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Calculator · IT cooling
Server room cooling calculator
A server room is cooled for the heat its equipment gives off, and nearly all of the power drawn becomes heat. This page adds the IT load, the UPS and distribution losses, the lighting at 1.33 W per sq ft and 100 W per person, and multiplies the watts by 3.412 to get BTU/h. 12 kW of IT load with a 20 kW power system in 400 sq ft and 2 people comes to 14,572 W, which is 49,720 BTU/h, 4.14 tons and 2,302 CFM at a 20 F rise. The unit rating adds up to 30 percent for mixed air, and N+1 adds a spare unit. Humidity control, walls and roof, and rack airflow are not modeled. The same load drives the HVAC CFM calculator and the BTU to tons converter.
- Give the IT load in kW, taken from metering or equipment data, and the rating of the UPS or power system. The page opens on 12 kW of IT load behind a 20 kW system.
- Enter the floor area, the most people in the room at once and whether the room sits inside an air-conditioned building. The default room is 400 sq ft with 2 people. An exposed room needs an envelope figure from an HVAC consultant.
- Choose a ducted return or mixed air, no spare or N+1, and the temperature rise across the equipment. The page returns the load, 49,720 BTU/h or 4.14 tons for the default room, with the unit rating, the installed capacity and 2,302 CFM at 20 F.
Example result for the default inputs
12 kW of IT load in 400 sq ft: 49,720 BTU/h of heat, 4.14 tons.
Cooling load
49,720 BTU/h
Tons
4.14 tons
Load in kilowatts
14.57 kW
Airflow
2,302 CFM
| IT equipment | 12,000 W, the power draw |
|---|---|
| UPS losses | 1,400 W, 0.04 x 20 kW + 0.05 x 12 kW |
| Power distribution | 440 W, 0.01 x 20 kW + 0.02 x 12 kW |
| Lighting | 532 W, 1.33 W x 400 sq ft |
| People | 200 W, 100 W x 2 |
| Heat inside the room | 14,572 W, 49,720 BTU/h at 3.412 BTU/h per W |
| Envelope heat added | 0 BTU/h |
| Cooling load | 49,720 BTU/h, 4.14 tons, 14.57 kW |
| Unit rating | 49,720 BTU/h, 4.14 tons, at 1 x the load |
| Installed capacity | 49,720 BTU/h, 4.14 tons, no spare unit |
| Airflow | 2,302 CFM at a 20 F rise |
- Heat from the IT equipment is its power draw, 12 kW. The UPS, the power distribution, the lighting and the people add 2.57 kW, so the IT equipment is 82.3 percent of the load.
- The room is taken as inside an air-conditioned building, so heat through walls, roof and windows is left out. That matches the source, and it does not hold for a room with outdoor exposure.
- No oversizing is added for humidity control, which the source allows only with a ducted return. A room with mixed air needs up to 30 percent more.
- Airflow is the sensible form, 1.08 x CFM x temperature rise, for air near sea level and room temperature. Latent heat, altitude and leakage are not in it.
- This is preliminary sizing from a quick worksheet. It is not a data center design, and it does not check humidity control, redundancy design or rack airflow. A mechanical engineer sizes the cooling system.
For estimation purposes only. Consult a licensed engineer for final designs.
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How the cooling load is worked out
This page adds up the heat released inside a server room or small data room and converts it to BTU/h, tons, kW and airflow. IT equipment gives off its power draw as heat. UPS and power distribution losses grow with the rating and with the load, lighting adds 1.33 W per sq ft, and each person adds 100 W. Multiplying the sum in watts by 3.412 gives BTU/h. Cooling units are then rated above that load for humidity control and for a spare unit. The worksheet is a public white paper from APC, now Schneider Electric, and it is a quick estimate for rooms under 4,000 sq ft, not a design standard.
