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