We're moving into a new office in an old building in London (that's England :) and are walling off a 2m x 1.3m area where the router & telephone equipment currently terminates to use as a server closet. The closet will contain:

  • 2 24-port switches
  • 1 router
  • 1 VSDL modem
  • 1 Dell desktop
  • 1 4-bay NAS
  • 1 HP micro-server
  • 1 UPS
  • Miscellaneous minor telephony boxes.

There is no central A/C in the office and there never will be. We can install ducting to the outside quite easily - it's only a couple of metres to the windows, which face a courtyard.

My question is whether installing an extractor fan with ducting to the window should be sufficient for cooling? Would an intake fan and intake duct (from the window, too) be required? We don't want to leave a gap in the closet door as that'll let noise out into the office. If we don't have to put a portable A/C unit into the closet, that'd be perfect.

The office has about 12 people; London is temperate, average maximum in August is 31 Celsius, 25 Celsius is more typical. The same equipment runs fine in our current office (same building as new office, also no A/C) but it isn't in an enclosed space.

I can see us putting say one Dell 2950 tower server into the closet, but no more than that. So, sustained power consumption in the closet would currently be about 800w (I'm guessing); possibly in the future 2kw.

The closet will have a ceiling and no windows and be well-insulated. We don't care if the equipment runs hot, so long as it runs and we don't hear it.

  • 3
    Why the downvote? I can see why the proposed idea isn't liked, but that doesn't make it a bad question! +1 to counteract the needless down vote.
    – Bryan
    Jan 18, 2012 at 14:01
  • 1
    For example, the VM on a desktop computer for your DC? What are you doing to keep that DC from being lost? Really you need at least two of them on separate systems for redundancy. There was just a guy with this issue in another question that fubared his only DC and had to rebuild from scratch. I'm all for budget solutions ($DEITY knows we do enough of them around here) but you're skimming really low in the "oh @#%!!" territory with that setup. Friendly caution, that's all... Jan 18, 2012 at 14:12
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    People and silence are FAR more important than extending the life of the equipment. Suggesting headphones as a solution is just bizarre. Jan 19, 2012 at 5:58
  • 4
    Keep in mind that this is simply your current setup. It's quite likely that equipment will be added to it over the next few years, and installing AC later is going to be significantly more painful than simply doing it right during the initial buildout.
    – Adam Davis
    Jan 19, 2012 at 21:21
  • 4
    My experience of bosses that tell you that silence (or cost or sticking with "crazy joe's place of cheap PCs" brand of home brew servers because the boss went to school with crazy joe or whatever) is more important than using the right gear will suddenly change their minds when a disaster strikes and not only decide that not only should the right gear should have been used all along, but that its your fault it wasn't for not framing that in language they can understand.
    – Rob Moir
    Jan 19, 2012 at 22:54

11 Answers 11


Well, let's work this out;

  • 2 x 24 port switches (say Cisco 3750-E's) can output 344 BTU/hr each so that's 688 in total
  • 1 x router (say a Cisco 2921) can output 1260 BTU/hr
  • 1 x VDSL modem (say a Draytek Vigor 2750) can output 120 BTU/hr
  • 1 x Desktop (say a Dell Optiplex 790, with monitor switched off) can output 850 BTU/hr
  • 1 x 4-Bay NAS (say a Netgear ReadyNAS Ultra 4 with 4 x 2TB disks) can output ~600 BTU/hr
  • 1 x HP Microserver can output 511 BTU/hr
  • 1 x UPS (say an APC Smart-UPS 2200VA that can handle the ~1.2Kw you may be drawing) can output 275 BTU/hr

That's 4300 BTU/hr.

You've got 5.2 cubic metres of space (minus the items inside it), so not including natural heat loss you're going to have to install a minimum 29cm fan with a 900 cubic metre per hour rating with 29cm conduit all the way to the room if you don't want to hit 42 degrees C (the lowest recommended highest temp of the kit listed above) from a nominal of 20C in 17 minutes.