- IT equipment
heat (W) = IT load (W)Power drawn from the mains is converted to heat, and the data lines carry a negligible share. 12 kW of IT load is 12,000 W of heat. Use the draw of the equipment and not the rating of the power system.- UPS and power distribution
UPS (W) = 0.04 x rating (W) + 0.05 x IT load (W); distribution (W) = 0.01 x rating (W) + 0.02 x IT load (W)With a 20 kW rating and 12 kW of IT load, the UPS gives off 1,400 W and the distribution 440 W. Each has a fixed part set by the rating and a part that follows the load. The rating leaves out a redundant UPS.- Lighting and people
lighting (W) = 1.33 x floor area (sq ft); people (W) = 100 x people400 sq ft of floor gives 532 W of lighting heat, and 2 people give 200 W. The lighting figure is the computer room power density that the white paper cites from ANSI/ASHRAE 90.1-2016.- Cooling load
load (BTU/h) = total (W) x 3.412 + envelope heat (BTU/h); tons = BTU/h / 12,000; kW = W / 1,000The default room adds up to 14,572 W, which is 49,720 BTU/h, 4.14 tons and 14.57 kW. White paper figures are 3.41 BTU/h per watt and 3,530 W per ton, and this page uses 3.412 and 12,000 BTU/h per ton, within half a percent of those.- Unit rating and spare
rating = load x 1 (ducted return) or x 1.3 (mixed air); installed = rating x (N + 1) / NDucted return needs no oversizing and high mixing needs up to 30 percent. The default load becomes 64,636 BTU/h, 5.39 tons, on mixed air. With two duty units and one spare, the default rating of 49,720 BTU/h is installed as 6.21 tons. The general rule of the white paper for rooms under 4,000 sq ft is a unit rated at 1.3 times the IT load plus any redundancy.- Airflow
CFM = load (BTU/h) / (1.08 x temperature rise, F)The 1.08 is 0.075 lb/ft3 of air x 0.24 BTU/lb-F x 60 min/h, for air near sea level and room temperature. 49,720 BTU/h at a 20 F rise needs 2,302 CFM. A rise of 10 F raises that to 4,604 CFM.
Worked examples
The default case is 12 kW of IT load on a 20 kW power system in 400 sq ft with 2 people. The IT equipment gives 12,000 W, the UPS 1,400 W, the distribution 440 W, the lighting 532 W and the people 200 W, 14,572 W in all. Convert it. That is 49,720 BTU/h, 4.14 tons and 14.57 kW, and the IT equipment is 82.3 percent of it. With mixed air the unit is rated at 64,636 BTU/h, 5.39 tons, and with N+1 on two duty units the installed capacity is 6.21 tons. Nothing more.
The white paper works a 5,000 sq ft room with a 250 kW rating, 75 kW of IT load and 20 people. Our worksheet gives 101.4 kW. The paper's own text gives 105 kW from an older lighting figure, and the difference is small. Next, the scaled case in the table below runs the same ratings in 4,000 sq ft, the largest room the page takes.
