Set Appliance State:

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

Measurement:

Allow the burner to ignite and appliance to run for at least 1 minute.
With the appliance still running measure the temperature differential across the plate heat exchanger.
Expect approximately 11°C-20°C