The air pressure switch test station often misjudges—sometimes the fan is not yet started before the device shows the air pressure switch is closed; Sometimes, when the fan is running normally, the equipment issues a warning of insufficient air pressure.

1. Analysis of the Cause of the Problem

The two opposing phenomena you encounter actually have different roots and need to be diagnosed separately.

• Phenomenon 1 (fan not started but shows closed): This is usually due to incorrect initial setting of the air pressure switch or abnormal pressure transmission pipelines. The air pressure switch should be normally open when stationary. If its microswitch remains closed due to mechanical jamming or contact adhesion, or if the device's input port produces false conduction signals due to damp leakage, it will result in a "false closure." Additionally, if the pressure transmission pipe is reversed or there is residual positive pressure in the pipeline, it may also be triggered falsely.

• Phenomenon 2 (fan running but alarm not closed): The most common reason is that the air pressure switch is set too high, causing the negative pressure generated by the fan to be insufficient to drive the diaphragm; Or the pressure transmission pipe may be blocked, loose, or bent, causing pressure to be unable to be effectively transmitted to the pressure sensor port of the air pressure switch; Another possibility is dust accumulation on the fan impeller or partial blockage in the flue, indicating that the actual negative pressure is indeed insufficient (not a misjudgment by the equipment).

2. Detailed Investigation and Solutions

Regarding Phenomenon 1: The fan closes before starting

1. Check the air pressure switch itself: remove the air pressure switch and gently suck the low-pressure port with your mouth (pay attention to hygiene). Under normal conditions, you should hear a "click" contact movement; After releasing it, it should be returned to its position immediately. If it does not release after being sucked or is delayed, it indicates that the internal diaphragm is aging or the reset spring has failed. Replace the air pressure switch directly (maintenance is not recommended).

2. Check the input channel of the testing equipment: Use the multimeter's resistance range to measure the two terminals of the air pressure switch; when stationary, it should be infinite (open circuit). If the measured resistance is close to 0Ω but returns to infinite after removing the switch, it indicates the switch itself is adherent; If after removing the switch, there is still a resistance of several tens of kΩ between the two terminals on the measurement end of the device, it indicates the internal circuit board is damp or dirty. Use anhydrous ethanol to clean the circuit board and dry it with a hair dryer on the cool setting.

3. Check the pressure transmission pipeline connections: Make sure the pressure transmission pipe is not connected in reverse (the "+" or "H" port of the air pressure switch is connected to the high-pressure side, and the "-" or "L" port is connected to the low-pressure side). At the same time, check whether condensation water has accumulated in low-lying areas of the pipeline forming a liquid seal, preventing pressure from dissipating. Operating method: Remove the pressure transfer pipe, blow through with 0.2MPa compressed air, and confirm the pipeline is unobstructed and free of standing water.

Regarding Phenomenon 2: The fan is running but the alarm is not closed

1. Measure the fan's negative pressure with a pressure gauge: Connect a digital pressure gauge (range± 500Pa) in parallel at the air pressure sampling port of the testing station. Start the fan and read the actual negative pressure value. For example: if the nameplate states the air pressure switch operating pressure is 150Pa, but the measured negative pressure is only 120Pa, then there is indeed a problem with the fan or flue; If the measured negative pressure reaches 180Pa but the air pressure switch is still closed, the switch setting value is too high.

2. Adjust the pressure of the air pressure switch (adjustable type only): Use a small flathead screwdriver to turn the adjustment screw on the air pressure switch. Rotate clockwise (increase spring compression) to increase the operating pressure, rotate counterclockwise to reduce the acting pressure. It is recommended to rotate 1/4 full turn each time and then retest until the fan starts, the switch is reliably closed, and the fan can be reliably disconnected after stopping.

3. Cleaning the fan impeller and flue: After long-term use, the fan impeller can accumulate oil and dust, causing airflow to drop. Remove the fan, clean the impeller with a soft brush and special cleaning agent, and check the exhaust pipe for bird nests or foreign objects blockage.

3. Daily preventive measures

• Pressure transmission pipelines are blown with compressed air every six months to prevent dust and insect nesting.

• For devices installed in humid environments (such as near bathrooms), it is recommended to apply conformal paint to the wiring of the air pressure switch to prevent false alarms from leakage.

• Each time the equipment starts up for self-check, it should automatically detect the initial state of the air pressure switch (it must be open). If it detects closure, an alarm will immediately alert the operator to inspect.