Hot Water System Differential Pressure setpoint reset control is poor

Objective/ Method/Outcome

Objective: Determines if the Hot Water System Differential Pressure (DP) control is efficient.

Method: Uses the pump speed and valve position to determine if DP is controlling well to setpoint. An insight is generated if a mismatch occurs for longer than the defined Threshold for Faulty Hours Per Day.

Outcome: This rule generates an insight.

Description

Reference: Reference: ASHRAE90.1-6.5.4.2/ASHRAE Guideline 36-5.21.6.6

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 heating, 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.

Default Priority

Medium

Diagnosis

  1. The best practice is to reset Differential Pressure by using Trim and Respond sequence.
  2. A alternative way is to reset Differential Pressure based on building schedule. For example, lower DP setpoint in unoccupied hours.

Message

Hot 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