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
- The boiler fans starts but the boiler does not ignite. When the fan starts during the Sequence of Boiler Operation, the boiler air pressure switch must signal the printed circuit board and then ignite. But (for example) if there is a blocked flue, the air pressure will not work correctly and remain closed – creating a fault.
- Boiler cycles on and off. This can happen if the signals to the PCB are not working efficiently from the air pressure switch.
- Quite often Error Codes are displayed on the boiler (modern boilers) which indicate a fault with the air pressure switch – lookup our boiler error codes on various boiler manufacturers to see more information.
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).