If your hot water works but your central heating doesn’t, the boiler is still able to produce heat, but something may be preventing the heating system from requesting or circulating it. The cause could be as simple as an incorrect thermostat or programmer setting, but faults with valves, controls or circulation can produce similar symptoms.

Start by checking that the heating is switched on, the thermostat is requesting heat and the boiler pressure is within its normal range. Also check whether all of your radiators are cold or only some of them, as this can help narrow down the problem.

What should you check first?

Before assuming that something inside the boiler has failed, rule out the simpler causes.

Check the following:

• Make sure central heating is switched on rather than the boiler being in hot-water-only or summer mode.
• Check that the programmer or timer is currently set to provide heating.
• Turn the room thermostat above the current room temperature and see whether the boiler responds.
• Replace the batteries if you have a wireless thermostat and its display is blank or unreliable.
• Check the boiler pressure against the range specified by the manufacturer.
• Look for a fault code or warning symbol on the boiler display.
• Check whether every radiator is affected.

Avoid repeatedly resetting the boiler if it continues displaying a fault. Note the code and check the manufacturer’s instructions instead.

Are all the radiators cold?

This is one of the most useful things to establish.

If all the radiators are cold, the problem is more likely to involve the heating controls, boiler pressure, a valve, circulation or another part of the central-heating system.

If only one or two radiators are affected while the others heat normally, the boiler is clearly supplying heat to the system. The problem is more likely to be local to those radiators.

Trapped air, closed or faulty valves, poor balancing and sludge are among the possible reasons for radiators not heating up properly.

This distinction can save you from looking for a boiler fault when the problem is actually elsewhere in the heating system.

Check the thermostat and heating programmer

If hot water is working normally but none of the radiators heat up, check whether the boiler is actually receiving a request for central heating.

Start with the room thermostat. Set it several degrees above the current room temperature. If it’s wireless, check its display and batteries.

Next, check the programmer or timer. A power cut, clock change or accidental adjustment can leave heating schedules set incorrectly. Make sure the current time is correct and that the heating is scheduled to be on.

Some boilers and heating controls also have a summer or hot-water-only mode that disables central heating while continuing to provide hot water.

If the controls appear correct but the boiler doesn’t respond when you request heating, there may be a fault with the thermostat, receiver, wiring or another control component.

Check the boiler pressure

Low pressure can prevent a sealed heating system from operating normally.

Look at the pressure gauge or digital display when the system is cool and compare the reading with the range specified in your boiler manual. Many systems operate at approximately 1–1.5 bar when cold, but the correct range varies between appliances.

If you’re unsure what your boiler pressure should be, check the manufacturer’s instructions before attempting to change it.

If the pressure is repeatedly falling rather than simply needing an occasional adjustment, investigate the cause instead of continually topping up the system. Repeated pressure loss can indicate a leak, pressure relief valve problem, expansion-vessel issue or another fault.

Combi boiler: could the diverter valve be faulty?

If you have a combi boiler, a diverter-valve problem is one possible explanation for having hot water but no central heating.

A combi has to provide both domestic hot water and central heating without using a separate hot-water cylinder. The boiler therefore needs to direct heat according to which service is required.

If the diverter mechanism becomes stuck or develops a fault, the boiler may continue providing hot water to your taps while failing to send heat to the radiators.

This symptom doesn’t prove that the diverter valve has failed. Thermostat, pressure and circulation problems can produce similar results, which is why the basic checks should come first.

The diverter valve is an internal boiler component. Diagnosis and repair should be left to a Gas Safe registered engineer rather than attempting to access it yourself.

System or conventional boiler: check the heating zone

Homes with system or conventional boilers usually have a hot-water cylinder, so the arrangement is different from a combi.

Many systems use motorised valves to control where heated water travels. Depending on the system design, a valve may direct flow towards the hot-water cylinder, central heating or both.

A faulty motorised valve or actuator can therefore result in the cylinder continuing to receive heat while the radiators don’t.

