Air Handler Filter Pressure Sensor and Low Static PressureAir Handler Filter Pressure Sensor - This is a pressure sensor. It tells a maintenance technician if the air filters are dirty and clogged. The pressure control is a low static pressure switch to trip the unit if the mixing box drops below the set point pressure. The pressure the switch is calibrated to. In addition, the low static pressure sensor protects the fan and the duct work including dampers from damage. If the economizer damper and the return air damper would fail closed (or another possible scenario) that would cause the unit to pull negative pressure in the mixing box.

Air Handler Filter Pressure Sensor – Pressure Controls

Pressure controls typically either protect the unit from a catastrophic failure or are used to control a fan or dampers. Pressure controls are either switches that trip at a calibrated pressure set point or transmitters that can transmit an analog signal. The signal is transmitted back to the controls systems to other components that control air flow. On this air handling unit there are four pressure sensors. The pressure sensors shown in the photo (Filter Pressure Sensor and Low Static Pressure) and two additional pressure sensors.

Air Handler Filter Pressure Sensor - Duct Static Pressure Sensor

This air handler provides air to DDC Variable Air Volume Systems. so the air flow provided by the Air Handlers will modulate based on what the need of the VAV boxes require and what the duct static pressure sensor is reading. The fan will modulate to maintain a pressure set point in the duct work. The fan modulates via a variable frequency drive. The signal for the frequency drive comes for the control and is an analog signal.

On one side of the DDC controller is the input coming from the pressure sensor. It tells the control program what the actual duct pressure is reading. The control program is set to maintain a certain pressure. The input from the duct static pressure sensor goes through the control program and program responds. It responds by sending a signal to the variable frequency drive (VFD) to modulate the fan to maintain the duct pressure set point.

What changes the pressure in the duct? VAV boxes have dampers and the dampers respond by temperature. If the room is warm more air flow is fed through the secondary duct. If the temperature is cool in the room (below set point) the damper closes off to a minimum position. Then the box goes into heating mode. If several VAV boxes, modulate at the same time, the dampers will cause a change in pressure in the duct. The fan compensates as described above to maintain a minimum amount of pressure in the duct work.

Air Handler Filter Pressure Sensor - High Duct Static Pressure Switch

The high duct static pressure switch protects the duct work but shutting down the supply fan (and any other fans associated with the air handler if it is so equipped) if a high pressure scenario was encountered. Several years ago before I learned Building Automation Systems I responded to a call. There were two rooftop units that were not working and the building engineer was frustrated trying to figure out the problem. I found that the high duct static pressure sensor (with auto reset) was tripping when the air handler started up.

Finally, a check of the VAV boxes found that the VAV boxes were on a scheduled and the schedule has the boxes in the unoccupied mode by an error in the program. When the VAV’s were in unoccupied mode they are sleeping and all dampers were closed. So the rooftop air handlers kept shutting down because the air had no place to go. A look at the program and a change in the schedule fixed the problem and everything went to normal but the high static pressure switches protected the duct work.

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