Heating but no hot water is a frustrating boiler problem, especially when your radiators are working perfectly. This issue is most common with combi boilers and is usually caused by a small number of identifiable faults rather than a complete boiler failure.

In many cases, the problem can be diagnosed quickly and repaired without replacing the boiler.

Step 1: Check the Basics First

Before assuming an internal fault, carry out these basic checks. Many combi boiler hot water faults are caused by simple setting or pressure issues.

• Confirm hot water is switched on in the boiler settings
• Increase the hot water temperature control
• Check that timers or programmers are not set incorrectly
• Reset the boiler using the reset button or power cycle
• Check the boiler pressure is between 1.0 and 1.5 bar

Low pressure can prevent hot water delivery even when the heating still operates.

Step 2: Heating, But No Hot water, and the Diverter Valve

The most common cause of heating but no hot water in a combi boiler is a faulty boiler diverter valve.

The diverter valve directs heated water to either:

• Central heating radiators, or
• Domestic hot water taps

If the valve becomes stuck in heating mode, hot water never reaches your taps.

Signs of a boiler diverter valve fault

• Radiators heat normally
• Taps only run cold or lukewarm
• Hot water only works when the heating is switched on
• The boiler sounds normal, but produces no hot water

This fault is extremely common in older and heavily used combi boilers.

Step 3: Other Causes of No Hot Water With Heating Working

If the diverter valve is operating correctly, other faults may be responsible.

Thermostat or sensor fault

The boiler may not detect hot water demand when taps are opened.

PCB (Printed Circuit Board) issue

A faulty PCB can prevent the boiler from switching into hot water mode.

Flow or pressure-related problems

Insufficient water flow or pressure can stop hot water production while heating continues to function.

These faults require professional testing.

Step 4: When to Call a Gas Safe Engineer

If you still have heating but no hot water after checking:

• Boiler pressure
• Settings and timers
• Performing a reset

Then an internal repair is likely required. Components such as diverter valves, sensors, and PCBs must only be worked on by a Gas Safe-registered engineer.

Using an unqualified engineer is unsafe and may breach UK gas safety regulations set out by the Health and Safety Executive (HSE):
https://www.hse.gov.uk/gas

Summary: Heating, But No Hot Water

When heating but no hot water occurs, the cause is usually a diverter valve or control issue rather than a full boiler breakdown. Early diagnosis helps avoid further damage and restores hot water quickly and safely.

If the issue persists, always use a Gas Safe engineer to ensure compliance with UK safety standards and manufacturer requirements