Set Appliance State:

Mains Power Off
Set Multimeter to DC Volts Set meter to measure DC Volts
Turn Hot Water Tap On
Boiler Operating Switch / Programmer set to WATER only
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

With the DHW tap fully open measure the DC voltage across connections 5 and 6 (Yellow wires) on connector X2 on the main PCB.
The voltage obtained should be greater than 20V DC.