No hot water but heating works — boiler checks & fixes

No hot water but heating works: quick guide to diagnosis and fixes

If your radiators are warm but the hot taps run cold, that mismatch usually points to a limited set of faults rather than a complete boiler failure. This guide explains the safe checks you can do, the common faults that cause "no hot water but heating works", and when to call a Gas Safe engineer for a prompt repair.

First checks to try yourself

Before booking a repair, take a calm, methodical approach and run these simple checks. Most modern boilers lock out parts of their functions if they detect a fault, so avoid repeatedly resetting or randomly changing controls.

• Check the boiler is set to the correct mode and that hot water is actually selected.
• Look for error codes or flashing lights on the boiler display and note any messages.
• If the manual allows, press the reset button once only and watch for changes.
• Check the system pressure gauge — it is usually between 1 and 1.5 bar when cold.
• Try more than one hot tap or the shower to confirm the problem is house‑wide.

If one of these checks restores hot water, monitor the system in case the fault returns. If nothing changes, the next step is to think about your boiler type and the faults most commonly linked to this symptom.

Common causes on combi boilers

Combi boilers heat hot water on demand from the mains and also supply radiators. When heating works but hot water doesn’t, an internal component that switches between the two functions is often at fault.

Diverter valve sticking in heating position

The diverter valve acts like a traffic controller, sending hot water either to the radiators or to the domestic taps. If it sticks or fails in the heating position the radiators will heat while taps remain cold. Typical signs include radiators warming whenever you open a hot tap, or hot water only being available when the heating is on. Repair or replacement of the diverter valve requires a qualified Gas Safe engineer because it involves opening the boiler and working on pressurised, gas‑fired components.

Plate heat exchanger blocked with limescale

The plate heat exchanger transfers heat from the boiler water to tap water without mixing the two circuits. In hard water areas it can slowly clog with limescale. Symptoms include lukewarm or fluctuating hot water, hot water only at very low flow rates, or hot water that cuts out after a short time. Flushing, descaling or replacing the plate heat exchanger is usually required, and regular servicing helps spot scale build‑up before it causes failure.

Flow sensor or related sensor faults

Combi boilers rely on a flow sensor to detect when a hot tap is opened. If the flow sensor fails or sticks, the boiler may not attempt to fire for hot water even though the heating circuit works normally. You might notice the boiler display does not react when you open a tap, or there is no burner indicator during tap flow. Sensor replacement and electrical diagnostics should be carried out by a trained engineer.

Causes on system and regular boilers

System and regular boilers heat a hot water cylinder that stores domestic hot water. When radiators work but the cylinder stays cold, faults are usually within the cylinder controls or the valves that direct heat to it.

Cylinder thermostat and programmer settings

Cylinders normally have their own thermostat and can be controlled by a wall programmer or time switch. If these controls are off, set incorrectly, or call for heat at the wrong times, the boiler will not heat the cylinder. Check the hot water programme is set to “on” or “timed” and that the cylinder thermostat is set to around 60 °C. If controls look correct but the cylinder remains cold and the boiler does not fire for hot water, an engineer will need to test the wiring and programmer outputs.

Motorised valve faults on the hot water circuit

Many systems use motorised valves to route boiler output to radiators, the cylinder, or both. If the valve for the hot water circuit fails, sticks, or its motor seizes, the boiler may only heat the radiators. You may hear clicking or buzzing from the valve or find that the boiler fires but cylinder pipework stays cold. Motorised valve replacement requires a heating engineer to handle the electrical and mechanical work safely.

Immersion heater and control confusion

Some cylinders contain an electric immersion heater as a supplementary heat source. If the immersion heater is off or faulty and the boiler has stopped heating the cylinder, you can be left with no hot water despite the heating working. Check labelled fused spurs and immersion switches if accessible, but do not remove covers or expose wiring. If you are uncertain which device heats your water at what times, a visit from an engineer can clarify and label the controls for future reference.

Airlocks and circulation problems

Air trapped in the cylinder coil or system can prevent hot water circulating through the cylinder properly. This can occur after draining the system or following maintenance. Signs include gurgling noises, pipes heating then cooling quickly, or radiators that need frequent bleeding. Removing airlocks and restoring correct circulation is a job for someone who understands the system layout and venting points.

Warning signs — stop and call an engineer

Certain symptoms indicate a potentially dangerous fault or a failing component that needs immediate professional attention. If you see any of the following, stop further checks and contact a qualified engineer:

• Burning, melting or electrical smells from the boiler or nearby wiring.
• Repeated lockouts or persistent error codes after resetting.
• Visible water leaks, staining or dripping from the boiler or cylinder.
• Loud banging, clanging or whistling noises from the boiler.
• Sooty marks, scorch marks or discolouration near the appliance.

If you suspect a gas leak or fumes, turn the boiler off, ventilate the area and contact the national gas emergency number immediately. Do not remove covers or attempt repairs on gas appliances yourself.

Reduce the risk with regular servicing

Many causes of “no hot water but heating works” develop gradually. Limescale, stiff valves and failing sensors are frequently identified during routine annual servicing. A scheduled service keeps components moving freely, checks safety devices, and helps maintain efficiency. In hard water areas your engineer can advise on scale control measures to protect the plate heat exchanger and other parts.

Find out about our ongoing protection options on our boiler service plans page: boiler service plans.

Next steps: diagnosis and booking a repair

If you’ve carried out the basic checks and still have no reliable hot water, arrange a professional diagnosis. An experienced heating engineer will pinpoint whether the issue lies with a combi diverter valve, a plate heat exchanger, a cylinder control, motorised valves or a sensor, and then recommend the safest repair.

To book a prompt boiler repair or schedule a service, use our online booking: book a boiler service or repair, or visit our contact page for further options. For an overview of what an engineer checks during a service, see our boiler service checklist: boiler service checklist.

Getting the fault diagnosed and repaired properly will restore your hot water and help keep your heating system safe and efficient.