Why Is My Boiler Giving Hot Water but No Heat?
If your boiler makes hot water but the radiators stay cold, the fault is usually on the heating side, often a stuck timer, thermostat, zone valve, or diverter valve. If you have a combi boiler and no space heating at all, start with the controls before you pay for parts. This guide starts with the safest, cheapest checks, then moves by boiler type.
Boiler type fault tree
Start with the boiler type. That decides which parts can fail while hot water still works, and it stops you chasing the wrong fault. A combi boiler makes domestic hot water on demand, has no separate hot water cylinder, and uses a diverter valve to switch between hot water and central heating. A system boiler uses a hot water cylinder, so stored hot water and heating can be affected separately.
| Boiler type | Symptom | Likely cause | Safe homeowner check | When to stop and call an engineer |
|---|---|---|---|---|
| Combi boiler | Taps run hot, radiators stay cold | Thermostat, programmer, summer mode, low pressure, or a diverter valve stuck partly or fully in hot-water position | Check the room thermostat, timer, boiler mode, and pressure gauge on the fascia | Call a Gas Safe registered engineer if the boiler fires but heating never starts, if pressure keeps dropping, or if the diverter valve is suspected |
| System boiler | Hot water cylinder is warm, but heating does not run | Programmer, thermostat, motorised valve or zone valve, pump, or air in the heating circuit | Check heating schedule, cylinder and room controls, and whether any valve has a manual lever | Stop if a valve will not open, the pump is noisy or hot, or the fault sits inside the boiler or sealed system |
| Oil boiler or heat-only setup | Stored hot water or taps still work, but radiators stay cold | Controls, circulation fault, motorised valve, pump, airlock, or oil boiler lockout on the heating side | Check timer, thermostat, pressure if fitted, and radiator condition | Call an engineer for lockouts, burner faults, leaks, repeated resets, or any internal repair |
Why is my boiler giving hot water but no heating?
Because hot water and central heating are often controlled by different parts of the system. On many boilers, the taps can still get hot even when the heating call is blocked by a timer, thermostat, valve, pump, or low pressure. The exact fault path depends on whether you have a combi, system, or oil boiler.
Combi boilers
A combi boiler heats domestic hot water directly when you open a tap. It does not store hot water in a cylinder. That means the heating circuit can fail on its own while tap water still works, especially if the diverter valve sticks, the pressure is low, or the controls are not asking for heat.
Say your combi boiler fills the kitchen sink with hot water, but the radiators stay cold even after the thermostat is turned up. That points first to the heating controls or diverter valve, not to the domestic hot water side.
System boilers
A system boiler heats a cylinder of stored hot water and can serve heating separately. So the cylinder may still provide hot water while the space-heating side has a separate fault. On these systems, motorised valves, zone valves, and schedules often decide whether the radiators receive heat.
Oil boilers and heat-only setups
Oil boilers and older heat-only systems often depend on external controls and circulation parts. The boiler may still heat water for one circuit while a zone valve, pump, or timer stops the radiators from receiving it. A burner fault can also affect only part of the heating cycle, so persistent lockouts need an engineer.
Is the heating switched off?
Yes, a control setting can stop heating while leaving hot water on. Check the thermostat, programmer, timer, summer mode, holiday mode, and any hot-water-only setting before you look for failed parts. These are the cheapest faults to rule out, and they are common after a clock change, power cut, or schedule change.
Check the thermostat and room setting
Make sure the room thermostat is set above the current room temperature. Dead batteries in a wireless or smart thermostat can also stop a heating call. If the display is blank, showing an error, or stuck on an old schedule, the boiler may never receive the request for space heating.
Check the programmer or timer mode
Many boilers let you control hot water and heating separately. If the programmer is set to hot water only, or the heating channel is off, the taps can work while the radiators stay cold. Holiday settings and temporary overrides can do the same thing.
- Check that heating is set to on or timed, not off.
- Check that hot water and heating are both enabled if your model separates them.
- Cancel any holiday, away, or override mode.
- Confirm the current clock time matches the schedule.
Check summer mode and heating-off settings
Some boilers have a summer mode that leaves domestic hot water available but disables central heating. That setting is easy to miss on a control panel or smart hub. If the boiler has a mode selector, confirm it is not set to summer, hot-water-only, or heating off.
Could low pressure be stopping the heating?
Yes. Low system pressure can stop central heating even when domestic hot water still works, especially on sealed systems and many combi boilers. Check the pressure gauge on the boiler fascia. A common cold pressure is often around 1.0 to 1.5 bar, though the exact target can vary by model.
Read the pressure gauge
If the needle is well below the usual cold range, the boiler may refuse to run the heating circuit. Some boilers will still make hot water with low pressure, which can make the fault look more confusing than it is. Repeated pressure loss suggests a leak or another fault that needs attention.
