Server and Comms Room Air Conditioning: why comfort cooling fails here

Why an IT room is a different problem

Every other space in a commercial building has a cooling load that rises and falls with occupancy and weather. A server or comms room does not. The heat comes almost entirely from the equipment, the equipment runs continuously, and the load is therefore roughly flat around the clock and around the year. The room needs as much cooling at 3am on a January night as it does at 3pm in July.

That single difference drives everything else. It means the plant runs continuously rather than cycling, so component wear accumulates faster. It means the system has to work in low outside ambient temperatures, which is precisely where standard comfort air conditioning stops behaving. And it means a failure is not a comfort complaint — it is a temperature rise that can force equipment shutdown within minutes in a well-insulated modern room.

The winter failure that catches people out

The most common bad outcome in this space is a comfort split system installed in a comms room that works beautifully all summer and then fails in the first cold snap. The reason is worth understanding, because it is not obvious.

A standard air conditioning system rejects heat outdoors and relies on the outdoor coil being warmer than the refrigerant. When the outside temperature drops toward or below freezing, condensing pressure falls too far, the system's low-pressure protection operates, and it trips out — exactly when the servers are still producing full heat. Some units simply refuse to run in cooling mode below a set outdoor temperature, because they were never designed to.

The fix is specifying equipment with low-ambient operation — head pressure control, variable-speed condenser fans, and in some cases wind baffles — rated down to the temperatures your site actually sees. This should be an explicit line in the specification, not an assumption. A unit sold as "suitable for server rooms" without a stated minimum operating ambient has not answered the question.

Close control versus a well-specified split

There are two legitimate answers here and the honest one depends on scale and consequence.

Close control units — also called precision cooling or CRAC units — are purpose-built. They hold temperature and humidity inside a tight band, run a high sensible heat ratio (they cool rather than dehumidify, which is what an IT room wants), are built for continuous duty, and come with monitoring and alarms as standard. They cost more and they need more space.

Well-specified splits or VRF, chosen with low-ambient capability and appropriate controls, are a reasonable answer for smaller comms rooms — a handful of kW serving a rack or two in an office building. Plenty of such rooms run happily this way for years.

The dividing line is consequence rather than a kW threshold. Ask what happens if the room reaches 35°C on a Sunday morning. If the answer is "someone notices on Monday", a good split is fine. If the answer is "the business stops", you are in close-control-and-N+1 territory regardless of how few kilowatts are involved.

Humidity matters more than people expect. Too dry brings electrostatic discharge risk; too damp brings condensation and corrosion. Comfort air conditioning controls temperature and lets humidity fall where it may — which is usually acceptable in a small room and increasingly not as density rises.

Resilience: what N+1 actually means

Single-unit cooling on a critical room is a single point of failure, and it is remarkable how many otherwise well-run buildings have exactly that. N+1 means providing one more cooling unit than the load requires, so any one unit can fail or be taken out for maintenance with no loss of cooling.

Getting it right involves more than buying two boxes. The units should rotate duty automatically so both accumulate similar running hours and neither sits idle for months and then fails on first demand. They should be on separate electrical circuits, because a shared circuit reintroduces the single point of failure you just paid to remove. And there should be monitoring with an alarm that reaches a human — a temperature alarm that sounds in an empty plant room at the weekend is not a control.

Where a room is genuinely business-critical, this is also a maintenance question. Continuous-duty plant justifies quarterly servicing rather than the twice-yearly comfort-cooling baseline — see AC servicing.

Sizing from IT load, and the airflow question

The arithmetic is unusually simple: essentially all the electrical energy the IT equipment consumes ends up as heat in the room. If the racks draw 12 kW, you have roughly 12 kW of heat, plus UPS conversion losses, plus lighting, plus a modest fabric gain.

The critical instruction is to measure the actual draw — at the distribution board, the PDU or via rack metering — rather than adding up nameplate ratings. Nameplate figures are maximum-possible values and are commonly double the real draw. Sizing to nameplate produces a system that is grossly oversized, short-cycles, wears itself out and controls temperature badly. Then add sensible headroom for planned growth, ideally by designing the plant space so a further unit can be added rather than by oversizing today.