Basically get an external A/C unit that can scrub 5k BTU/hr ok - a fan's going to literally and figuratively suck :)

  • 12
    That's a fantastic answer (though not what I wanted to hear! :) - thanks for taking the time to analyse the problem.
    – JamesCo
    Jan 18, 2012 at 14:01
  • 5
    Really technical answer and really this guy should be installing this for you. But i suspect getting a fan with temperature regulated speed (and 1 or 2 sensors in the cabinet) with possibly alarm will be good. Because UK is mostly cool during the year try to intake air from outside, via a HEPA filter to remove dust particles. Disposed air should go outside too as it can cause office sickness because of the dryness of the air, but its not necessary. Remember air flow should be cold intake on bottom, warm outlet on top. We have run similar ones in Manchester without AC.. no probs.
    – Piotr Kula
    Jan 18, 2012 at 14:15
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    Could you add the formulas you used to calculate the rate of temperature rise without ventilation, and how you calculated the fan size needed to maintain the temperature within the hardware limits. Jan 18, 2012 at 14:48
  • 11
    Those are maximums. If you're consuming 800W of power, you're not going to produce more than 800W of heat. 2700BTU. By my rough calculation, you'd need to exchange 223cu m/hour of air to keep the rise at 10 degrees Celsius. With those numbers, it's roughly a medium sized kitchen range fan (running perfectly with a very short hose!), and there would certainly need to be an equivalent intake fan to keep the air supply going. 100% agreed that A/C is necessary to futureproof, but if they're not going over 800W, they'll be okay with fans. Please correct me if I'm wrong(!)
    – mgjk
    Jan 18, 2012 at 15:20
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    @DanNeely - to be fair, after I worked out those basics (no shortcutting that I'm afraid) I put it into a Computational Fluid Dynamics package designed for data centres and it did the rest - can't take full credit for it.
    – Chopper3
    Jan 18, 2012 at 18:03

Get a small split system AC unit, like the Mitsubishi MSY series (no affiliation, used them before and like them). They're very small, quiet, reliable, and reasonably affordable. Two pipes will have to be run outside (roof or similar), they're small (3/4" and 1/4" roughly). The unit will cost around $2000 USD, and less than half that again for installation (depends on the specifics but it could be much cheaper).

  • 5
    +1 for this. I use a split Mitsubishi system in our comms room and it's as good as silent next to the hum of the servers themselves. They're also rated for a 24-hour duty cycle, something that you might not be able to say for a typical extractor fan or combined unit. Jan 18, 2012 at 13:46

When keeping things absolutely quiet is the top priority over keeping your equipment in good working shape, and the proper housing of equipment can't be considered, I'd strongly look at outsourcing your server needs or running fiber/high speed connections to a proper data center or facility.

Otherwise you risk looking at costing more in periodically replacing equipment than it would have cost to properly protect it in the first place.

It will be nearly impossible for someone to tell you it'll be just fine with a general statement of "it's usually this temp in the city each year," etc. because what about airflow? What about what your office mates insist on setting the thermostat in the office, and that temp flows into the closet? What about processor load on your switch and servers?

Even with a server of XYZ specs, the temp it runs at differs based on airflow through a rack, through the room, and what load we place on the power supplies/processor/hard disks. In other words, we wouldn't know until it's in the room running in real-world conditions. And you're describing a situation where you can easily find yourself overheating equipment because someone doesn't like the humming noise from the room.

  • That's a good idea for outsourcing any server needs. Our Dell desktop serves as a local domain controller (running in a VM) which is the only essential onsite server. We can install ducting for extraction and intake to the window - it seems a stupid question, but both are necessary, if the closet is otherwise air-tight?
    – JamesCo
    Jan 18, 2012 at 13:32
  • +1 I know a lot of smaller companies do this not only due issues with space, but because they usually move their offices a lot more than larger companies. Needinig to move and set-up your servers everytime you move is more trouble than it's worth.
    – Wipqozn
    Jan 18, 2012 at 17:21

Unfortunately, the realities of thermodynamics are crashing full speed into your environment. If the closet isn't big enough to absorb and radiate out the heat, then you need to mechanically get rid of it. There are, in fact, some experimental data centers that do simply cycle exterior air without additional cooling, though they tend to be in rather temperate and dry climates. London, however, is not exactly known for its low-humidity and mountain fresh air.

