No Hot Water but Heating Works? Common Causes and Solutions Explained

When the radiators are warming up nicely but the hot tap runs stone cold, it can be confusing and frustrating. The good news is that this pattern usually points to a handful of common faults that an engineer can diagnose and fix fairly quickly.

First checks if heating works but no hot water

Before you assume the boiler has completely failed, there are a few simple checks you can safely do. These can rule out minor issues and give your engineer clearer information if you do need a visit.

Most modern boilers will shut down part or all of their functions if they detect a problem, low pressure or a control setting that does not make sense. Take a calm, methodical approach and avoid repeatedly resetting or turning knobs at random.

• Check the boiler is in the correct mode and the hot water is actually “on”
• Look for error codes or flashing lights on the boiler display
• Press the reset button once only, if the manual allows it
• Check the system pressure gauge is usually between 1 and 1.5 bar when cold
• Try more than one hot tap or shower to confirm the problem is house‑wide

If any of these checks solve the problem, keep an eye on the system in case the fault returns. If nothing changes, the next step is to think about the type of boiler you have and the faults most often linked to this situation.

Common causes on combi boilers

A combi boiler heats water directly from the mains for taps and showers, and also feeds your radiators. When the heating works but the hot water does not, it often means an internal component that switches between the two has failed or is sticking.

Diverter valve not sending heat to hot water

The diverter valve is like a traffic controller, directing hot water from the boiler either to the radiators or to the hot taps. If it sticks in the heating position, the radiators will get hot but the taps will not.

Typical signs include radiators warming up whenever you run a hot tap, or hot water only working when the heating is on. A diverter valve problem is a job for a Gas Safe engineer, as it involves opening the boiler and dealing with pressurised, gas‑fed parts.

Plate heat exchanger blocked with limescale

The plate heat exchanger transfers heat from the boiler water to your tap water without mixing the two. In hard water areas, this component can slowly clog with limescale inside.

Symptoms can be lukewarm or fluctuating hot water, taps that only run hot at very low flow rates, or hot water that cuts out after a short time. Flushing, cleaning or replacing the plate heat exchanger is usually needed, and regular boiler servicing can help pick up early signs before it fails completely.

Flow sensor and sensor faults

Combi boilers use a flow sensor to detect when you turn on a hot tap. If this sensor fails or sticks, the boiler might not even attempt to fire for hot water, even though heating works normally.

You may notice the boiler display does not change when you open a hot tap, or there is no burner symbol even though water is running. This is an internal electrical component and needs an engineer to test and replace safely.

System and regular boilers

If you have a hot water cylinder in an airing cupboard, you likely have a system or regular boiler. In this setup, the boiler heats water which is then stored in the cylinder for taps, while radiators are fed on a separate circuit.

When the radiators work but the hot water from the cylinder does not, the fault is often in the cylinder controls or the valves that direct water to heat it.

Cylinder thermostat and programmer settings

Your cylinder usually has its own thermostat and time controls, either on the wall near the boiler, on the cylinder itself, or built into a digital programmer. If these are set incorrectly, hot water may never be “requested” from the boiler.

Check that the hot water programme is actually set to “on” or “timed” and that the cylinder temperature is set around 60 °C. If the controls look correct but the cylinder feels cold and the boiler does not fire when hot water is called, an engineer will need to test the wiring and controls.

Motorised valve faults on hot water circuits

Many systems use motorised valves to send boiler water either to the radiators, the cylinder, or both. If the hot water valve fails, sticks, or the motor burns out, the boiler may heat radiators only.

You might hear the boiler fire when hot water is “on” but the cylinder pipes stay cold, or you may hear a valve clicking or buzzing. Motorised valves involve electrical work and should be replaced by a qualified heating engineer.

Immersion heater and control confusion

Some cylinders also have an electric immersion heater. If the immersion is off or faulty and the boiler has stopped heating the cylinder, you could end up with no hot water at all, even though the radiators are fine.

Check any immersion switches and labelled fused spurs nearby, but do not remove covers or expose wiring. If you are unsure which device heats your water at which times, an engineer can explain and label the controls during a visit.

Airlocks and circulation problems

Occasionally, air in the pipework can stop hot water circulating through the coil inside the cylinder, so the cylinder never heats up properly. This is more common after draining or work on the system.

Signs can include gurgling noises, pipes that get hot then cool quickly, or radiators needing frequent bleeding. Removing airlocks often means venting specific points in the system, which is best done by someone who understands the layout.

Warning signs to stop and call an engineer

While basic checks are fine, some symptoms mean you should stop troubleshooting and get professional help straight away. Ignoring them can put your home and family at risk.

• Burning, melting or electrical smells from the boiler or wiring
• Repeated lockouts or error codes after a reset
• Visible water leaks, staining or dripping from the boiler or cylinder
• Boiler making loud banging, clanging or whistling noises
• Sooty marks, scorch marks or discoloured surfaces near the boiler

If you suspect a gas smell or fumes, turn the boiler off, ventilate the area and contact the gas emergency number immediately. Do not attempt to remove any covers or interfere with flues or gas pipework yourself.

Reducing the risk with regular servicing

Many of the faults behind “heating works but no hot water” build up over time. Limescale in the plate heat exchanger, stiff diverter valves and failing sensors are often caught early during routine boiler servicing.

Annual servicing helps keep components moving freely, checks safety devices, and can improve efficiency at the same time. In hard water areas especially, your engineer can advise on scale control to protect the boiler and hot water system.

Next steps and booking a repair

If you have been through the simple checks and still have no reliable hot water, it is time to arrange a professional diagnosis. An experienced heating engineer can quickly pinpoint whether the problem is with your combi boiler, cylinder controls, motorised valves or another part of the system.

To arrange a prompt gas boiler repair or to ask about preventative boiler servicing, contact MBW Plumbing & Heating on 07712 508313. Getting the fault sorted properly will restore your hot water and help keep your heating system running safely and efficiently.

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