Again, working hot water doesn’t establish that the valve is definitely responsible. The programmer, thermostat and other controls involved in requesting central heating should also be checked.

If the fault involves electrical controls or a motorised valve, it should be diagnosed and repaired by an appropriately qualified professional.

Could there be a circulation problem?

The boiler can receive a heating demand and produce heat, but that heat still needs to circulate through the radiators.

A circulation-pump fault is one possible reason why this doesn’t happen. Blockages, sludge and other circulation problems can also restrict movement through the heating circuit.

Possible signs include the boiler becoming hot while the radiators remain cold, unusual noises or some parts of the system heating while others don’t.

However, these symptoms can have several causes. Don’t remove the boiler casing or attempt to test internal pumps or electrical components yourself.

If only certain radiators are affected, the problem may instead involve trapped air or poor distribution. Balancing your radiators can help when some radiators heat much faster than others, but it won’t fix a failed pump, valve or heavily restricted system.

What can the symptoms tell you?

Different patterns can help narrow down where to look:

What you notice	Possible cause
Hot water works, all radiators cold	Controls, pressure, valve or circulation problem
Combi provides hot water but no heating	Diverter valve is one possibility
Cylinder heats but radiators don’t	Motorised/zone valve or heating controls may be involved
Only one radiator is cold	Air, TRV, valve or local blockage
Some radiators heat much faster than others	System may need balancing
Boiler pressure is very low	Heating may not operate normally
Boiler displays a fault code	Check the manufacturer’s instructions
Boiler fires but repeatedly stops	Controls, circulation or another fault may need investigation

These symptoms aren’t enough to diagnose a component conclusively, but they’re useful information to provide to an engineer.

What if the boiler keeps firing and stopping?

If the boiler responds when you request heating but repeatedly starts and stops without warming the radiators properly, the issue may be different from a boiler that doesn’t respond at all.

Restricted circulation, controls, boiler sizing and several faults can contribute to frequent cycling. Our guide to why a boiler keeps turning on and off explains that symptom in more detail.

If the boiler displays an error code when it stops, check the manufacturer’s instructions rather than repeatedly resetting it.

When should you call a heating engineer?

Call an engineer if the basic settings are correct but the heating still doesn’t work, particularly if every radiator is cold.

Professional diagnosis is also appropriate if you suspect a diverter valve, motorised valve, circulation pump or internal control fault; the boiler repeatedly locks out; pressure won’t remain stable; or you notice unusual noises, leaks or fault codes.

Gas boiler work should be carried out by a Gas Safe registered engineer who is qualified for the appliance and work involved.

Before the engineer arrives, note whether you have a combi or a boiler with a hot-water cylinder, whether every radiator is affected, the boiler pressure, any fault code and what happens when you increase the thermostat setting. These details can make troubleshooting easier.

Frequently asked questions

Why do I have hot water but no heating with a combi boiler?

Start by checking the thermostat, heating schedule, boiler mode and pressure. If these are correct, possible causes include a diverter-valve problem, circulation fault or heating-control issue.

Does working hot water mean the boiler isn’t broken?

No. It shows that the system can still produce domestic hot water, but it doesn’t prove that every component required for central heating is working correctly.

Why does my boiler fire for hot water but not heating?

The boiler may not be receiving a heating demand because of a thermostat, programmer or control problem. A valve or another component involved specifically in central heating may also be faulty.

Why is only one radiator cold when my hot water works?

If the other radiators heat normally, the problem is more likely to be local to that radiator. Possible causes include trapped air, a closed or faulty valve, or a blockage.

Can I fix hot water but no heating myself?

You can safely check accessible controls, thermostat settings, boiler mode, pressure and fault codes according to the manufacturer’s instructions. Internal boiler, gas, electrical, valve and pump repairs should be handled by an appropriately qualified professional.

Sources

HSE, Gas safety – Home owners (https://www.hse.gov.uk/gas/domestic/faqownerocc.htm)

Accurate as of: 15 September 2026