Variable Speed Fan Drives


  • CRAC (computer room air conditioning) unit fans consume a lot of power and tend to account for 5% to 10% of a data center's total energy use. (Typically, only cooling compressors use more energy in a data center.) Most CRAC units are unable vary their fan speeds with the data center server load, which tends to fluctuate. Because data center environments constantly change, variable-speed fan drives (or VSDs for short) should be used wherever possible. Retrofits of many CRAC and CRAH (computer room air handling) units are available.
  • To sufficiently cool equipment and provide more than one backup unit, data centers concurrently operate multiple CRACs around the clock. This can cause short-cycled cooling and extreme static pressures in the plenum, wasting energy. (A CRAC unit compressor that short-cycles turns itself on and off too frequently—thus reducing efficiency.) VSDs save energy when the data center load fluctuates.
  • Many energy-efficiency measures—hot aisle/cold aisle, containment, free cooling—depend on VSDs being installed on CRAC and CRAH units to fully realize their energy-saving potential.

Savings and Costs

  • A reduction of 10% in fan speed reduces that fan's use of electricity by approximately 25%. A 20% speed reduction yields electrical savings of roughly 45%.
  • Four ENERGY STAR certified data centers — BNY Mellon, Kaiser Permanente, RagingWire, and Target — employed variable speed drives to save hundreds of thousands of dollars annually and had paybacks ranging from 0.54 to 1.7 years.
  • EBay recently completed a case study (PDF, 578.49 KB) with EPA ENERGY STAR that showed the following paybacks for ultrasonic humidification and variable speed drives retrofits at their Phoenix data center:

    EBay Data Center Retrofit Measure Paybacks

    Measure Payback without APS Incentive (years) Payback with APS Incentive (years)
    Ultrasonic Humidification 1.9 0.5
    Variable Speed Drives 2.6 1.6
  • Retrofit kits for Liebert CW and Liebert Deluxe System/3 chilled water systems make it easy to upgrade, with an average payback of usually less than two years.20
  • The Green Grid recently completed a  comprehensive analysis (PDF, 2 KB)   of return on investment and power usage effectiveness at a data center for installing variable speed fans, adding baffles and blanking panels, repositioning temperature/humidity sensors, and adjusting temperature setpoints. Payback for VSDs was 16 months on capital costs alone.
  • The Green Grid’s  Chiller System Optimization: Project Impacts on Data Center Sustainability (PDF, 1 KB) examines the ROI of variable speed drives installed on 6 chiller pumps resulting in reduced pump speeds.


  • Not all manufacturers have retrofit kits for their older CRAC and CRAH units. The best option may be to ensure that your next CRAC or CRAH comes with variable speed fans.
  • When retrofitting, reduced fan speed can lead to condensation and freezing on the coils if not closely monitored. Newer CRACs and CRAHs with variable-speed fans use digital compressors that slow down with fan speed.