Why Adjacent Data Center CRAC Units Fight Each Other on Humidity—Teamwork Fix

Resolving simultaneous humidification and dehumidification in CRAC units through coordinated dewpoint control strategies.

Direct Engineering Summary

Adjacent Data Center CRAC units often fight over humidity due to independent relative humidity (RH) control loops, leading to simultaneous humidification and dehumidification. This inefficiency is resolved by implementing a master-slave control strategy, typically via a BACnet or Liebert iCOM bus, where one unit acts as the dewpoint master, dictating the setpoint for all slave units. This ensures a unified approach to humidity management, maintaining precise ASHRAE 90.1-2019 thermal guidelines. Western Mechanical, Inc. (CSLB #618138) employs UA Local 250 journeymen to engineer and implement these advanced control solutions, preventing energy waste and ensuring optimal data center environments. Contact us at (661) 251-0111.

Why Adjacent Data Center CRAC Units Fight Each Other on Humidity—Teamwork Fix - Western Mechanical, Inc. CSLB #618138

Engineering Tolerances & Governing Standards

Parameter	Specification	Standard
ASHRAE TC9.9 Recommended RH Range	8% to 80% (dewpoint 41.9°F to 59°F)	ASHRAE 90.1-2019
Typical Data Center Dewpoint Setpoint	55°F (12.8°C)	ASHRAE 90.1-2019
Relative Humidity Control Deadband	±2% RH	Liebert iCOM Best Practices
Dewpoint Sensor Accuracy	±0.9°F (±0.5°C)	NIST Traceable
BACnet MS/TP Baud Rate	38.4 kbps to 76.8 kbps	ANSI/ASHRAE 135-2016

Understanding the Inefficiency of Independent RH Control

The fundamental problem of adjacent Computer Room Air Conditioner (CRAC) units battling over humidity stems from their independent relative humidity (RH) control algorithms. Each CRAC unit, operating autonomously, senses the return air RH and attempts to maintain its individual setpoint. If Unit A's sensor reads slightly below its RH setpoint, it will energize its humidifier, introducing steam into the data center. Simultaneously, if Unit B's sensor reads slightly above its RH setpoint, it will engage its cooling coil to subcool the air below its dewpoint, thereby dehumidifying the space. This direct opposition creates a thermodynamic tug-of-war, wasting significant electrical energy on both humidification and latent cooling.

This scenario is particularly prevalent in Southern California's varied climate, where ambient conditions can shift rapidly, influencing internal data center environments. The constant cycling of humidifiers and cooling coils not only inflates operational costs but also increases wear and tear on critical components, reducing equipment lifespan. Furthermore, the rapid swings in localized humidity can stress sensitive IT equipment, leading to potential failures. Western Mechanical, Inc. diagnoses these inefficiencies by analyzing historical trend data from Building Management Systems (BMS) to identify patterns of simultaneous heating and cooling, which are clear indicators of this control conflict.

Implementing Master-Slave Dewpoint Coordination via BACnet

The most effective solution to prevent CRAC units from fighting over humidity is to transition from independent RH control to a coordinated dewpoint master-slave strategy. Instead of each unit reacting to RH, a designated 'master' CRAC unit or a central controller measures the return air dewpoint and broadcasts this value, along with a unified dewpoint setpoint, to all 'slave' CRAC units via a robust communication protocol like BACnet MS/TP or Liebert iCOM. This ensures that all units operate under a single, consistent humidity objective, eliminating conflicting commands.

Under this master-slave architecture, if the master unit determines that the dewpoint is too low, it will command only the necessary humidifiers (often from a single unit or a shared system) to activate. Conversely, if the dewpoint is too high, it will instruct cooling coils to engage for dehumidification in a coordinated manner, preventing simultaneous humidification elsewhere. This precise control maintains the critical ASHRAE 90.1-2019 recommended dewpoint range (41.9°F to 59°F), optimizing energy consumption and prolonging equipment life. Our UA Local 250 journeymen specialize in integrating and commissioning these advanced control sequences for maximum efficiency.

Optimizing Data Center Energy Efficiency and Reliability

Beyond merely resolving the humidity conflict, implementing a master-slave dewpoint control strategy significantly enhances overall data center energy efficiency. By eliminating redundant humidification and dehumidification cycles, facilities can realize substantial reductions in electrical consumption, particularly from humidifier resistive heaters and compressor loads. This directly translates into lower utility bills and a reduced carbon footprint, aligning with sustainability goals and Title 24 Part 6 energy efficiency standards prevalent in California. The improved stability of the environmental conditions also reduces thermal stress on IT hardware, leading to greater uptime and reliability.

Western Mechanical, Inc. approaches these upgrades with a comprehensive lifecycle cost analysis, demonstrating the tangible return on investment from such control system modernizations. Our expertise extends to selecting and integrating high-accuracy dewpoint sensors, configuring complex BACnet network architectures, and programming sophisticated control logic to ensure seamless operation. By proactively addressing these common control inefficiencies, we help data center operators in Southern California achieve optimal PUE (Power Usage Effectiveness) values, extend the operational life of their CRAC units, and maintain a highly stable environment for their mission-critical IT infrastructure.

UA Local 250 Certified Field Diagnostic & Commissioning Protocol

Verify BACnet MS/TP network integrity and baud rate (e.g., 38.4 kbps) across all CRAC units using a network analyzer.
Calibrate return air dewpoint sensors in each CRAC unit to NIST traceable standards, ensuring ±0.9°F accuracy.
Confirm master-slave control logic programming, ensuring only the designated master CRAC unit or central controller dictates dewpoint setpoint.
Trend return air RH, supply air RH, return air dewpoint, and humidifier/dehumidifier status for 72 hours to confirm coordinated operation.
Measure humidifier amperage draw and compressor run times before and after control optimization to quantify energy savings.

Facility Director & Chief Engineer FAQs

What is the primary cause of CRAC units fighting over humidity?

The primary cause is independent relative humidity (RH) control loops in adjacent CRAC units. Each unit attempts to maintain its own RH setpoint, often leading to one unit humidifying while another simultaneously dehumidifies, wasting energy.

How does dewpoint master-slave control resolve this issue?

Dewpoint master-slave control designates one unit or a central controller to measure and dictate the dewpoint setpoint for all CRAC units. This unified command prevents conflicting actions, ensuring coordinated humidification or dehumidification across the data center.

What communication protocols support this coordinated control?

Common communication protocols that support coordinated dewpoint control include BACnet MS/TP (ANSI/ASHRAE 135-2016) and proprietary systems like Liebert iCOM. These protocols allow CRAC units to share data and receive unified commands from a master controller.

What are the benefits of optimizing CRAC humidity control?

Optimizing CRAC humidity control leads to significant energy savings by eliminating redundant operations, extended equipment lifespan due to reduced wear, and a more stable, reliable environment for critical IT infrastructure, improving PUE.

Acton Headquarters & 24/7 Southern California Dispatch

Western Mechanical, Inc. • 2210 Soledad Canyon Road, Suite O, Acton, CA 93510 • Primary Phone: (661) 251-0111 • 24/7 Toll-Free: (800) 585-5085 • CSLB #618138 (C-20, C-38, C-4)