| Case | Setup | Result |
|---|---|---|
| Default room, 12 kW of IT load | 12 kW IT, 20 kW rated, 400 sq ft, 2 people, UPS in the room, interior room, ducted return, no spare, 20 F rise | 49,720 BTU/h, 4.14 tons, 2,302 CFM, unit 4.14 tons, installed 4.14 tons |
| Source example scaled to 4,000 sq ft | 75 kW IT, 250 kW rated, 4,000 sq ft, 20 people, UPS in the room, interior room, ducted return, no spare, 20 F rise | 341,439 BTU/h, 28.45 tons, 15,807 CFM, unit 28.45 tons, installed 28.45 tons |
| Network closet, no one working in it | 3 kW IT, 5 kW rated, 100 sq ft, 0 people, UPS in the room, interior room, ducted return, no spare, 20 F rise | 12,259 BTU/h, 1.02 tons, 568 CFM, unit 1.02 tons, installed 1.02 tons |
| Default room with mixed supply and return air | 12 kW IT, 20 kW rated, 400 sq ft, 2 people, UPS in the room, interior room, mixed air, no spare, 20 F rise | 49,720 BTU/h, 4.14 tons, 2,302 CFM, unit 5.39 tons, installed 5.39 tons |
| Default room with N+1, two duty units | 12 kW IT, 20 kW rated, 400 sq ft, 2 people, UPS in the room, interior room, ducted return, N+1 with N = 2, 20 F rise | 49,720 BTU/h, 4.14 tons, 2,302 CFM, unit 4.14 tons, installed 6.21 tons |
| Default room on an outside wall, 15,000 BTU/h through the envelope | 12 kW IT, 20 kW rated, 400 sq ft, 2 people, UPS in the room, 15,000 BTU/h of envelope heat, ducted return, no spare, 20 F rise | 64,720 BTU/h, 5.39 tons, 2,996 CFM, unit 5.39 tons, installed 5.39 tons |
| UPS outside the room, 8 kW of IT load | 8 kW IT, 10 kW rated, 200 sq ft, 1 people, UPS outside, interior room, ducted return, no spare, 20 F rise | 29,432 BTU/h, 2.45 tons, 1,363 CFM, unit 2.45 tons, installed 2.45 tons |
| Dense room, mixed air, N+1 with three duty units | 40 kW IT, 60 kW rated, 600 sq ft, 2 people, UPS in the room, interior room, mixed air, N+1 with N = 3, 25 F rise | 159,675 BTU/h, 13.31 tons, 5,914 CFM, unit 17.30 tons, installed 23.06 tons |
Sources and data
- Schneider Electric (APC), White Paper 25, Calculating Total Cooling Requirements for Data Centers - The heat output worksheet, with the IT load as heat, the UPS at 0.04 x rating plus 0.05 x load, distribution at 0.01 and 0.02, lighting at 1.33 W per sq ft and 100 W per person. Also its oversizing of up to 30 percent for humidification, its quick method for rooms under 4,000 sq ft, and its example of a 5,000 sq ft room with a 250 kW rating, which this page reproduces at 101.4 kW. The text of the paper gives 105 kW for that example, which matches an older lighting figure of 2 W per sq ft.
- ACCA, The Physics of Heat and Air Flow - The sensible heat equation BTU/h = 1.08 x CFM x temperature rise, with the 1.08 built from an air density of 0.075, a specific heat of 0.24 and 60 minutes per hour, and the caution that it holds for air near sea level and room temperature.
Limitations of this calculator
- The method is a quick worksheet from a public white paper and not a standard. It suits rooms under 4,000 sq ft, and a larger area is refused.
- Heat through walls, roof and windows is not calculated. A room inside an air-conditioned building can leave it out, as the white paper allows, and a room with outdoor exposure needs a figure from an HVAC consultant, which the page adds as BTU/h.
- Latent load and humidification are not modeled. The page only raises the unit rating by up to 30 percent for mixed air, which is the range in the white paper, and it does not size a humidifier or check a humidity band.
- UPS and distribution losses are the white paper's averages across brands. Efficiency at light load, a modular UPS or a rack-level power unit differ, so use the maker's loss data when you have it.
- Airflow is the sensible form on the total heat at one temperature rise. Altitude, leakage, bypass air and the airflow each device needs are not checked, and the 1.08 factor changes with air density.
- N+1 is the arithmetic of one extra unit of the same size. Fault tolerance, the power paths to the cooling units and failover control are not designed here.
- Rack-level airflow, hot and cold aisle containment and raised floor design are not covered. A room total can be right while one rack runs hot.
- Heat from the fans and compressors of the cooling units is left out because it is rejected outdoors, and the electrical load of the cooling system is not calculated.
- Growth in the IT load, a larger UPS later or a changed lighting level moves the load, and future requirements are a separate allowance in the white paper that this page does not add.
