Cooling a server room or comms cabinet: why a comfort split is the wrong machine

Updated · Claude AC · 3 sources cited

Contents (6)
  1. The load is an electricity reading, not a room calculation
  2. It has to run in January, and that is where comfort kit breaks
  3. One machine is a single point of failure
  4. Temperature, humidity and the drain
  5. The regulatory wrapper is commercial, not domestic
  6. Frequently asked questions

Somewhere in most European small businesses there is a cupboard with a rack in it, a bundle of patch leads, a UPS and a wall-mounted air conditioner chosen because a domestic installer had one in the van. It works for a year. Then it fails on a Sunday in January, or it does not restart after a power cut, or its condensate tray overflows into the bottom of the rack. None of those are bad luck. They are the predictable consequences of putting a comfort machine on a process cooling job.

A server room is not a small office. Its load has nothing to do with people, its operating season is twelve months long, and the cost of an outage is not discomfort. Four decisions separate a room that runs quietly for a decade from one that generates emergencies.

The load is an electricity reading, not a room calculation

Almost all the electricity entering IT equipment leaves it as heat. One kilowatt drawn at the rack is one kilowatt to be removed, which makes sizing unusually straightforward: read the load rather than estimate it. The right figure comes from the UPS display or a metered PDU over a busy period, not from adding up nameplate power supplies, which overstates real draw by a wide margin.

InstallationIndicative heat loadUsual consequence
Single comms cabinet: switch, router, firewall, small NAS0.3–1.5 kWoften solvable by ventilation rather than refrigeration
Small business room, one or two half-populated racks1.5–5 kWa dedicated, properly specified split is the normal answer
Three or more racks with virtualisation hosts and storage5–15 kW and upwardsredundancy and airflow management stop being optional

To that, add the losses of the UPS itself — it sits in the room, is rarely more than a rounding error, and is always forgotten — plus any solar or fabric gain if the room has an external wall or, worse, a window. The people who walk in occasionally contribute almost nothing and should not be part of the calculation at all.

One subtlety costs more projects than any other. A comfort air conditioner’s rated capacity is total cooling: sensible cooling plus moisture removal. In a server room there is no moisture to remove, the sensible heat ratio is effectively 1.0, and the machine’s usable sensible capacity is typically only around three quarters of the headline figure. Sizing a 3.5 kW split against a 3 kW IT load looks comfortable on paper and is marginal in reality.

It has to run in January, and that is where comfort kit breaks

A domestic split is designed to cool between roughly May and September. A server room needs cooling at −5 °C outdoors at three in the morning, and a standard unit asked to do that behaves badly: condensing pressure collapses, refrigerant migrates, the oil dilutes and the compressor eventually fails. The specification that prevents it is ordinary and cheap when ordered with the unit, and expensive to retrofit afterwards.

One machine is a single point of failure

If the room matters, two units matter. The standard arrangement is N+1: two systems, each sized for the full load, alternating on a weekly duty rotation so both stay exercised and neither accumulates all the running hours. Many small controllers do this natively; where they cannot, a simple rotation controller does. The failure being bought out is not exotic — it is one compressor dying in August, on a Friday, with a week’s lead time on the part.

Below roughly a kilowatt, a single cabinet in a ventilated room may not need refrigeration at all. Filtered forced ventilation on a thermostat, drawing from a cooler adjoining space, is cheaper, simpler and does not fail in a way that stops the network — provided the source air is genuinely cooler than the target temperature, which rules it out in a top-floor plant room in July.

Temperature, humidity and the drain

Running a server room at 16 °C is a habit, not a requirement. The widely used ASHRAE thermal guidelines put the recommended IT equipment inlet range at roughly 18–27 °C, and modern hardware is entirely comfortable in the upper half of that band. A setpoint of 22–24 °C is cheaper to hold, leaves more margin before an alarm, and reduces the risk of condensation on cold surfaces in a humid room.

Condensate is the physical risk that actually materialises. A wall unit mounted above a rack, draining into a blocked or frozen pipe, puts water exactly where it must never go. The rules are simple: never site the indoor unit directly above equipment, prefer a gravity drain with a proper fall over a pump, and if a pump is unavoidable fit one with a high-level alarm that stops the unit. Standing water in a tray is also a biological problem, covered in our guide to an air conditioner leaking water indoors.

Alarms that do not depend on the air conditioner

The air conditioner cannot tell you it has stopped. An independent temperature sensor — a cheap networked probe or an environmental monitor in the rack — should raise an alert on rising temperature, ideally with a second threshold and a door-open contact. Set the first alarm low enough that someone can react before the servers throttle or shut down, and route it somewhere that is read out of hours. Rooms are lost to a silent failure far more often than to a violent one.

The regulatory wrapper is commercial, not domestic

Installation involves fluorinated refrigerant, so it is work for a certified operative, and the system carries leak-checking obligations at intervals set by its CO₂-equivalent charge along with a maintained record — that framework sits in the EU F-gas Regulation and is summarised in our guide to air conditioning maintenance rules. Above an effective rated output of 70 kW, national transpositions of the energy performance of buildings directive add periodic inspection duties: that is a large room rather than a cupboard, but it arrives sooner than expected once several systems are aggregated on one site. For anything approaching a real data room, the European Commission’s Code of Conduct for data centres is the accessible reference on airflow separation and setpoints, and the sizing logic for the rest of the premises is in our guide to cooling shops and small offices.

Frequently asked questions

Can I just use a normal domestic split in a server room?

The same physical box is often used, but it must be ordered with low-ambient operation, a crankcase heater and automatic restart, and sized on sensible capacity rather than on the headline rating. A standard consumer installation left on factory settings will usually survive one summer and fail in winter or after the first power cut.

What temperature should a small server room be kept at?

An inlet temperature in the low twenties suits nearly all modern equipment and sits comfortably inside the recommended ASHRAE envelope of about 18–27 °C. Going colder buys very little reliability and costs continuously; the useful margin comes from alarms and redundancy, not from a low setpoint.

Is a portable air conditioner acceptable as a backup?

Only as a deliberate, tested emergency measure, never as the plan. A monobloc needs a permanent duct route, it depressurises the room and pulls warm air in from the rest of the building, and its condensate tray fills unattended. If it is the contingency, run it once a year to confirm it actually works.

Do I need two air conditioners for one rack?

It depends on what an outage costs. A single cabinet holding a switch and a firewall, in a room that can be propped open with a fan for a day, does not justify duplication. A rack running the systems the business invoices from does — and the second unit is cheaper than the first, because the pipework route, power supply and drainage are already there.

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