Knowledge Center

The Knowledge Center is created by Computer Room Uptime to be used as a technical resuource for IT Administrators and facility managers.  We will post in-depth technical topics from time to time that will help explain complex computer room concepts.  IT design methodologies change quickly in our industry to keep up with the ever changing IT equipment located in computer rooms.  Our goal will be to keep this page updated with the latest information to help you improve your network's uptime and server room's efficiency.



is the measure of time a computer network has been functioning or available for use.  It replaces the term downtime, time when a system is not operational, to describe the availability and reliability of a computer network.  The uptime of computer/communication facilities are often measured by the percentage, "Nines", in which the network is up and running.  For example "Five Nines" or 99.999% availability results in a total downtime of 5 minutes and 16 seconds per year.  Different businesses and organizations require different levels of network availability.  It is acceptable for most businesses to have planned network shutdowns during the night or weekends, to limit the added expense of keeping your network running continuously.  Unfortuantely the uplanned shutdown that happen during business hours kills productivity, lose sales, damage equipment, corrupt files and impact your company's reputation.

  • Local A/C contracting company does not need 99.999% availability
    • IT primarily enhances internal business process and merely uses a "web presence" as a passive marketing tool
  • 911 call center needs 99.999% availability
    • Network and communication equipment runs 24/7/365

Computer Room Uptime's goal is to educate our customers by applying proven data center design philosophy to their small and medium sized computer rooms, and help them make the right decision for their business.








Per Day

Per Month

Per Year


0.9 secs

26 secs

5 mins 16 secs


9 secs

4 mins 23 secs

52 mins 36 secs


1 min 26 secs

43 mins 50 secs

8 hrs 45 mins 57secs


14 mins 24 secs

7 hrs 18 mins 17 secs

87 hrs 39 mins 30 secs



Critical Temperature Measurements inside a Data Center 


Executive Summary

Understanding the health of your data center is easy by taking (4) simple temperature measurements.  These measurements will help you understand the efficiency of your data center, the performance of your computer room air conditioner and problems with the layout of your data center.



Measuring temperatures inside your server room is an asset to understanding the health of your network and the supporting computer room infrastructure.  This white paper will provide practical knowledge on where to take temperature measurements and how to interpret them.  To provide maximum  network uptime, it is critical to understand temperature dynamics within your data center and take proactive steps to prevent unplanned network downtimes.  Understanding the temperature results will provide insight to the risk of unacceptable high temperatures, data center efficiency, computer room air conditioner (CRAC) performance and infrastructure layout improvements.  There are four critical temperature measurements that should be taken from any size server room/data center (see Figure 1 & 2):

1.  Server Inlet Temperature

      Temperature entering into the front of your servers

2.  Server Outlet Temperature

      Temperature  exiting the back of your servers

3.  CRAC - Cold Supply Air

      Temperature exiting your computer room air conditioner

4.  CRAC - Warm Return Air

      Temperature returning back to your computer room air conditioner


Figure 1



Figure 2



Temperature Measurement Location


Server Inlet Temperature

Measuring the server inlet temperature should be done at the front face of the computer rack at three separate locations:  top, middle, and bottom.  Typically, the top measurement will be the warmest because hot air rises, and there might be recirculation of the warm air from the back of the computer rack (server exhaust being drawn over the server cabinet to the front face and into the server inlet).  Ideally these measurements should be taken at every computer rack, but if that is not feasible be sure a good enough sample is taken around the room.


Server Outlet Temperature

Measuring the server outlet temperature should be done at the back face of the computer rack at three separate locations:  top, middle, and bottom.  The warmest measurement should be at the top of the server cabinet, as a result of the warmest server inlet temperature described above.  It is important to take a good sample of measurements around the room.


CRAC - Cold Supply Air

Measuring the cold air supply from the computer room air conditioner should be done where the air is exiting the air conditioner.  If the computer room air conditioner is a down-flow configuration that is blowing into a raised floor plenum, make sure you take the measurement under the floor next to the computer room air conditioner to get the most accurate reading.  Taking temperature measurements next to a computer room air conditioner, eliminates warm air mixing with the cold, therefore providing a more accurate reading.


