This product has been discontinued with no replacement. The Baxi air pressure switch is designed to work with Baxi Main HE, Performa SL and Suprima boilers. An air pressure switch is a critical safety device that ensures the boiler fan is operating properly and prevents the accumulation of toxic gases in the event of a fan failure. The printed circuit board of the unit closely monitors the air pressure switch to ensure it is fully functional before powering on the system. This prevents the appliance from starting without the presence and operation of critical safety features.