Differential Pressure Setpoint Reset (CPO) - Johnson Controls - Metasys - LIT-12011147 - Software Application - Controller Configuration Tool - 13.1

Controller Tool Help

Johnson Controls
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Controller Configuration Tool
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User Guide
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The Differential Pressure Setpoint Reset option loads the Pump Differential Pressure Setpoint Reset module, which resets the dP setpoint based on the building differential pressure (when the By coil position option is not selected).

By coil position: This option loads the dP Setpoint Reset by Coil Position module, which resets the differential pressure setpoint based on the valve position on the cooling coils.

You can reset the variable speed primary chilled water pump dP setpoint based on one of two methods, reset by building dP or reset by coil position. When reset is based on building dP, the pump dP setpoint incrementally increases (stage up) if the building dP is below the building dP setpoint minus deadband (default = 2 psi), and incrementally decreases (stage down) if the building dP is above that setpoint plus deadband. When reset is based on chilled water coil valve position, a number of valve positions (default of four) are monitored and a statistical algorithm is applied to determine if the pump dP setpoint is incrementally increased (stage up), decreased (stage down), or held at its present value.

The incremental stage up and stage down thresholds should be set to values that are proportionate to the expected operating point of the dP across the building. The default values of 3 psi and 2 psi for the incremental increase and decrease thresholds respectively, are appropriate for a building dP of 20 psi. As a rule of thumb, the increase and decrease thresholds should be approximately 10% of the building dP operating point.

Set the minimum differential setpoint to a value that is large enough so that the fully open bypass valve meets all the minimum flow requirements when the building pressure is at the minimum differential setpoint:

Figure 1. Minimum Differential Point Equation

where dP min is the minimum differential pressure setpoint, flow min is the largest minimum flow requirement (all chillers) and CV is the flow coefficient of the bypass valve (when open).

The pump dP setpoint is not reset to a value that exceeds the maximum dP setpoint (default = 40 psi).

Figure 2. Differential Pressure Setpoint Reset by Building Differential Setpoint