Thermostat shows domestic hot water as ON but hot water does not heat as expected.

Safety first: Hot water controls involve mains voltage and hot pipework. Isolate power and allow components to cool before working.

Steps to diagnose:

• 1. Confirm demand: Ensure the thermostat or app shows a hot water demand and that the receiver’s hot water LED is lit.
• 2. Check zone valve: At the cylinder, feel the hot water zone valve head when demand is present; it should motor open and the pipe after the valve should get hot.
• 3. Inspect wiring: Verify that the hot water relay output from the Baxi controller is wired correctly to the zone valve and boiler call-for-heat circuit.
• 4. Test cylinder sensor/thermostat: If a Baxi cylinder sensor is used, test its resistance. If a mechanical cylinder stat is used, check its set temperature and operation.

When to call a professional: Diagnosing and replacing valves or sensors should be done by a heating engineer.