Set Appliance State:
User Control to Off Position
Set Multimeter to AC Volts 
Isolate Mains Power to appliance as per TOPs P4
Ensure DHW is calling for heat
Gain Access to PCB
Ensure DHW is calling for heat
Gain Access to PCB
| WARNING! - Mains voltage is expected during test follow TOPs P4 live testing procedure |
Measurement:
User control switch to ON position.
Allow fan to run.
Measure voltage across connection 3 (grey lead, marked Ing) on connector CN1 on the control board and the neutral (connection marked N, blue wire) on the main terminal strip.
Isolate as per TOPs P4
and refit as necessary