Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Attach temperature clamps to primary flow and return as shown
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

DHW temperature set to MAX.
Once the burner has lit and established measure the temperature differential between the flow and return pipes for at least 2 minutes.
The differential should be approximately 25°C or below.
Additionally the boiler should not modulate excessively during the test.