In addition to understanding how the PID module and PRAC+ function, other parameter settings within the CCT/PCT application can affect how well a mechanical system controls. Certain parameters, if not set correctly, could create situations where a mechanical system behaves poorly, resulting in a control loop that cannot tune itself correctly, or system performance that is worse. See Table x for a list of critical data points to keep in mind when you use adaptive tuning.
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Issue |
Application |
Variables |
Notes |
|---|---|---|---|
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Slow control loop response to a setpoint change |
VAV Occupied Zone Sequencing (See Figure 1.) |
Fast Sequence Enabled Setpoint Threshold |
Ensure that Fast Sequence Enabled is set to True. This setting allows the user to enable or disable the transition to satisfied when the setpoint changes by more than the Setpoint Threshold. Ensure that the Setpoint Threshold is less than 0.5 degrees if Fast Switch Enabled is set to True to ensure a fast response on zone temperature changes. |
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Figure 1. VAV Occupied Zone Sequencing
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Zone recovers slowly or responds poorly to large setpoint changes |
Box Heating/Discharge Air Reset by Zone (See Figure 2.) |
Discharge Air Setpoint Cooling Min Discharge Air Setpoint Heating Max Discharge Air Setpoint Zone Satisfied |
Set the minimum and maximum discharge air temperatures of the VAV Box so that these match the minimum and maximum discharge air temperatures of the fan system. To limit heat in the zone, lower the maximum discharge air setpoint to prevent overheating. |
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Figure 2. Box Heating/Discharge Air Reset by Zone
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Dampers open too quickly in cold weather and AHU hits low temperature limit |
Mixed Air System/Damper Control/ Cold Weather (See Figure 3.) |
Damper Ramp Rate |
Change the Damper Ramp Rate from the default of 50% Change per Minute to 5% in order to slow down how quickly the dampers open in cold weather and ensure successful startup of the AHU. |
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Figure 3. Mixed Air System/Damper Control/Cold Weather
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Airflow variable must be reset from default |
OA Flow (See Figure 4.) |
Minimum OA Flow Range |
Set the Minimum OA Flow Range variable to the value expected from the minimum default outside air flow station. The default value is 5,000 cfm, but actual airflow values may be upwards of 20,000 cfm. |
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Figure 4. Outside Air Flow
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Hot air is sitting in duct work which causes dampers to open |
Cold Weather/DAT Start Stop Sequencing (See Figure 5.) |
Stabilize Delay |
Change the Stabilize Delay from 60 seconds to 120 seconds at minimum to help start up equipment in cold weather conditions, where heating coils are left in control and hot air is sitting in duct work. |
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Figure 5. Cold Weather/DAT Start Stop Sequencing
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Startup not stable |
Air Handling Unit (See Figure 6.) |
SF Ramp Rate Filter Setup |
Lower the SF Ramp Rate to 20 or 30% Change per Minute. For static pressure input, change the filter setup to VAV Flow Control if default filter is slow. |
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Figure 6. Air Handling Unit
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