Airflow matters as much as capacity, and it is the part most often ignored in small rooms. Cold air must reach the front of the equipment and hot exhaust must get back to the cooling unit without mixing. In practice that means paying attention to rack orientation — hot aisle and cold aisle rather than all racks facing the same way — fitting blanking panels in unused rack spaces so hot air cannot short-circuit back through the rack, and not siting the cooling unit where it blows straight into a hot exhaust. A 20 kW system with poor airflow management will underperform a well-arranged 15 kW one.

Free cooling, and the energy question

Because the load is continuous, a server room's cooling runs up a meaningful annual electricity bill, which makes efficiency measures pay back faster here than almost anywhere else in a building.

The relevant technique is free cooling: for much of the UK year the outside air is cooler than the return air from the room, so some or all of the cooling can be done without running the compressor. Implementations vary — indirect free cooling through a dry cooler and a heat exchanger, integrated free-cooling coils within a close control unit, or on larger installations a free-cooling chiller. Direct fresh-air economisers exist but bring filtration and humidity control complications that make them a poor fit for small rooms.

Given the UK climate, the number of hours a year in which partial free cooling is available is substantial, and that is the single biggest efficiency lever available on this type of plant. On larger installations the same logic scales into chilled water — see commercial chillers, which covers free cooling in more depth.

The other efficiency lever is simply not overcooling. Equipment intake temperature guidance has relaxed considerably over the years, and rooms held at temperatures set decades ago are often being cooled harder than the hardware requires. Raising a setpoint by a couple of degrees, where the equipment manufacturer permits it, is free money — but it should be a documented decision made against the equipment's stated intake requirements, not a guess.

Refrigerant and compliance in a small room

Two points specific to this application. First, comms rooms are frequently small, and small room volume is exactly what triggers BS EN 378 refrigerant charge limits for the mildly flammable A2L refrigerants now standard in new equipment. A system that would be straightforward in an open-plan office may need reconfiguring, leak detection or a different architecture in a 12 m² comms cupboard. This is covered on the replacement page, and it is a common reason a like-for-like swap of old plant is not permissible.

Second, IT room cooling counts toward the building's air conditioning for TM44 purposes. Comms room units are among the most commonly forgotten when someone totals up effective rated output — and because the threshold aggregates across the building, they can be what pushes it over 12 kW. See servicing and TM44.

Frequently asked questions

Can I use a normal air conditioning unit in a server room?

Sometimes, for a small comms room, provided it is specified for low-ambient operation so it can still cool when the outside temperature drops below freezing, and provided you accept that humidity is not controlled. Standard comfort units without low-ambient capability commonly trip out in winter — exactly when the equipment is still producing full heat.

How do I size cooling for a server room?

Almost all the electrical power the IT equipment draws becomes heat, so measure the actual draw at the distribution board or PDU and use that as the load, adding UPS losses, lighting and a modest fabric gain. Do not add up nameplate ratings — they are maximum-possible figures, often around double the real draw, and sizing to them produces an oversized system that short-cycles.

What is N+1 cooling and do I need it?

N+1 means installing one more cooling unit than the load requires, so any single unit can fail or be serviced without losing cooling. Whether you need it depends on consequence rather than size: if the room reaching 35°C on a Sunday would stop the business, you need it even on a small room. The units should rotate duty automatically and sit on separate electrical circuits.

Why does my comms room air conditioning fail in winter?

Because standard comfort air conditioning is not designed to reject heat when the outside air is very cold. Condensing pressure falls too low, the low-pressure protection operates and the unit trips — or it simply refuses to run in cooling mode below a set outdoor temperature. The fix is specifying equipment with head pressure control and low-ambient operation down to the temperatures your site actually sees.

What temperature should a server room be kept at?

Follow the equipment manufacturer's stated intake temperature requirements. Guidance has relaxed considerably over the years, and many rooms are held at setpoints chosen decades ago and are being cooled harder than the hardware needs. Where the manufacturer permits it, raising the setpoint a little reduces running cost — but make it a documented decision against the equipment specification.

We design and install close control and precision cooling for server rooms, comms rooms and small data suites across the UK. See chilled water and free cooling for larger installations, servicing and TM44 for the maintenance regime, and commercial air conditioning for background. Or request a quote.