When your hot water is working but the radiators stay cold, it is not only frustrating, it is also confusing. The boiler clearly runs, yet the central heating side of the system is not doing its job. A few simple checks can rule out easy fixes and help you decide when it is time to call a heating engineer.
How your system gives priority to hot water and heating
Modern boilers and controls can send heat in different directions depending on demand. With a system or conventional boiler, motorised zone valves send hot water either to your hot water cylinder or to the radiators, or sometimes both.
With a combi boiler, a diverter valve inside the boiler switches heat between your hot water taps and the radiators. If any of these components stick or fail, you may get hot water only while the radiators stay cold.
Quick homeowner checklist before calling for help
Before assuming there is a serious fault, work through a few simple checks. These are safe for most homeowners and can save you time when speaking to an engineer.
Confirm the central heating is actually set to "on" and timed correctly
Turn the room thermostat up several degrees higher than the current room temperature
Check the boiler pressure gauge (sealed systems) and top up if safely able
Make sure radiator valves are fully open on a couple of test radiators
Note any error lights or codes showing on the boiler or controls
If at any point you feel unsure, stop and contact a professional. Do not remove boiler casings or covers, as this is not safe and, for gas appliances, should only be done by a Gas Safe registered engineer.
Controls and thermostat issues that stop heating
Surprisingly often, the central heating simply is not being asked to come on. Check your programmer or timer first. The heating channel should be set to "on" or to a schedule that matches the time you are testing.
Next, look at your room thermostat. Turn it up above the current room temperature and listen for a click. On wireless stats, check the batteries and make sure the receiver by the boiler is showing a normal signal light. A failed or flat thermostat can stop the boiler from starting the heating, even though hot water still works.
Zone valves and diverter valves preventing radiator heat
Motorised zone valves on system boilers
On system or conventional boilers with a hot water cylinder, motorised zone valves control where the hot water from the boiler flows. One valve usually feeds the radiators, and another feeds the cylinder. If the heating valve sticks shut or its head fails, only the cylinder will heat up.
Signs of a stuck heating zone valve include hot pipes to the cylinder but cold flow pipes to the radiators when heating is "on". You may also hear the boiler cycle briefly then switch off, as water cannot circulate through the radiators.
Diverter valves on combi boilers
Combi boilers use a diverter valve to swap between hot water for taps and heating water for radiators. When this valve sticks in the hot water position, you will still get showers and taps, but your radiators stay cold.
This is an internal boiler component and should not be accessed by a homeowner. A Gas Safe engineer can test and replace it if needed, often after checking that controls and pressure are all in order.
Pressure, airlocks and pump problems
Most sealed systems rely on correct water pressure to push hot water around. Check the boiler pressure gauge when the system is cold. It is usually marked with a green band around 1 to 1.5 bar. If it is below this and you know how to use your filling loop, you may carefully top it up following the manufacturer’s instructions.
A failed or stuck circulation pump can also leave radiators cold while the boiler fires briefly then shuts down. You might notice the boiler getting hot very quickly, unusual noises, or a lack of the gentle humming you expect from the pump.
Airlocks and sludge can restrict flow too. Air tends to collect at the top of radiators, causing the top section to be cool while the bottom is hotter. Sludge usually does the opposite, causing cold spots at the bottom and noisy pumps or pipes.
Safe bleeding and radiator valve checks
If you are confident, you can try bleeding just one radiator with a radiator key and a cloth. Turn the heating off first and have a towel ready. Release a small amount of air until water appears, then close the valve firmly.
After bleeding, check your boiler pressure again, as it may drop slightly. Also make sure the valves at each end of the radiators are fully open, especially after decorating or any recent work where they might have been turned down.
When symptoms suggest sludge and circulation issues
If some radiators heat up and others stay cold or only get warm at the top, sludge or poor system balancing may be the culprit. You may hear the pump sounding rough or the boiler kettling, which is a loud boiling or banging noise.
In these cases, a professional power flush or targeted cleaning may be recommended, along with checking radiator valves and balancing the system so each radiator gets its fair share of flow.
What your heating engineer will test
Once you have done the basic checks, a qualified engineer can carry out more detailed diagnostics. For gas boilers, they must be Gas Safe registered to remove the casing and work on internal components legally and safely.
They will typically check:
Pump operation, speed settings and electrical supply
Motorised zone valve or diverter valve movement and power to the valve
Flow and return temperatures across the boiler heat exchanger
System balancing and whether certain radiators are starved of flow
The engineer can then advise on repair options, from replacing a faulty valve or pump to cleaning the system and adjusting controls so everything works together properly.
Staying safe and knowing when to call in help
It is important not to remove boiler covers, bypass safety devices or attempt gas work yourself. Apart from being unsafe, this can also invalidate warranties and break the law. Your role as a homeowner is to carry out simple external checks and provide clear information to your engineer.
If your hot water works but the radiators will not heat after you have checked controls, pressure and valves, it is time to book a professional visit.
Need help with no heating problems?
If you are still without heating, or you suspect a stuck valve, pump fault or sludge in the system, it is best to get expert support. For friendly, professional boiler repair, contact Beales Services on 02034880883 and we will help get your heating running properly again.
If you have cold spots on radiators, noisy pipework or a pump that sounds strained, ask Beales Services about a power flush assessment at the same time. Tackling circulation issues early can improve comfort, cut running costs and extend the life of your central heating system.
