Set Appliance State:

Mains Power Off
CH Demand
CH Selector in Maximum position
ON/OFF Switch in ON position
Summer/Winter in Winter Position (IN)
Timer Set to ON Position
Ensure DHW heat exchanger is cool (this can be achieved by running the DHW with the gas turned off)

WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow boiler to fire and run for a few minutes.
Does the DHW Heat Exchanger remain cool?