Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to DC Volts Set meter to measure DC Volts
Gain access to DHW flow sensor connections
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Test 1.
With No DHW demand present measure for DC voltage across the flow sensor connections.
Expected voltage approximately 5 Vdc.
Test 2.
Turn hot water tap On, repeat voltage measurement as per test 1.
Expected voltage approximately 0 Vdc.