Air Pressure Switches (General)

Air pressure switches

(operating @ approx. 0.5 – 4 m/bar pressures)

See also: How to test your Air Pressure Switch with a multimeter

Reasons an air pressure switch will fail to start:

  • Loose electrical connections
  • Kinked or split rubber hoses / loose hose connections
  • Condensation within rubber hoses
  • Blocked venturi / melted venturi within fan assembly
  • Reversed rubber hose connections
  • Internal rubber diaphragm perished / damaged
  • Poorly installed flue seals
  • Flue way blocked
  • Faulty fan assembly (slow running)
  • Incorrect flue lengths
  • Incorrect or no restrictor rings in flue assembly
  • Inadequately sealed combustion chamber (poor casing seals)
  • Faulty electrical micro switch assembly

The pressure differential created by the fan is transmitted via the pressure hoses and creates a difference in pressure on top and bottom of the diaphragm, this in turn, pushes the diaphragm and an attached pin and lever.

When the diaphragm has fully reached its proved air flow position, the pin and lever operate a micro switch, sending a signal to the printed circuit board to commence ignition sequence (by either a 2 or 3 wire configuration).

When the fan has stopped, a force (usually a spring or gravity) returns the diaphragm to its rest/no air flow position.

Typical indicators that the Air Pressure Switch is faulty

2 wire configuration:

  • C – Common
  • NC – Normally closed

3 Wire configuration:

  • C – Common
  • NC – Normally closed
  • NO – Normally open


C – Common Printed circuit board signal (voltage) to the air pressure switch
Normally Open (NO) Air pressure switch returns signal (voltage) to printed circuit board, once the fan is operating and a proved air flow is established.
Normally Closed (NC) Rest mode / no air flow and the return from the NC terminal must be proved at the printed circuit board before next ignition sequence takes place.


When the fan operates effectively and air is proved at the air pressure switch, the micro switch moves to the proved air flow position NO terminal.

Some air pressure switches are marked with +ve or 1 and –ve or 2.
(1 being positive and 2 being negative).

Some air pressure switches are marked with L and H.
(L being negative and H being positive).