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This York air handling unit serves several VAV boxes and runs on a static pressure control setpoint determined by a DDC program. This program has a static pressure control setpoint and sends a signal to the variable frequency drive to speed up or slow down the blower to maintain the proper static pressure control setpoint. When the VAV boxes need more air the drive speeds the blower up. When the VAV boxes need little the drive slows the blower down. The VAV boxes are controlled by a thermostat in their respective zones.
When the space is too warm the thermostat sends a signal to the controller. The controller gets this input from the thermostat and determines that more air flow is needed. It opens the damper more using more air supplied by the air handler blower. The static pressure transducer reads this loss of static pressure and sends a signal to the DDC controller. The DDC controller sees that the duct static pressure is below the setpoint and sends an analog signal to the variable frequency drive. This analog signal tells the variable frequency drive to speed up to make up for the loss in duct static pressure as determined by the static pressure transducer. This same scenario is reproduced over and over again during the occupied hours that the building is open. During the unoccupied hours, which is programmed into the building automation system, the entire system shuts down.
