Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Selector Switch in Summer (I) position.

Measurement:

Turn on Hot Water Tap and set flow rate to approximately 2.5 gal/min (11.4 litres/min) OR 1 pint in 3 seconds.
This should give a temperature rise of approximately 30 Degrees C.