Refill only if the manual allows it
Only top up pressure if the boiler manual says you can do that yourself. If you do not know the correct method, stop and ask for help. Never force valves or guess at the filling loop. If pressure falls again soon after topping up, the fault is not solved.
Picture a boiler reading 0.4 bar on a cold morning. The hot taps still work, but the radiators stay cold and the boiler will not start the heating cycle. In that case, low pressure is a far better first suspect than a failed pump.
What parts fail when hot water still works
When controls and pressure look normal, the fault often moves to the parts that switch or circulate heat. The key is to match the boiler type to the part that can fail while leaving domestic hot water available.
Diverter valve on a combi boiler
A diverter valve redirects heated water to either the taps or the radiators. On a combi boiler, it is a common failure point when hot water works but heating does not. The valve may stick partly or fully in one position, so taps still get hot while the heating side never receives flow.
That fault is internal. It needs a Gas Safe registered engineer.
Motorised valve or zone valve on a system boiler
System boilers often use a motorised valve or zone valve to open the heating circuit. If that valve fails, the cylinder can still heat while the radiators stay cold. Some valves have a manual lever that can show whether the valve is opening, but that is only a temporary clue, not a repair.
Central heating pump and circulation problems
The pump moves hot water through the heating circuit. If it has an airlock, starts to seize, or runs hot and noisy, the boiler may still make hot water but fail to push heat through the radiators. A seized or failing pump needs an engineer. Do not open the casing or touch electrical parts.
Why are the radiators still cold?
Radiators can stay cold even when the boiler is firing. Sometimes the issue is trapped air or a closed valve rather than the boiler itself. Start with the radiators in the rooms that are coldest, and check whether the tops are cooler than the bottoms, which often points to air.
Bleed trapped air
If a radiator has a cold top and a warmer bottom, trapped air is a likely cause. Bleeding can restore circulation if the system pressure stays within range afterward. If pressure drops sharply after bleeding, stop and check for a leak or a filling problem.
Check thermostatic radiator valves and balancing
Thermostatic radiator valves can be turned down or stuck closed. Make sure the valve heads are open enough for heat to pass. If some radiators are hot and others are cold, the system may need balancing, but that is a follow-up job after the basic checks.
Look for sludge, airlocks, or uneven heating
If only certain radiators warm up, sludge or an airlock may be restricting flow. A heating pump that is struggling can also create uneven heat. These faults are not fixed by thermostat changes, so if the boiler still gives hot water but the circuit stays cold, the next step is an engineer.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if simple checks do not restore heating, or if the fault suggests an internal boiler, gas, electrical, or sealed-system problem. That includes repeated lockouts, pressure loss, ignition faults, visible leaks, scorching, unusual smells, or a pump or valve that appears to have failed.
Do not keep resetting a gas boiler that will not heat the radiators. Do not open the case. Do not attempt internal repairs. Those jobs are for a qualified engineer.
Is it safe to keep using the boiler if only hot water works?
Usually, domestic hot water use is not the same as a major emergency, but you should not assume the boiler is healthy. If the boiler is leaking, locking out, making unusual noises, losing pressure, or showing a fault code, stop using it and get it checked. If you smell gas, leave the area and follow emergency gas advice immediately.
Frequently asked questions
Why is my combi boiler giving hot water but no heating?
A combi boiler can still make tap water while the heating side fails. Check the room thermostat, programmer, heating mode, and pressure first. If those are fine, a diverter valve that sticks in the hot-water position is a common internal fault and needs an engineer.
Why do I have hot water but no central heating?
That usually means one circuit is working and the heating circuit is not being called or circulated. A system boiler may still heat the cylinder while a valve or pump blocks the radiators. On a combi, the problem is often a control setting, low pressure, or diverter valve.
What switch or setting could stop the heating while keeping hot water on?
Look for summer mode, hot-water-only mode, heating off, holiday mode, or a timer that controls hot water and heating separately. A room thermostat set too low or a smart thermostat with dead batteries can also stop the heating call while leaving hot water available.
Can a diverter valve cause hot water with no radiators?
Yes. On a combi boiler, the diverter valve can stick partly or fully in the hot-water position, so taps still get hot while the radiators stay cold. That is a common internal fault and should be diagnosed by a Gas Safe registered engineer, not repaired by a homeowner.
When should I call a Gas Safe engineer for no heating?
Call one after you have checked the thermostat, timer, boiler mode, and pressure and the radiators are still cold. Call sooner if there is a leak, repeated lockout, ignition fault, scorching, unusual noise, or any sign that the diverter valve, pump, or zone valve has failed.
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