Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to DC Volts Set meter to measure DC Volts
Turn Hot Water Tap On
Boiler Switch in ON position
Central Heating switch in OFF (central) position
DHW Selector turned to maximum heat
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the DC voltage across the red (+ve) and black (-ve) wires of GV3.