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Commercial · Server Rooms & IT ClosetsServer Room & IT Closet Cooling
Size server room and IT closet cooling on the watts the equipment draws, not on floor area: every kilowatt of IT load needs about 3,400 BTU/h of sensible cooling, around the clock, twelve months a year. The right equipment is a dedicated ductless mini-split or a small precision unit that holds the server inlet air inside ASHRAE's recommended 18–27 °C range and has a low-ambient kit so it still cools at −20 °C outside. A branch off the office rooftop unit fails on nights, weekends and all winter.
Start with the load. Add up the watts on the UPS display or the rack power strips, multiply by 3.412 to get BTU/h, and add a margin for lights, a person working in the room and any wall that sees sun; a closet drawing 3 kW needs roughly 10,000 BTU/h before that margin. Then choose the unit on its sensible capacity from the manufacturer's data, because a comfort air conditioner spends part of its nominal rating wringing out moisture that a closed server room does not have. Floor area tells you almost nothing — a closet the size of a pantry can need more cooling than the open office beside it.
The usual shortcut is a duct branch or a transfer grille off the office system, and it fails for three reasons. The office rooftop unit goes into unoccupied setback at night and on weekends while the servers keep running. In winter that unit is in heating mode, so the branch delivers warm air into the hottest room in the building. And the thermostat that controls it hangs on a wall out in the office, with no idea what the closet is doing.
Low-ambient operation is where most server-room splits in Ontario go wrong. A standard mini-split is built to cool in summer; ask it to cool at −20 °C and the refrigerant pressure on the outdoor side collapses, the indoor coil ices over and the unit trips on a protection fault, usually on the coldest night in January. A proper install has three parts: a low-ambient control that slows or cycles the outdoor fan to hold head pressure, a crankcase heater that keeps liquid refrigerant out of the compressor oil, and a wind baffle so a north wind does not over-cool the outdoor coil. Check the cooling operating range on the submittal sheet before you approve a quote, because many models sold for houses stop well short of an Ontario cold snap.
Reliability is the rest of the design. For a room the business cannot run without, we recommend N+1 — a second unit able to carry the load alone, on lead/lag rotation so both share run time and a failure on one starts the other — plus a high-temperature alarm contact wired to the IT monitoring system and a condensate pump with an overflow safety switch. A single-zone ductless mini-split runs $4,000–$7,000 installed in our published pricing; the low-ambient kit, alarm wiring and a second unit are quoted in writing on top of that. If an existing unit is already tripping, start at mini-split repair; for the rest of the building see office HVAC and commercial HVAC, or contact the shop with the UPS load reading.
Equipment we install and service
An inverter wall or ceiling-cassette unit on its own circuit, serving only the server room and running every hour of the year. The indoor head is placed to deliver cold air to the front of the rack where the equipment draws it in, not to blow across the hot exhaust at the back.
A unit built for continuous duty with a high sensible heat ratio and tighter temperature control than comfort equipment. It costs more and is chosen when the load is several racks, the room has no tolerance for swings, or the cost of an outage justifies it.
Wind baffle, crankcase heater and a head-pressure control on the outdoor fan. Without all three a split system that cooled perfectly in July faults in January, and the outdoor unit also has to sit above the snow line on a wall bracket or roof stand.
A dry contact that closes on high room temperature and reports to the monitoring system, a pump that lifts condensate to a drain and shuts the unit down before it overflows, and a controller that alternates two units and brings on the standby when the lead one fails.
What goes wrong in these buildings
- The closet reaches 32°C every Saturday afternoon because the building air shuts off on weekends.
- The split on the wall worked all summer and threw a fault code in the first cold week of January.
- We keep the closet door propped open with a fan in it, which is also our security problem.
- Nobody knew the unit had quit until the servers shut themselves down on a Sunday.
- Water dripped out of the indoor unit onto the top of the rack.
- We added a second rack and a bigger UPS, and the old unit now runs flat out without catching up.
How it works
UPS readout, rack inventory and any growth planned for the next few years, then the room itself: wall and ceiling construction, where an outdoor unit can sit, the line-set route, the condensate route and the electrical panel. The quote states the sensible capacity being supplied against the load measured.
A dedicated circuit by a licensed electrician under an ESA notification, the outdoor unit mounted clear of drifting snow, low-ambient controls fitted and the condensate pump wired to its safety switch. Refrigerant work is done by an ODP-certified technician and the line set is pressure-tested before charging.
We trip the high-temperature alarm and confirm IT receives it, switch off the lead unit to prove the standby starts, lift the pump float to prove the shutdown, and cut power to confirm the unit restarts unattended. Settings and model numbers are left on a sheet inside the room.
Common questions
What temperature should a server room be kept at?
ASHRAE's recommended range is 18–27 °C measured at the air inlet of the equipment. Small rooms are usually set in the low twenties to leave a buffer if cooling stops. Running colder than 18 °C wastes electricity and buys nothing. The reading that counts is at the front of the rack near the top, not at a thermostat on the far wall.
Can I use a portable air conditioner in a server closet?
As a stopgap for a few days, yes. As the permanent plan, no. A portable unit has to dump its heat somewhere, a single-hose model pulls warm replacement air into the closet as it exhausts, its condensate tank fills and shuts the unit off, and it has no alarm output. It also restarts unreliably after a power interruption.
Why did my server room mini-split stop working in winter?
Almost always because it is cooling in outdoor temperatures it was not equipped for. A technician reads the fault history, checks for an iced indoor coil, tests the outdoor fan control and looks for a crankcase heater. If the model accepts a low-ambient kit, that is the repair; if it does not, the dependable fix is a unit rated for cold-weather cooling.
Do we really need a second cooling unit?
Use this test: if a cooling failure at the start of a long weekend would cost more than the second unit, install it. A room with one unit is a single point of failure no matter how good the unit is. If the budget only allows one, the high-temperature alarm stops being optional and someone has to own the response plan.
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