Heating Works but No Hot Water? Likely Causes Explained

If your central heating is working but you have no hot water, it can be both confusing and frustrating, especially when you rely on hot water daily for showers, washing up, or cleaning. This issue often feels like a sudden disruption in your home’s comfort, leaving you unsure where to start troubleshooting. The good news is that this specific symptom usually points to a distinct set of common faults within your heating system. Because the problem is typically isolated to the hot water function, a skilled heating engineer can quickly identify and fix the root cause without unnecessary guesswork.

This targeted approach means faster repairs, reducing the time you’re left without hot water and restoring your system’s full functionality efficiently.

How heating can work, but hot water does not

Your boiler sends heat to different parts of your system depending on whether you want hot water, heating, or both. When the radiators are working properly but the hot taps are not, it often means the boiler itself can produce heat, but it is not being directed correctly to your hot water.

This is different from a full boiler failure, where you would normally lose both heating and hot water. Here, the issue is usually with control valves, sensors or heat exchangers that handle the hot water side of the system.

Quick checks you can do safely first

Before you assume there is a serious fault, it is worth running through a few simple checks. These do not involve opening the boiler or touching any internal parts, so they are safe for a householder.

• Check your programmer or timer: make sure hot water is turned on, not just the heating.

• Look at your thermostats: turn the cylinder or hot water temperature control up slightly and listen for a click.

• Note any boiler display messages: fault codes, flashing lights or low pressure warnings are all useful clues.

• See whether the problem is constant or intermittent: for example, hot water sometimes works, then cuts out.

It also helps to confirm that all or most radiators heat up as normal. If your heating is weak in some rooms as well as having no hot water, you may have a broader circulation or sludge issue rather than a fault only on the hot water side.

Common causes on combi boilers

With a combi boiler, you do not have a hot water cylinder. The boiler heats water on demand when you open a tap, so if heating works but hot water does not, the problem is often within the boiler controls or internal components.

Diverter valve sticking or failed

The diverter valve is a key part in a combi boiler. It directs the heat either to your radiators or to the hot water supply when you turn on a tap.

If it sticks in the heating position, the boiler will happily warm your radiators but will not send heat to the hot water circuit. You may notice the boiler fires when a hot tap is opened, but the water stays lukewarm or cold while some radiators warm up slightly instead.

Plate heat exchanger scaled or blocked

Combi boilers use a plate heat exchanger to transfer heat from the boiler water into your tap water. Over time, limescale and system sludge can build up and restrict this component.

When that happens, the boiler may appear to run normally but cannot transfer enough heat into the hot water. You might get a brief burst of hot water followed by it turning tepid, or the temperature may fluctuate during a shower.

Flow sensor or demand detection issues

The boiler relies on a flow sensor to detect when you open a hot tap. If the sensor is faulty, the boiler may not recognise that you are asking for hot water at all.

In that case, you will find the heating still comes on when programmed, but there is no response on the boiler when you run a tap. Sometimes this shows up as no flame icon or no change in boiler display when you open a hot outlet.

System boilers and cylinders with no hot water

In homes with a hot water cylinder, the boiler heats water in the cylinder, which is then stored ready for use. When you have heating but no hot water from this type of setup, the boiler is usually fine, but something between the boiler and the cylinder is not working.

Motorised valve not opening for hot water

Many systems use a motorised valve to control where the hot water from the boiler goes. One position sends heat to radiators, another sends it to the cylinder coil, and some valves can do both.

If the hot water side of that valve fails, the cylinder will not heat, even though the radiators are fine. The valve head can stick, the motor can fail, or the microswitch inside can stop sending the right signal.

Cylinder thermostat or programmer settings

A cylinder thermostat tells the system when the stored hot water has reached temperature. If it is set too low, faulty, or has failed completely, the boiler may not be asked to heat the cylinder.

Similarly, if the programmer or smart controls are not set to provide hot water, or a schedule has been changed accidentally, your heating can carry on as normal while your cylinder stays cold. It is always worth double checking the time settings and hot water mode before assuming a component has failed.

How an engineer tests these components

A qualified heating engineer will start by confirming the controls are calling for hot water, using a multimeter or test tools to check for power at the relevant terminals. They will observe the boiler firing pattern and see whether it behaves differently for heating and hot water demand.

On combis, they may test the diverter valve movement, check the resistance of sensors, inspect filters, and measure temperature rise across the plate heat exchanger. On system boilers with cylinders, they will check the motorised valve operation, cylinder thermostat and wiring, and ensure the cylinder coil is actually heating when the boiler runs.

Preventing the problem coming back

While individual parts sometimes fail with age, many hot water issues are made worse by lack of servicing, sludge, or scale. Taking care of the system can reduce breakdowns and keep efficiency higher.

• Book an annual boiler service so moving parts like diverter and motorised valves are checked and kept in good order.

• Use correct water treatment, including inhibitor, to limit corrosion and sludge build up in radiators and pipework.

• In hard water areas, speak to your engineer about scale protection for plate heat exchangers and cylinders.

• If sludge is suspected, consider system cleaning so new parts do not quickly become blocked again.

Pay attention to early warning signs such as noisy radiators, frequent topping up of boiler pressure, or water that runs hot then suddenly goes cold. Sorting these issues promptly can often prevent a full loss of hot water later.