# How do you compute the AC capacity needed for a server room?

We've got a small server room ( approximately 600 square feet in area ), and three partially-filled racks of servers, switches, et cetera.

What's the best-practice way of computing the AC capacity needed to hold temperature/humidity conditions constant in a server environment? Our current AC unit was installed about 10 years and 12 servers ago, and despite regular visits from the helpful physical facilities staff-- it's unable to hold temp/humidity without overheating the compressor.

Right now, I've got the estimated power-draw of the servers, switches, etc--- but I can't find a good way to take that power usage and determine cooling needs from that. Any help/directions would be appreciated.

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Make sure that the unit is getting regular servicing; this should not be a wait until it breaks situation. You clearly need an outside company; contact the manufacturer and get a couple of recommendations. We had similar problems until we found (after 3 tries) a service company that knew what they were doing. We get them in every 6 months or so to top off the coolant and check the unit. – tomjedrz Jun 8 '09 at 15:18
IMHO - a 10 year old unit should not be totally overmatched unless you have dramatically increased the equipment in the room or the unit was undersized at the beginning. However, like cars, they do age and need more attention over time. – tomjedrz Jun 8 '09 at 15:21
In re: AC service contract. The contracting company should own a temporary unit big enough to do the job and be able to deploy it on site as well. – dmckee Jun 8 '09 at 15:57

Just convert the estimated draw from Watts to BTUs: To calculate the BTUs/hour, multiply the watts by 3.413. 1 ton of cooling is equivalent to 12,000 BTU/hour.

Most of the energy drawn will be converted to waste heat.

From what I am reading, a pro would tell you to have someone come in and do the full calculation. That would take into effect any odd heat sources (solar for example) and your humidity concern.

In practice - I have just oversized mine by 25% or so in the past - to allow for new equipment. (In a small equipment room!)

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Upvoted, great solution! – Greg Meehan Jun 8 '09 at 14:20

I would also STRONGLY recommend you pick up this book:

Enterprise Data Center Design and Methodology = http://www.amazon.com/Enterprise-Methodology-BluePrints-Official-Microsystems/dp/0130473936/ref=sr_1_1?ie=UTF8&s=books&qid=1244469855&sr=8-1

Even though it is from 2002, it is still very relevant today. It is all about A/C, power, rack load, rack placement (cold aisle/hot aisle), good housekeeping, cabling runs, etc. for a datacenter.

Excellent reference book.

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Yours is a good question..

To be completely honest, this is something that I've seen a.) tackled by server admins and IT managers with the best of intentions yet yielding the worst results :) and b.) honestly better left up to HVAC/building engineering/facilities professionals who know the right questions to ask and what to think of.

I do, of course realize that you may not have that option though.

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Our problem right now seems to be that the physical facilities staff is more familiar with carpentry/AC wiring that goes on in a dorm,rather than actual HVAC work. The two different outside contractors that have been called in have both stated that our unit is "probably undersized for load". My concern is that I'd like to be able to figure out the load myself, so that if the powers-that-be actually authorize a new system, we won't be buying underrated equipment. – Bill B Jun 8 '09 at 14:00
Yeah, I've totally been in that boat before. :) Either way, good luck mate, hope those links can serve some use. – Greg Meehan Jun 8 '09 at 14:06
If your facilities staff aren't in familiar territory, make sure that they know these will be continuous run systems (even in the winter). I have ran in to an issue before where the outside compressor could not run in sub-zero temperatures. I know it may be cold outside, but our servers are still generating 60,000 BTU/hour of heat. – Doug Luxem Jun 8 '09 at 14:08
We've recently hired an Actual HVAC guy --- but his experience is all large buildings/residential, not data centers. He's well aware of the 24x7 requirements, and has even asked that his email be added to the notification alias for temp warnings. – Bill B Jun 8 '09 at 16:58

Please consider that many now believe that it's cheaper, quicker and more environmentally-sound to use fans to extract warm air from a data centre and draw in ambient air from the outside (via some form of filtration at least). Most, if not all, IT kit is designed to be ran at ambient temperatures, not the 17-10C we all seem to feed them.

As an example look at Switch Communications new enormous data centre - there's no AC at all, it saves them a fortune - and they're in Nevada so it clearly works even in hot and dusty surroundings.

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That greatly depends on where you are, though. Here in the PacNW it makes sense 9-10 months a year, so long as there is efficient air circulation in the room itself, and the incoming air is dehumidified a bit. Other parts of the country aren't so lucky to have the 'natural airconditioning' we get. – sysadmin1138 Jun 8 '09 at 14:46
Couldn't agree more, but it's such a great idea it would be negligent to not consider it, even if only for a moment. – Chopper3 Jun 8 '09 at 15:02
We're in the basement of a building, with no easy way to run ductings/vents to the outside. – Bill B Jun 8 '09 at 16:59