Thermostat shows B6 and reports that the maximum flow temperature limit has been exceeded; heating is reduced or stopped.

Safety first: Turn off power before touching any valves or pumps that may be hot. Do not open the boiler casing unless qualified.

• Check radiator/underfloor valves: Ensure that enough radiators or underfloor loops are open so that heat can be dissipated. Closed valves can cause high flow temperatures.
• Verify pump operation: Listen for the circulation pump running when there is a heat demand. If silent, a technician should inspect the pump.
• Adjust boiler flow temperature: On the boiler control, reduce the maximum flow temperature setting to a value compatible with your emitters (e.g. 60–70°C for radiators, 35–45°C for underfloor).
• Check thermostat max limit: In the thermostat settings, ensure the maximum allowed flow or room temperature is set appropriately.
• Call technician if recurring: Repeated B6 errors may indicate a faulty boiler sensor or circulation issue that requires professional diagnosis.