Thermostat displays E5 and fails to control heating; may occur after a power surge or wiring change.

Safety first: Isolate mains power before opening the thermostat or receiver unit.

Steps to check:

• Identify the device: On Salus RF systems (e.g. RT510RF, RT520RF, RT310i), E5 may relate to the receiver relay rather than the thermostat head. Check both units for error indications.
• Listen for relay click: With power on and calling for heat, listen for a distinct click from the receiver or thermostat. No click suggests a failed relay coil or driver; continuous click or buzzing suggests a mechanical fault.
• Check load wiring: Confirm that the boiler or zone valve is wired to the correct COM/NO terminals and that the load current does not exceed the rated relay capacity (often 3A or 5A resistive). Overloading can weld contacts.
• Test output: With a multimeter, measure voltage across the switched output when the thermostat calls for heat. If the relay indicates ON but no voltage is present, the relay or driver is faulty.

Resolution: Replace the Salus receiver unit or thermostat containing the faulty relay. If the relay failed due to overload, correct the wiring and ensure the connected device is within specification before fitting the new unit.