Hot water but no central heating: what to check, and when to stop
If the taps run hot but the radiators stay cold, the problem is likely to be on the heating side: the programmer, the thermostat, the pressure, the radiator TRVs, or a valve or pump that needs an engineer. Run the safe checks below first. If the radiators are still cold, request a repair callback or call 0333 772 6247. One-off repair visits run Monday to Friday, 09:00 to 17:00, and a £95 call-out fee applies, paid in advance.
The short answer
On a combi, hot water at the taps tells you the boiler has power and a gas supply and can light, so the fault is not the gas or electricity supply. It is in one of three places. The heating is not being asked for, because a programmer, timer or thermostat is not calling for it. The heat is being sent the wrong way, because a valve that chooses between hot water and heating is stuck. Or the heat is not getting round, because the pressure is low, the radiator TRVs are turned down or the pump is not moving the water.
The first group you can check yourself in a few minutes. The last two groups mostly need a Gas Safe registered engineer, and the checks below tell you which side of that line you are on.
If you have a hot water cylinder, hot taps prove less. The cylinder stores hot water, so it can stay hot for hours after the boiler stops, and many cylinders also have an electric immersion heater. Check whether the immersion is switched on, and whether the boiler fires when the programmer calls for hot water, before you rule the boiler out.
Central heating not working but hot water is? What it can mean
| What you are seeing | Possible cause | What to do |
|---|---|---|
| Radiators cold, hot taps fine, the programmer shows hot water only | The heating is not switched on at the programmer or timer, sometimes left that way after summer | Set the programmer or timer so the heating is on, then turn the thermostat up. |
| Radiators cold, thermostat display blank or not calling for heat | Flat batteries or a thermostat set below room temperature | Change the batteries and set it a few degrees above the room. Our thermostat guide covers the rest. |
| Pressure gauge well below the level your manual gives | Low system pressure. On many boilers this stops the heating and hot water together, so check the gauge anyway | Top it up only as your boiler manual sets out. If it keeps dropping, book an engineer. |
| One or two radiators cold, the rest warm | A TRV turned down or stuck, trapped air or sludge in those panels, not the boiler | Check the TRV on each cold radiator is turned up. Our cold radiator guide explains the patterns. |
| Every radiator cold, all controls calling for heat, combi boiler | A diverter valve stuck on the hot water side, inside the boiler | Engineer job. Do not open the casing. |
| Every radiator cold, all controls calling for heat, boiler with a cylinder | A motorised valve on the pipework, the pump or the controls wiring | Engineer job. Describe exactly what the controls show when you book. |
| A fault or lockout code on the display | Depends on the code and the manufacturer | Look it up in your own boiler manual. Reset once only if the manual says so. |
Safe checks you can do yourself
Work through these in order. None of them involves opening the boiler casing or touching any gas part, and you should not go further than this list.
- The programmer or timer. Check the heating channel is switched on, not only the hot water channel, and that the clock is set to the right time. A heating channel switched off over the summer is easy to miss.
- The room thermostat. Turn it up a few degrees above the current room temperature. If it is wireless and the display is blank or faint, change the batteries first.
- The heating setting on the boiler itself. Some boilers have their own heating dial or a hot water only mode. Check it against your boiler manual and set it so the heating is on.
- The pressure gauge. With the system cold, compare the reading with the figure in your boiler manual. If it is low, top it up only in the way your manual describes, using the filling loop. If you cannot find the loop, or the pressure drops again, stop and book an engineer.
- The radiator valves. Check the thermostatic valve (TRV) on each cold radiator is turned up. A TRV pin can stick after a summer switched off. The valve at the other end (the lockshield) is set by the installer, so leave it to an engineer.
- One reset, if the display shows a lockout. Press the reset button once, and only if your manual says a reset is the right step for that code. If the fault comes back, do not keep resetting it.
When the checks do not help
If the controls are all calling for heat, the pressure is right and the TRVs are turned up, but the radiators still stay cold while the taps run hot, the fault is inside the heating system. What it is depends on the type of boiler you have.
On a combi, the part to suspect is the diverter valve. It is the part inside the boiler that sends the heat either to the radiators or to the taps, and when it sticks on the hot water side the boiler keeps making heat that never reaches the heating circuit. Our guide to a faulty diverter valve, linked below, explains how the valve works and how an engineer tells it apart from a thermistor or control board fault.
On a system or heat-only boiler with a hot water cylinder, the job is usually done by a motorised valve out on the pipework rather than inside the boiler. The pump that pushes water round the radiators, and the wiring that links the controls to the boiler and valves, can produce the same cold radiators too. All of these involve mains wiring or parts of the heating system that are not homeowner jobs.
Whichever it is, leave the casing closed. Work on a gas appliance must be done by a Gas Safe registered engineer, and you can check any engineer's registration on the Gas Safe Register before they start.
What to tell us when you request a callback
A clear description saves time on the first visit. Have these ready when you request a repair callback or call 0333 772 6247.
- Whether you have a combi, or a boiler with a separate hot water cylinder.
- The make and model from the boiler's data badge, if you can read it without opening anything.
