Set Appliance State:

Mains Power Off
CH Demand
ON/OFF switch in ON position
CH temp selector to MAX
DHW temp selector to MIN
Ensure DHW heat exchanger is cool (this can be achieved by running the DHW with the gas turned off)

Gas On
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?