Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Attach temperature clamps as indicated by the red arrows
Boiler DHW temperature set to MAX
Gas On
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

Measure the temperature differential after the appliance has been running for at least 2 minutes.
Expected differential approximately 22°C or less