- Any code on the display, written down exactly as it shows.
- Whether the radiators stay cold even when no hot tap is running and the thermostat is calling for heat.
- The pressure reading, and whether you have had to top it up recently.
- Which of the checks on this page you have already tried.
If you smell gas, stop
None of the checks on this page applies if you can smell gas, hear hissing near the boiler or pipework, or your carbon monoxide alarm is sounding. Do not touch any electrical switch. Turn the gas off at the meter if it is safe to, open the doors and windows, leave the property and call the National Gas Emergency Service on 0800 111 999. Do not smoke or use a naked flame. Only go back in once you have been told it is safe. If your carbon monoxide alarm has sounded, or anyone has headaches, dizziness or nausea, switch the appliance off if it is safe to, get fresh air and see a doctor, telling them you may have been exposed to carbon monoxide.
Getting it fixed today, and cover for next time
For the fault you have now, the route is a paid one-off repair. Request a repair callback or call 0333 772 6247. One-off repair visits run Monday to Friday, 09:00 to 17:00, and a £95 call-out fee applies, paid in advance. The engineer diagnoses the fault and tells you what is needed before any further work goes ahead. A one-off repair is a pay-per-visit service and carries no workmanship guarantee. You do not need a cover plan to book one.
A cover plan is for breakdowns you have not had yet. A fault the boiler is already showing counts as pre-existing, so cold radiators today are a repair decision, not a cover decision. For the future, the boiler and central heating module pays parts and labour up to £500 in each 12-month period while the boiler is under 7 years old, or £200 once it is over 7. Smart Plan is a service plan, not insurance.
Hot water but no heating: FAQs
Why is my central heating not working but the hot water is?
Because the boiler is lighting but the heat is not reaching the radiators. The simple causes are a programmer or timer not set to heating, a thermostat turned down or with flat batteries, low boiler pressure, or radiator TRVs turned down or stuck. If those checks do not help, the part to suspect on a combi is a diverter valve stuck on the hot water side, and on a boiler with a cylinder a motorised valve, the pump or the controls wiring. Those need an engineer.
Can I fix hot water but no heating myself?
You can check the programmer or timer, the thermostat and its batteries, the heating setting on the boiler, the pressure gauge, and the TRVs on the radiators. You can top up the pressure and press reset once, but only as your boiler manual describes. Never open the boiler casing or touch any gas part. Anything inside the boiler must be done by a Gas Safe registered engineer.
Is it the diverter valve?
On a combi, once the simple checks are ruled out, the diverter valve is the part that decides where heat goes, so a valve stuck on the hot water side gives exactly this symptom. It sits inside the boiler, so only a Gas Safe registered engineer can confirm it. Boilers with a separate cylinder usually use a motorised valve on the pipework instead.
Can low boiler pressure cause no heating but hot water?
It can contribute, although on many combis low pressure stops the hot water too. Check the gauge with the system cold and compare it with the figure in your boiler manual. Top it up only as the manual describes. If the pressure keeps falling after a top-up, there is a leak or a faulty part somewhere, and that needs an engineer rather than another top-up.
Boiler working but not heating the radiators?
If the boiler fires and the taps run hot but the radiators stay cold, work through the same checks: the programmer or timer heating channel, the room thermostat and its batteries, the pressure gauge against your boiler manual, and the TRVs on the radiators. If the controls are calling for heat and the radiators are still cold, the fault is likely a diverter valve on a combi, or a motorised valve, the pump or the controls wiring on a boiler with a cylinder. Those are jobs for a Gas Safe registered engineer.
How much does it cost to get an engineer out?
A one-off repair visit carries a £95 call-out fee, paid in advance, and visits run Monday to Friday, 09:00 to 17:00. The engineer diagnoses the fault and tells you what is needed before any further work goes ahead. The cost of the repair itself depends on what the engineer finds.
Will a cover plan pay for this fault?
Not if the fault is already showing, because a problem the boiler already has counts as pre-existing. For future breakdowns, the boiler and central heating module pays parts and labour up to £500 in a 12-month period while the boiler is under 7 years old, or £200 once it is over 7, subject to the module's exclusions. Smart Plan is a service plan, not insurance.
Radiators still cold? Request a repair callback
If the safe checks have not brought the heating back, request a repair callback or call 0333 772 6247. One-off repair visits run Monday to Friday, 09:00 to 17:00, and a £95 call-out fee applies, paid in advance. For future breakdowns, the boiler and central heating module pays parts and labour up to £500 in each 12-month period while the boiler is under 7 years old, or £200 once it is over 7. Under a cover plan we aim to get an engineer out to you within two working days, with call-outs Monday to Friday, 08:00 to 18:00, through a national network of thousands of Gas Safe registered engineers we work with. A £95 call-out fee, paid in advance, applies in your first 30 days and on faults the plan does not cover, and cover still applies in those first 30 days. Using a service starts a 12-month agreement period. Smart Plan is a service plan, not insurance, from UK Boiler Company Ltd, trading since 2014. If you smell gas, call the National Gas Emergency Service on 0800 111 999 first.
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