In theory it’s easy to calculate the size of the air con unit needed for your server room, you add together all heat sources and install an air con unit that can remove that much. In practice it’s rather more complicated. This guide doesnt go into every detail of possible heat sources, it should be used to give you an idea of the amount of cooling you might need.
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 critical 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:
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The floor area of the room
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The size and position of windows, and whether they have blinds or shades
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The number of room occupants (if any)
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The heat generated by equipment
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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
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 11000 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.
Ever thought of using free cooling?
If your reading this from the UK you'll know that theres a large time of year when our weather is less than hot. The use of ambient air circulation at times of the year where the outside temperature is low enough to cool your equipment will save you money. Ambient air cooling isn't a replacement for air conditioning and thermastatically controlled cooling but can provide a money saving break from the air con at relevant times of the year.
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.
