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Record Seattle heatwave pushes server room AC limits

With all the record breaking hot weather recently in the Seattle area (up to 109F!), I’ve had to reassess the cooling systems in some of the various server rooms and data closets I’m in charge of. Like many admins, I happened to take over responsibility for most of these server rooms at some point after the room had been designed and AC installed. I found out these last few weeks that most server room AC unit sizes were just guessed at. There are many resources online that go over how to calculate AC unit tonnage for server rooms, but I figured why not toss another page on the pile?

Because running a server is pretty much equivalent to running an electric heater, you are pretty safe doing a straight wattage to BTU conversion. This equation looks something like this: 1W of electricity is equal to 3.412141633 BTU/Hr. This means if your server’s maximum wattage rating is 650W and you run it for one hour, then it will have pumped out 2217.8 BTU (650W x 3.412 BTU).

Now, smaller AC units are sold based on how many BTU they can reduce. I say reduce because AC units typically can’t fully negate the number of BTUs it is rated for. That’s to say if you have a heater putting out 1000BTU and an AC unit rated for 1000BTU, you will still end up with excess heat. Larger AC systems designed for server rooms and industrial applications are sold by the “ton”, where a ton is equal to 12,000 BTU. These larger systems can be sold in any number of configurations, but most are sold as single units that cool up to several tons, or sold in one ton modules that stack together.

A typical server rack has a 20A circuit maximum, and at 120V that means a maximum of 2400W (20A x 120V). So with a maximum of 2400W, I’m looking at cooling 8189 BTU, which means I could get away with a 1 TON AC unit. However, as I stated above, an AC unit rated for one ton (12,000 BTU) can not totally negate the 8189 BTUs put out by the server equipment. There are many reasons for this fact, including things like the heat generated by the AC unit itself. Other reasons I’ve seen include heat radiated back into the server room from uninsulated or badly insulated exhaust ducting. To help remedy these issues, it’s best to multiply the maximum wattage by 3.5. So, my 2400W 8189 BTU server system suddenly jumps to a 8400W 28,661 BTU system that requires a 2.5 ton AC system to cool (or 3 tons if you are buying 1 ton modules).

This adjusted calculation holds true to real world examples. I recently inherited a simple 2 post rack running 1800W worth of server equipment running in a very small room/closet. The room contains a directly vented (no ducting) one ton (12,000 BTU) AC unit, which according to the straight across conversion of 1800W to 6141.6 BTU should be more than enough, but it barely keeps up. On those record breaking hot Seattle days we had recently, I had to shut down all the servers in the room! Even with everything turned off, the room’s internal temperature was still about 85F. That whole room’s design is now under review.

If you are interested in calculating a very accurate BTU rating for things that generate heat, from south facing windows to servers to people, check out this excellent post: http://www.openxtra.co.uk/articles/calculating-heat-load

Calculating The Size of a Server Room Air Conditioner
Sun, 2005/02/20 – 17:44 — dlaverty

A quick guide to show you 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


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


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


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 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.

3 Responses to “Record Seattle heatwave pushes server room AC limits”

  1. yves says:


    I need to know the meaning of the constant 337 in the room sizing.

  2. yves says:

    Also I need to knows the meaning of 3.5 constant in the formula of the equipment output heat.

  3. Amal says:

    The 337 is a generic offset used to adjust the BTU requirements for a typical room of specific dimensions. The 3.5 value used when calculating BTU output for equipment is, from what I gather, a safety buffer used to overestimate the BTU value per watt. Straight conversion of watts to BTU does not give you a realistic number to work with when it comes to AC systems and cooling server racks.

    Check out this post for my thoughts on passive cooling techniques: http://blog.amal.net/?p=1207

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