In the low-efficiency type C boiler, the air pressure switch detects the operation of the flue fan (exhaust fan). It consists of two tubes connected to the venturi ports of the extractor, functioning as pressure and vacuum sensors.
This safety feature is essential not only for detecting the proper operation of the extractor, but it also perceives its effective work and prevents the boiler from operating in case of chimney obstructions.
The function of the two different-colored tubes connected to the extractor is to move the membrane inside the air pressure switch and trigger the microswitch. Depending on the board version, the air pressure switch is designed for either simple control or double control.
Thus, it can be seen connected with only two connectors or with three connectors (Common / Normally Closed / Normally Open)
- Common (C): It is the standard connection
- Normally Closed (NC): To be used with the extractor at rest, not operating.
- Normally Open (NO): Contact used when the extractor is in operation.
Connections
This component is managed by the ignition and detection board.
- In older boiler models, it is often managed by a separate control unit from the main board, such as the BRAHMA TM 31
- In more recent boiler models, it is managed by the main boiler board, which now handles the entire control and ignition phase.
Issues
The component is not free from problems, which usually occur when the flue length in the type C boiler exceeds the lengths imposed by the manufacturer.
The connecting tubes fill with water
This happens due to the flue length exceeding approximately 18 meters, where the water vapor in the flue gases condenses before evacuation at the chimney and falls back into the boiler. Returning to the extractor, this can enter the connecting tubes, preventing the boiler from igniting the burner.
The flue length calculation should also include the intake duct, with each elbow empirically worth 1 meter, and horizontal sections worth twice the meters.
Maintenance and Replacement
Regular maintenance of the air pressure switch is crucial to avoid malfunctions. This includes cleaning the connecting tubes and checking the elasticity of the internal membrane. In case of failure, such as worn fan bearings or water infiltration, the entire component may need replacement.
The replacement should be carried out by a qualified technician who will dismantle and clean the pressure switch, fan, and flue, ensuring all connections are correct and free of dirt or debris.