Cooling a server room that used to be a cupboard

The comms room in most offices is whatever room was free when the network went in. It has a door, a power socket and no window, and it was fine with one switch in it. Then it acquired a server, a NAS, a recorder and a UPS, and now it is thirty-four degrees with the door propped open.

What heat actually does

Nothing dramatic. Servers throttle rather than fail, so performance drops and nobody connects it to the room. Disks fail earlier — the failure-rate curves published by the large operators are unambiguous above about thirty degrees. UPS batteries lose half their life for every ten degrees over their rated ambient. The room is not causing an outage; it is shortening the interval between them.

The arithmetic, again

Every watt drawn is a watt of heat. A rack drawing 1200 W is a 1200 W heater running around the clock, and it needs 1200 W of cooling — plus the heat that comes through the walls and the door. In air-conditioning terms that is a little over 4,000 BTU, which is small: a domestic split unit of 9,000 BTU is comfortably enough for most comms rooms, with margin.

A split unit, not a portable one

Portable air conditioners exhaust through a hose to somewhere, and in a room with no window that somewhere is the ceiling void, from where the heat comes back. They also need their condensate emptied. A wall-mounted split unit with the outdoor unit outside is the correct answer and it costs about the same as the portable unit that will be replaced by it anyway.

Two things about the split unit that the installer will not think of unless asked: it must be set to restart automatically after a power cut, and it must be on a circuit that is not the UPS. A unit that stays off after an outage cooks the room while everything else comes back up.

Air has to move through the equipment

Servers and switches draw cool air in at the front and push hot air out of the back. A rack pushed against the wall recirculates its own exhaust. Leave a gap behind it, and do not point the air conditioner's cold stream at the back of the rack — it goes in the front.

Know before it is a problem

A temperature sensor in the rack that sends an alert costs less than a disk. Most UPS network cards have a sensor input; most switches report their internal temperature over SNMP. Set an alert at twenty-eight degrees and the first sign of a failed air conditioner is a message, not a smell.

The door

Propping the comms room door open cools it with the office's air conditioning, which works until the office's air conditioning switches off at seven. It also means the room is not locked. Close the door and cool the room on its own terms.