CRAC - Warm Return Air

Measuring the warm return air from the computer room air conditioner should be done where the air is entering the air conditioner.  If the computer room air conditioner is a down-flow configuration that is blowing into a raised floor plenum, the return air will be entering the air conditioner from the top of the unit.  In most server rooms / data centers the suspended drop ceiling will the return plenum.  The return plenum is where all the warm server exhaust air is routed back to the computer room air conditioner.


Interpreting Temperature Measurements


1.  Server Inlet Temperature < 81°F

  • American Society of Heating Refrigeration Air Conditioner Engineers (ASHRAE)
    • Thermal Guidlines for Data Processing Environments (2008)
      • Server inlet temperatures need to be below 81°F
      • "Recommend Environmental Envelope" is acceptable to all IT manufacturers
        • Maintaining all (3) server inlet temperatures (top/middle/bottom) below this threshold will increase the network's reliability, longevity, and speed
  • Server inlet temperature is the most critical of all measurements
    • Network's uptime is in jeopardy if servers are receiving air that is warmer than 81°F
2008 Recommended Environmental Envelope Data

Low-End Temperature          64°F

High-End Temperature         81°F

Low-End Moisture               42°F Dew Point

High-End Moisture               60% RH & 59°F DP

2.  Server Outlet Temperature ~ 15°F - 35°F warmer than inlet

  • Temperature rise through electronic equipment (servers, storage devices, telecom devices, UPS, etc...) should be between 15°F and 35°F
    • Temperature depends on the equipment installed in a server cabinet
      • High density 1U servers will have a higher temperature rise
      • Older multiple "U" equipment will have a lower temperature rise
  • Server outlet temperatures less than 15°F
    • Caused be cold air bypassing equipment and going straight through racks that are not fully populated
      • Installing blanking panels in unused portions of the computer rack will separate the cold air in the front of the rack from the warm air in the back
3.  CRAC Cold Supply Air < 75°F
  • Cold air less than 75°F
    • Server inlet temperatures will less than 81°F if there is no mixing of cold and warm air
      • Above 81°F will damage and slow-down the network
    • Energy use will decrease as the cold air temperature increases toward 75°F
      • Reaction time to data center emergencies will decrease when the computer air conditioner is running at a higher set point (i.e. 75°F)
4.  CRAC Warm Return Air ~ Server Outlet Temperature
  •  Computer Room Air Conditioner Performance
    • CRAC temperature split:  15°F - 25°F    (Return Air Temperature - Cold Air Supply Temperature)
  • Cold and warm air mixing
    • If the return air is cooler than the server outlet temperature, than the cold air is mixing with the warm return air before it reaches the computer room air conditioner
      • Lowers the efficiency of your computer room air conditioner
      • Creates "hot spots" within your data center


Taking temperature measurements within the data center / server room should be done periodically.  The initial temperature measurements will provide a baseline for the data center.  The goal is to improve the data center's efficiency, while ensuring the servers always receive air below 81°F.  The best way to improve your data center's efficiency is to separate the warm air from the cold air with a cold aisle / hot aisle layout.  This layout can be modeled in both small and large data centers.  In addition to the cold aisle / hot aisle layout, physical barriers within the data center to separate air are important as well.


Physical Barriers

  • Hot Aisle Containment / Cold Aisle Containment above the racks
    • Use a manufactured containment system or a customized vinyl containment system
  • Blanking panels within computer racks
    • Use blanking panels within computer racks that are not fully populated
  • Raised Floor
    • Seal all cable openings to eliminate cold air escaping into hot aisles
As the cold air and hot air are separated by the computer rack layout and the physical barriers listed above are implemented, the efficiency of the computer room air conditioning system will increase.  The benefits are significant in both the network's increased uptime and reduction in capital/operating expenses.


Efficiency Benefits

  • Increased cooling capacity
    • CRAC system will receive higher return temperatures, which will allow the air conditioners to operate more efficiently at a higher cooling capacity
    • Existing CRACs will be able to cool more computer equipment
  • Reduction in operating expenses
    • Operating at a higher CRAC set point will use less energy
  • Elimination of "Ho Spots" within the data center
    • Separation of cold and hot air ensure inlet server temperatures below 81°F