Thermostat displays E09 and cannot control domestic hot water schedule or temperature.

Safety first: Isolate mains power to the heating system before opening any wiring centers or receiver units.

Steps to check:

• 1. Verify system configuration: In the thermostat setup, confirm the correct boiler type (combi or system) and that hot water control is enabled only when a separate cylinder is present.
• 2. Inspect wiring center: At the wiring center or junction box, confirm the hot water zone valve and cylinder sensor are wired according to the Baxi wiring diagram and that connections are tight.
• 3. Test hot water relay: With power on and cover fitted, call for hot water from the thermostat and listen for the receiver relay click. If no click, the relay or control circuit may be faulty.
• 4. Check cylinder sensor: If a Baxi cylinder sensor is used, test its resistance with a multimeter and compare to Baxi’s temperature/resistance chart.

When to call a professional: Hot water control involves mains wiring and motorised valves; if you are not experienced, have a heating engineer diagnose and replace the faulty relay, valve, or sensor.