Set Appliance State:

Mains Power Off
Gas Off
CH Demand
CH temperature control to max
Selector switch set to ch/dhw
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Does the fan run?
Note.
If power has been turned off to the appliance then approximately
10-seconds will elapse before fan will start.