Combi Boiler Not Producing Hot Water
When a combi boiler stops producing hot water, the problem is usually in the domestic hot water (DHW) circuit of the boiler. Because the heating and hot water functions share some components, identifying the specific cause requires systematic testing.
Common Combi Boiler Hot-Water Failures
The most common combi boiler hot-water problems involve the diverter valve (which switches between heating and hot water), the plate heat exchanger (which transfers heat to the domestic water), the flow sensor or turbine, or the DHW thermistor.
When Heating Works But Hot Water Does Not
If your heating works normally but hot water is not produced, the problem is likely in the DHW-specific components: the diverter valve is stuck, the flow sensor is not detecting water flow, or the plate heat exchanger is blocked.
When Both Heating and Hot Water Are Affected
If neither heating nor hot water works, the problem is more general: the boiler itself may be in lockout, there may be a gas supply issue, or a primary component like the pump or main PCB may have failed.
Safety Reminder
If you smell gas, suspect carbon monoxide or believe there is an immediate danger, leave the property and contact emergency services or the appropriate gas emergency authority. Do not remain inside — exit the building immediately and call for help from outside.