Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Attach a temperature clamp to the flow pipe, as close to the flow sensor / stat as possible

Observation:

This appliance features built in frost protection which activates a CH demand when the primary heat exchanger is in danger of freezing (approximately 5°C).
Measure the temperature of the flow pipe on the primary heat exchanger near to the flow sensor / stat.
Is the temperature of the appliance above 5°C ?