Set Appliance State:

Mains Power Off
Set Multimeter to DC Volts Set meter to measure DC Volts
Turn Hot Water Tap On
Selector Switch in HOT WATER position
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 the Brown wire and Black wire on the Modureg. The voltage obtained should be approximately 24 V DC.
Note: Ensure multimeter is in DC Volts, other meter settings may cause damage to the PCB.