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Off: This is the Off state. Here, the fans are off, and the dampers are closed. The unit waits for the signal to begin the start up sequence. The start sequence begins if heating or cooling is required. In addition to these, the Supply Fan Output Controller must respond to this sequencer.
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Start Fans: This state starts both fans. No delay is needed between supply or exhaust fans because there is no connecting duct between them.
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Ramp SF: A variable capacity supply fan is set to ramp so it slowly speeds up and slowly increases the duct static pressure. The FSM switches to the On state when the duct static pressure is above the duct static pressure setpoint. Constant Capacity fans are kept in this state for the Stabilize Delay to allow the outside temperature to accurately sense the outside conditions prior to determining whether a Cold Start should become active.
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Cold Start: This state allows preheat to come into control while limiting the rate at which it can close to prevent tripping the low limit.
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Normal Start: This state is activated when it is determined that the OA temperature is not cold. It is active for a single execution to turn off preheat prior to releasing the unit to sequenced control.
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On: The fans and dampers are sequenced properly for a startup condition. Control of the dampers is released to a lower level state machine. The AHU is allowed to run.
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Stop Fans: This state is used to turn off the fans.
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Fan Fault: The FSM enters this state after proof of airflow fault. The FSM remains in this state until airflow is proven. Once airflow is proven, the FSM transitions to the RAMP SF state to restart the supply fan.