Set Appliance State:

Mains Power Off
Gas Off
Turn Hot Water Tap On
DHW temperature control set to maximum
Ensure no demand for CH
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Appliance On/Off control set to ON position.
Reset lockout as necessary.
With a DHW demand does the fan run for longer than 35 seconds?
Note:
Fan speed may change during test.