Set Appliance State:
Mains Power Off
Set Boiler Control Thermostat to MAX position
Set Multimeter to AC Volts 
CH Demand
Ensure external controls are calling for heat
Use insulation mat as required
Use insulation mat as required
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
Measure the voltage between the N/C terminal on the air pressure switch (orange wire) and the neutral (N) connection (blue wire) at the mains input terminal block.
Mains voltage expected.
Mains voltage expected.
Isolate as per TOPs P4
and refit as necessary