- This is preliminary sizing and not a design drawing. A mechanical engineer and the equipment supplier decide the cooling system, and local code and the insurer may set further requirements.
Common server room cooling mistakes
- Using the power system rating as the IT load. Heat output is the power the IT equipment draws, 12 kW here. Reading the 20 kW rating of the power system as the IT load would count 68,240 BTU/h of IT heat and not 40,944.
- Leaving out the UPS, the distribution and the lighting. IT heat alone is 40,944 BTU/h for 12 kW. The UPS, the distribution, the lighting and 2 people raise the load to 49,720 BTU/h, 21.4 percent more.
- Ignoring the walls and roof of an exposed room. A room against an outside wall or under a roof gains heat that the worksheet leaves out. Adding 15,000 BTU/h for such a room moves the default load from 49,720 to 64,720 BTU/h, and the page raises an alert while that figure is missing.
- Mixing up watts, BTU/h and tons. The default room is 14,572 W, which is 49,720 BTU/h and 4.14 tons. Reading 14,572 as BTU/h would undersize it by a factor of 3.412, and reading 14.57 kW as tons would oversize it by a factor of 3.5.
- Skipping the oversizing for mixed air. A unit rated at the bare load has no margin for humidification when supply and return air mix. For the default room the load is 49,720 BTU/h and the mixed-air rating is 64,636 BTU/h, 5.39 tons.
- Using the wrong temperature rise for airflow. Airflow scales inversely with the rise. 49,720 BTU/h needs 2,302 CFM at 20 F and 4,604 CFM at 10 F. Take the rise from the equipment data sheets, and not from the setpoints of the room.
Common questions about server room cooling
How do you calculate server room cooling load?
Add the heat of the IT equipment, the UPS and distribution losses, the lighting and the people, then multiply the watts by 3.412. 12 kW of IT load with a 20 kW power system in 400 sq ft and 2 people comes to 14,572 W, or 49,720 BTU/h. Rate the cooling unit above that.
How many BTU per hour does 1 kW of server load produce?
Each kilowatt of IT power draw is 3,412 BTU/h of heat, because nearly all the power becomes heat. The UPS, distribution, lighting and people add to that, so the default room runs at 4,143 BTU/h per kW of IT load.
How many tons of cooling does a server room need?
Divide the load in BTU/h by 12,000. The default room is 49,720 BTU/h, which is 4.14 tons, and 5.39 tons with mixed air. Unit capacities come in set sizes, so the supplier picks the next size up from the rated figure.
How much airflow does a server room need?
Divide the load by 1.08 times the temperature rise across the equipment. The default load of 49,720 BTU/h at 20 F needs 2,302 CFM, and a 10 F rise needs 4,604 CFM. Air at altitude is thinner, so the factor changes there.
What does N+1 cooling mean?
N units carry the load and one more unit of the same size stands by. With N = 2, the rated 49,720 BTU/h is split between two units of 2.07 tons each and a third is added, for 6.21 tons installed. The white paper only says to add capacity for redundancy, so this is the page's arithmetic.
Does the calculator include heat from walls and windows?
Not by calculation. The white paper leaves it out for a room inside an air-conditioned building and sends an exposed room to an HVAC consultant. Choose outdoor exposure and type the consultant's figure in BTU/h, and the page adds it to the load.
How big a room does this method cover?
The white paper says the quick method works well for rooms under 4,000 sq ft. Above that, the walls, the roof and air recirculation matter, so the page refuses a larger area and any case above 1,000 W of IT load per sq ft of floor.
Is there a limit on how many times I can run this?
No. This calculator runs in your browser with no daily limit and no signup. Run it as often as you like, for each room on the list this week.
Used on the same job
Meter the load, then size the unit
The IT load comes first, from metering or equipment data, and the room sets the lighting and the people. The load in BTU/h then gives the tons and the airflow, and the unit rating and the spare are added on top. For the electrical side, see the kW to amps calculator. You can also ask the assistant to go over a room layout.
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