What you really need to do is install an air conditioning unit for that room. There are lots of solutions out there. The one I have the most experience with is the Mitsubishi Mr Slim line. We've used them to great effect in exactly the same situation you're finding yourself in.


The 2950 is a 2u rack server, I'm wondering if you are meaning the 2900?

I've run both of these at home in a room (converted garage) with no a/c or ventilation and they both warmed the room noticably after a few hours of usage.

For a production envoironment, I would have to suggest a/c, even if it is tricky - think about an unexpectedly hot summer day.... (Really for a production envoironment you should also have a backup a/c unit). The poweredge rack mount units are really designed to be in a data centre with decent cooling.

As an absolute minimum, seperate intake/extract fans, although it's not unknown for summer to hit 37/38°C in London, and there could be localised hotspots being slightly higher.

I believe there is an overheat threshold of around 65°C on some device in those servers (you may wish to verify the exact temp yourself), and 40°C air will not cool them down at all well compared to nice air conditioned air (the 9G servers also had FB-DIMM's which can run pretty hot, SAS drives if fitted also add a fair bit of heat compared to SATA).

If you really are not allowed to install a/c, I would suggest a decent envoironmental monitor which can send you a SMS message in case of problems, and settle on your callout rate to attend site out of hours due to thermal issues. Once this price is settled, suddenly the cost of a/c may not seem so bad.

  • Yes, you're right, I meant the 2900. Central A/C just isn't an option, but if required then a portable A/C unit could be put in the closet, but we'd just like to avoid that due to noise & maintenance.
    – JamesCo
    Jan 18, 2012 at 13:37

A/C would be nice.

You can blow cold air in with a fan or pull hot air out. The latter might give problems as you will pull out air and not feed any new air in it.

You could put a fan on the bottom of your closset to suck in cold air, and then in the ceiling put another fan to suck out the hot air.

What also could be very silent is a big tank of oil where you put all your equipment in. Oil doesn't conduct, so your equipment is safe and it will normally keep it quite cool.

  • 1
    An oil bath is too ambitious for our small office, and some stuff (e.g. the telephony) is bolted to the wall. If we vent at the bottom of the closet then it means venting from the general office-space, which we want to avoid as it'll mean noise escaping. Thanks for the answer!
    – JamesCo
    Jan 18, 2012 at 13:30
  • 1
    Two fans at the top of the closet, and run ducting from the suck-cold-air-in one down to floor level?
    – nickgrim
    Jan 18, 2012 at 16:57
  • Why do you want a fan at the bottom? If you dump cold air in on top, it will naturally sink.
    – Ben Voigt
    Jan 19, 2012 at 20:40
  • yes but you will suck out some of it as well, it's just better to suck in cold air or have a hole in the bottom than another hole in the ceiling. Jan 19, 2012 at 20:43

That's not a lot of equipment. Up until recently I was running 6 2U servers in a room with nothing but an exhaust fan. Yes, it was sub-optimal, but it worked well enough to keep the room around 80F and under, except on very hot days. This was a 12" fan with a variable speed control always at 100%. Sorry, I don't know the RPMs or more technical data. We quickly outgrew this and put in a A/C which costs us at least 10x more.

My concern is that any fan job will require proper engineering, which is costly. Getting the airflow right will be tricky. Your input air needs to match your output air. Are you going to pull from the inside office and push out into the outdoors? Are you then just fighting your own heaters in winter? Are there building or safety codes in play here? What about filtering? Or humidity? A/C units have all stuff built in and they're practically turn-key solutions here. Just shove one into a hole in the wall or take a portable unit and cut a hole for it and call it a day. Small shops like yours don't need $20,000 cooling solutions.

