Set Appliance State:

Set Multimeter to AC Volts Set meter to measure AC Volts
CH Demand
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

With the fan running measure the voltage between the yellow wire marked "NO" at the plug for the air pressure switch (RHS, refer to pictures) to a suitable earth point.
Mains voltage should be present.