On" or in an active time period
The clock is showing the correct time and day
Holiday, frost or eco modes are not cancelling your normal schedule
Now check your room thermostat. Set it a few degrees higher than the current room temperature, for example from 18°C up to 22°C, and wait a couple of minutes to see if the boiler fires up. If your thermostat is battery powered, replace the batteries, as weak batteries can stop it sending a signal even if the display still lights up.
If, after these checks, the boiler does not try to start for heating at all, there may be a control or wiring issue that needs a professional diagnosis.
Motorised valve checks you can do safely
In many homes with hot water cylinders, a motorised valve decides whether hot water from the boiler goes to the cylinder, the radiators or both. If the hot water side works but the radiators stay cold, the valve may be stuck or its motor (actuator) may have failed.
You will usually find the motorised valve near the hot water cylinder or close to where several pipes join. It often has a small metal or plastic box on top with wires coming out.
Things you can safely observe without opening anything include:
Listen and feel: When the heating is calling, you may hear or feel a gentle whir or click from the valve head as it tries to move. If there is complete silence, the motor may not be working.
Check the manual lever: Many valves have a small lever underneath or on the side marked "Auto" and "Man". With the power off at the fused spur, you can gently move this. If it is very stiff or will not move, the valve body may be stuck. Do not force it, and do not remove covers or screws, as this can expose electrical parts or release water.
If the valve feels stuck, or the boiler fires only for hot water but not for heating, that is a strong clue the motorised valve or its actuator needs attention from a heating engineer.
System pressure and water flow
If you have a combi boiler or a sealed system, there will usually be a pressure gauge on the front of the boiler or nearby pipework. This shows the water pressure inside the heating system, not your mains water pressure.
When the system is cold, most boilers want the pressure to be around 1.0 to 1.5 bar. If it has dropped close to zero, the boiler may refuse to run the heating, or it may start and then shut down. Hot water can sometimes still work because it uses a different circuit.
If your manufacturer's instructions show how to top up the pressure using a filling loop, you can follow that step by step. Only do this if you are confident, and never leave the filling loop open or exceed the recommended pressure. If the pressure keeps dropping, there may be a leak or expansion vessel problem that needs professional help.
On older open-vented systems with a small tank in the loft, low water level or a stuck ball valve can also affect heating flow. You should not walk in a loft without safe access and lighting, so if you suspect this, it is better to call an engineer.
Radiator valves and TRVs after summer
It is very common for radiator valves, especially thermostatic radiator valves (TRVs), to stick shut after months of being turned off in warmer weather. The boiler may be working, but heat cannot circulate into the radiators.
Check that each radiator has both valves open. The TRV (with numbers 0 to 5) should be turned up to a higher setting. The lockshield valve at the other end is usually set with a plastic cap or square spindle; make sure it has not been accidentally closed.
If a TRV head turns freely but the radiator stays stone cold while others warm, the pin underneath may be stuck. You can often remove the plastic TRV head by unscrewing a ring or clip, then gently press the small metal pin down with a blunt object. It should spring back up. Do not pull the pin out, and do not hit it.
If several radiators are cold or only warm at the bottom, there may be sludge in the system. In that case, a professional power flush can help restore proper flow. If you are in Bletchley, it is worth looking at our Bletchley power flushing service for more information on clearing sludge from heating systems.
Pump or airlock clues
The circulation pump moves hot water from the boiler around the radiators. If it is not running properly, some or all radiators will stay cold even though the boiler fires.
Signs that point towards a pump or airlock issue include:
Gurgling or rushing water sounds in pipes or radiators
Only the closest radiators to the boiler heat up; distant ones stay cold
The pump feels very hot to the touch but pipes after it are cool
Most modern pumps are inside the boiler casing or in nearby pipework. You should not remove any covers, but you can listen. A healthy pump usually has a gentle hum or whir when the heating is on. Loud grinding, buzzing or silence when heating is on can suggest a fault.
Air trapped in the system can also stop circulation. Bleeding radiators with a proper radiator key can help, but always protect carpets, have a cloth ready, and do not over-bleed, or you can drop the system pressure too far. If you keep getting air, or the same radiators never heat properly, have an engineer check for a deeper issue.
Stop and call a Gas Safe engineer if…
Some warning signs mean you should stop troubleshooting and get a professional to attend. Safety always comes first with gas and pressurised systems.
You should contact a Gas Safe registered engineer immediately if you notice:
Smell of gas anywhere in the property or at the boiler
Boiler error codes that you cannot clear with a single reset following the manual
Repeated lockouts or the boiler constantly shutting itself down
Visible leaks, dripping joints or staining around the boiler or cylinder
If you ever smell gas, turn off the gas at the meter if it is safe to do so, open windows, and call the National Gas Emergency Service immediately on 0800 111 999.
Next steps if your heating still will not start
If you have worked through these checks and your central heating is still not working, there is likely a fault with a component such as the motorised valve, pump, controls or boiler internals. At this stage, it is more efficient and safer to have a professional take over.
For homeowners in Milton Keynes, Newport Pagnell and Bletchley, B.Brill Plumbing & Heating LTD can carry out expert diagnostics on all types of central heating systems. We can pinpoint issues with controls, valves, pumps, sludge build-up and boiler faults, and advise on repair options that suit your home and budget.
If you suspect a boiler issue, you can book a visit via our boiler repair service page. For wider system problems such as cold radiators, circulation faults or suspected sludge, see our central heating repair options and our specialist Bletchley power flushing service.
To get your heating back up and running quickly and safely, call B.Brill Plumbing & Heating LTD on 01908410370 and our team will be happy to help.
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