Calculating The Size of a Server Room Air Conditioner

This article is a quick guide to how to work out your requirements for an air conditioning unit for your Server Room or Data Center*.
In principle it’s easy to calculate the size of air conditioning unit you need for your Server Room, just add together all the sources of heat and install an air conditioning unit that can remove that much. In practice it’s rather more complicated.

Fire regulations often require that Server Rooms have levels of insulation far above that of a normal office. Providing sufficient cooling is essential to ensure reliable running of servers, routers, switches and other key equipment. Failure of the air conditioning can have serious consequences for the equipment itself and for your company. Early warning of problems and spare capacity in the cooling system are both highly desirable.

Calculating Heat Load
The amount of heat generated is known as the heat gain or heat load. Heat is measured in either British Thermal Units (BTU) or Kilowatts (KW). 1KW is equivalent to 3412BTUs.
The heat load depends on a number of factors, by taking into account those that apply in your circumstances and adding them together a reasonably accurate measure of the total heat can be calculated*.

Factors include:
The floor area of the room
The size and position of windows, and whether they have blinds or shades
The number of room occupants (if any)
The heat generated by equipment
The heat generated by lighting

Floor Area of Room
The amount of cooling required depends on the area of the room. To calculate the area in square metres:

Room Area BTU = Length (m) x Width (m) x 337

Window Size and Position
If, as is quite common, your Server Room has no windows, you can ignore this part of the calculation. If, however there are windows you need to take the size and orientation into account.
South Window BTU = South Facing window Length (m) x Width (m) x 870
North Window BTU = North Facing windows Length (m) x Width (m) x 165

If there are no blinds on the windows multiply the result(s) by 1.5.

Obviously if you are in the Southern Hemisphere you would swap the conversion factors as the heat on North facing windows is then greatest.
Add together all the BTUs for the windows.
Windows BTU = South Window(s) BTU + North Window(s) BTU

Occupants
Purpose built Server Rooms don’t normally have people working in them, but if people do regularly work in your Server Room you will have to take that into account. The heat output is around 400 BTU per person.

Total Occupant BTU = Number of occupants x 400

Equipment
Clearly most heat in a Server Room is generated by the equipment. This is trickier to calculate that you might think. The wattage on equipment is the maximum power consumption rating, the actual power consumed may be less. However it is probably safer to overestimate the wattage than underestimate it.

Add together all the wattages for Servers, Switches, Routers and multiply by 3.5.
Equipment BTU = Total wattage for all equipment x 3.5

Lighting
Take the total wattage of the lighting and multiply by 4.25.
Lighting BTU = Total wattage for all lighting x 4.25
Total Cooling Required
Add all the BTUs together.

Total Heat Load = Room Area BTU + Windows BTU + Total Occupant BTU + Equipment BTU + Lighting BTU

Total Heat Load x 2 (Factor of Safety) is the Cooling Load


This is the amount of cooling required so you need one or more air conditioning units to handle that amount of heat.
So what size of unit do I need?
Small air conditioning units have a cooling capacity of between 5000 and 10000 BTUs. Small units may fit in windows, venting to the outside world.
Larger units may be rated in tons of cooling. 1 ton of cooling is equivalent to 12 thousand BTUs.

* Disclaimer: This calculation is intended as a rough guide only. Complete accuracy cannot be guaranteed. Before you decide on an air conditioning unit you should commission an audit from a suitably qualified air conditioning equipment specialist or installer.

About the Author
Denis Laverty possesses more than 17 years experience in network management and communications, Denis has been involved with network management applications from the early DOS days; as product trainer, technical author and QA Director. In 2003 he co-founded OPENXTRA together with Jack Hughes and serves as its Managing Director.
About OPENXTRAOPENXTRA Limited is a privately held network management VAR based in the UK. This blog is here to help you get the most out of our products and network management in general. Read more about OPENXTRA and this blog.
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