The Worcester 240 boiler is fitted with an air pressure switch, which enables the boiler to activate.
The air pressure switch is located in the bottom section of the boiler, behind the grey panel, to the left of the two circuit boards.
The air pressure switch is connected by three wires, and has two nozzles (P1 and P2) coming out towards the back of the boiler. Air pipes are connected to the two nozzles.
Nozzle P1 is connected to the flue. Nozzle P2 goes into the burner.
Air pressure on the P1 nozzle throws the switch, which enables the boiler to operate.
The switch is connected by three wires as follows:
Terminal 1 - White
Terminal 2 - Grey
Terminal 3 - Brown
Supply power from the brown wire is fed to Terminal 3.
In the switch idle position, this power is output from terminal 1 (White) via the switch.
Demand switches on an extractor fan at the top of the boiler, near the flue. This causes a vacuum, throwing the air pressure switch, causing power to be output instead to terminal 2 (grey).