blockEnable

Information

Block that generates chiller plant enabling signals and outputs the initial plant stage, according to ASHRAE Guideline 36-2021, section 5.20.2.1, 5.20.2.2 and 5.20.2.3.

The chiller plant should be enabled and disabled according to the following sequences:

1. An enabling schedule should be included to allow operators to lock out the chiller plant during off-hours, e.g. to allow off-hour operation of HVAC systems except the chiller plant. The default schedule shall be 24/7 and adjustable.
2. The plant should be enabled in the lowest stage when the plant has been disabled for at least plaThrTim, e.g., 15 minutes, and:
   • Number of chiller plant requests > ignReq (ignReq should default be 0 and adjustable), and,
   • Outdoor air temperature is greater than chiller lockout temperature, TOut > TChiLocOut, and,
   • The chiller enable schedule is active.
3. The plant should be disabled when it has been enabled for at least plaThrTim, e.g., 15 minutes, and:
   • Number of chiller plant requests ≤ ignReq for reqThrTim, or,
   • Outdoor air temperature is 1 °F less than chiller lockout temperature, TOut < TChiLocOut - 1 °F, or,
   • The chiller enable schedule is inactive.

The following state machine chart illustrates the transitions between plant enable and plant disable:

Parameters

Connectors

Revisions

• January 19, 2021, by Milica Grahovac:
  Added state chart illustration.
• March 12, 2020, by Milica Grahovac:
  Removed initial stage determination as it is implemented as a separate sequence. This is for issue 1831.
• January 24, 2019, by Jianjun Hu:
  First implementation.