If the space is has access to an exterior wall, then purely from a cost/headache perspective I imagine you can get a decent window unit, cut a hole, and not worry about it. The AC unit will recycle air and a window unit will drop out the condensate outdoors. I've done this in the past for small server closets and have had a lot of luck. If you can't get a wall/window unit and need to go with a portable cooler then you need to engineer a drain pipe for your condensate. Assigning some office manager or janitorial staff the job to clean out the condensate bucket nightly will end in disaster.

I'd look at 10k BTU minimum for that space. Always build more capacity than you need, especially when you're dealing with small business. 10k units come at really good pricepoints too. Set your thermostat to 70F. Log into your server and check the temps in the case as well as the temp in the room. Spend a little money on a sensor to alert you when the room gets too hot. I'd also spring for a water detector sensor as well. A lot of shops sell environmental packages that can do both over ethernet for a few hundred.


In our company we have central A/C, but it doesn't run into our "server room" (Telecom closet, what ever you want to call it. Small space, 1 42U 2post rack). I bought a Tripp Lite SRCOOL12K Portable Air Condintioner for the room because it puts out a lot of cooling and does NOT require water drainage (it evaporates the water into the exhaust air). We ran the flex tube that the unit came with into the plenum and haven't had a problem since.

The room used to run into the low 90s F (32C) and the A/C unit now keeps the room down to 73F (22C) per our setting. The A/C even turns it self off on occasion because it doesn't need to run 24/7 to keep that temp.

In the room we've got 4 switches, a couple of rack servers, a couple of desktops sitting in the rack, a UPS and a bunch of Cisco network equipment.

  • 1
    Oh, and to address noise, it's not that bad. I spend time in the room without hearing protection and can still hear myself think/talk just fine. We've got an office that shares a wall with the "server room" (with next to no sound proofing) and you can hear a faint fan noise, nothing major though.
    – Jon
    Jan 18, 2012 at 22:31

I've dome alot of work at hotels where they tend to stick servers wherever there happens to be extra room in the floor plan for a closet that someone else also uses. I've measured regular room temps (mostly out of curiosity) of 120 degrees Fahrenheit for years. I was astounded. (servers were stacked on top of the floor transformer in that one- they never could figure out why the tape backups kept failing...). The biggest thing to be afraid of is temperature change - not how much heat. If you are buying new you can get racks from APC that can integrate power and cooling(which also helps with noise) (see thermal containment)


Even in London you're going to need an air conditioning unit for this closet, otherwise are you going to shut your business down in late July and early August? When using a portable air conditioning unit there's no need for ventilation of the indoor air in the closet. The unit will circulate the room's air in-place, and the only ventilation that is required is the exhaust heat and condensation from the unit to the outdoors or to a plenum space. This airflow is a rather small fraction of the air in the room and doesn't require any special consideration for air pressure or duct work. The air that already seeps in through the door, wall fixtures, and ceiling is more than enough. These units are readily available in both the rental and retail market.

What features to consider: It's very important to use an air conditioning unit with automatic condensate evaporation. These units send the water vapor that was removed from the air out through the ventilation duct and totally eliminate the need to drain water from the unit. This exhaust is sent through any kind of pipe or duct but it ought to be an insulated one to avoid condensation in colder weather. A suitable pipe would be the kind that is used with a clothes dryer and is easily found at the local hardware store.

A more expensive solution is to use an outdoor condensor/compressor with a heat exchanger indoors. Unfortunately, in most localities, this would require installation by a licensed plumber and/or HVAC technician and requires regular maintenance. The advantage of a portable unit is that maintenance is as simple as rolling in a replacement unit if it fails.

In the U.K., we used a number of different units that we procured from a local rental firm for a nominal fee. I don't recall the model numbers. Some had condensate evaporators and some had condensate tanks.

Back home in the USA, I use a SHARP CV-2P12SX 11,500 BTU portable air conditioner from Newegg which has an automatic condensate evaporator.

Duty cycles call for unattended, 24/7 use.


You could also look at housing your equipment offsite aside from a hardware vpn/firewall setup to tie the two sites together. Your options are fairly limited if you want to keep everything on site: the server would be the loudest piece of equipment. If you were able to keep the on site equipment limited to a firewall and switch, you would have the least amount of sound and heat generated.

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