Chilled Water System Differential Pressure setpoint reset control is poor

Objective/ Method/Outcome

Objective: Determine if the Chilled Water System setpoint reset control is effective.

Method: The Chilled Water System DP setpoint is compared to the AHU valve opening to determine if the setpoint reset control is poor.

Outcome: An insight is generated when the reset control is determined to be poor.

Description

Reference: ASHRAE90.1-6.5.4.2/ASHRAE Guideline 36-5.20.5

This rule determines if the DP setpoint can be reset based on the AHUs valve position. Pump will have a lower speed when DP setpoint is reset lower.

To ensure all AHUs are providing enough cooling,  proper DP setpoint can be as low as possible to ensure the maximum AHU valve position is nearly fully open. General speaking, this rule is applicable for pumps with DP control. For example, VFD primary pumps in primary-only system and VFD secondary pumps in Primary/Secondary system can apply this rule.  

This rule is part of the CopperTree Standard rule-set. (Aliased Celsius / Aliased Fahrenheit)

Default Priority

Medium

Diagnosis

  1. Check the building load.
  2. Verify pump is running in accordance with heating load.
  3. Inspect primary HVAC equipment.

Message

Chilled water differential pressure could be reset based on valve position.

Rule Template

Insight Type: Energy

Trigger Type: Active High

Notes

1. Expected values for “Pump Status” inputs are “0” or “1”.

2. Expected values for “Pump Speed” inputs are “100” if they are constant speed pumps or not available.

3. The input(s) that is(are) not used should be set to “0”.

4. The equations used in the rule were generated from the following article’s graph:

https://control.com/textbook/control-valves/control-valve-characterization/

Default Values:

Const. – Target Maximum Valve Position [%]: 85

Const. – Pump Power Rating [HP]: 10

Const. – Threshold for Faulty Daily Hours of Valve Position [hours]: 2

Const. – Energy Rate [$/kWh]: 0.1

Const. – Weeks of Operation: 26