Set Appliance State:
Gas Off
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Continuity 
Turn Hot Water Tap On
Selector Switch in DHW Summer position
Hot Water Thermostat set to Maximum
Remove all three wires from the Air Pressure Switch and make safe
Hot Water Thermostat set to Maximum
Remove all three wires from the Air Pressure Switch and make safe
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
Using crocodile clips connect meter to COM and NO terminals of Air Pressure Switch.
When Fan starts can a continuous tone be heard from the meter?
Replace all three wires back on Air Pressure Switch as follows:
COM - Red/Black
NC - Brown
NO - White
When Fan starts can a continuous tone be heard from the meter?
Replace all three wires back on Air Pressure Switch as follows:
COM - Red/Black
NC - Brown
NO - White
Isolate as per TOPs P4
and refit as necessary