Server room and telecom closet cooling

Most organizations do not have a data centre, but almost all of them have a server room, a main telecom room or a network closet on every floor. These spaces hold equipment that runs 24 hours a day, 365 days a year, in rooms that were often designed as storage closets and cooled by whatever building system happened to be nearby.

The result is familiar to facility managers: a closet that overheats on summer weekends, a portable air conditioner with a hose taped to the ceiling, or a switch that fails every few years. This guide explains how to size and select cooling for small IT rooms properly, with attention to Canadian winters.

Start with the real heat load

Nearly all the electrical power consumed by IT equipment ends up as heat in the room. The most reliable way to estimate the load is to measure it.

1. Read the actual power draw from the UPS display or smart power distribution units. This is better than nameplate ratings, which often overstate real consumption by a wide margin.
2. Add the losses of any UPS in the room, typically a few percent of the IT load for online units.
3. Add lighting, people (if the room is regularly occupied) and any heat gain through exterior walls or glazing.
4. Allow for growth over the life of the cooling equipment, based on a realistic IT plan rather than a blanket multiplier.

Useful conversions: 1 kW of IT load is about 3,412 BTU/h, and 1 ton of refrigeration is 3.517 kW (12,000 BTU/h). A closet with two 5 kW racks therefore needs roughly 10 kW (about 3 tons) of sensible cooling, before redundancy.

Sensible heat ratio

IT equipment produces almost entirely sensible heat. Comfort air conditioners are designed to remove a meaningful share of moisture, so their sensible capacity can be noticeably lower than their total rating. Always compare equipment on sensible capacity at the expected return air conditions. The guide on CRAC units vs comfort cooling explains this in more detail.

What temperature to hold

The ASHRAE recommended range for IT inlet air is 18 to 27 C (64 to 81 F), with broader allowable ranges depending on equipment class, as described in the ASHRAE TC 9.9 thermal guidelines. The temperature that matters is at the front of the equipment, not at the thermostat. Many small rooms are kept much colder than necessary because the thermostat sits in a hot spot or near the cooling unit's discharge.

Battery-backed UPS units are the exception. If lead-acid batteries share the room, holding about 25 C (77 F) or lower protects battery life.

Cooling options compared

For many offices, a properly selected ductless system is the right answer for a closet. Make sure it is rated for continuous operation and that its controls restart automatically after a power interruption. The guide to ductless mini-splits in commercial applications covers selection.

Avoid relying on the building system alone

A central system serving offices is scheduled around occupancy. When it shuts down at 6 p.m., the closet keeps generating heat. Even when it runs, a VAV box serving a closet may go into heating mode in winter because the zone it shares is calling for heat. If the building system must serve the room, give it a dedicated zone with its own controls and a 24/7 schedule, and add a backup.

Portable air conditioners

Portable units are acceptable as temporary relief during a failure. They are not a permanent solution: they have limited capacity, need condensate handling, often exhaust through a ceiling tile into a return plenum, and fail without alarm.

Winter operation in Canada

Server rooms need cooling when it is minus 30 C outside. Air-cooled equipment with outdoor condensers must be rated and configured for low-ambient cooling, using fan speed control, head pressure control or crankcase heaters as the manufacturer requires. Many standard residential-grade heat pumps limit or stop cooling below a certain outdoor temperature. Check the published cooling operating range, not the heating range.

Other cold-climate details:

• Locate outdoor units where snow drift and ice from roofs will not bury or damage them, and raise them on stands.
• Protect condensate lines from freezing; route them to interior drains or use heat trace.
• Where the room has an exterior wall, include winter heat loss in the calculation so the room does not get cold at very low IT loads.

Free cooling with outdoor air can work for larger rooms but needs filtration, humidity control and careful mixing to avoid very cold, dry air reaching equipment. For most closets it is not worth the complexity.

Redundancy and power

Decide how critical the room really is. A main server room supporting an entire business deserves N+1 cooling, meaning one extra unit so that any single unit can fail or be serviced without overheating. A floor network closet with a few switches may only need monitoring and a plan.

Where the IT equipment is on a UPS and the building has a generator, confirm that the cooling equipment is on the generator circuit. A room that loses cooling during a long outage will overheat even while the servers stay powered. Small rooms have little thermal mass, and temperature can rise several degrees within minutes.

Airflow inside the room

Good cooling capacity is wasted if hot exhaust air recirculates to equipment inlets.

• Install racks so all equipment breathes front to back, with fronts facing the cooling supply.
• Fit blanking panels in empty rack spaces.
• Seal cable openings in rack tops and floors.
• Return air to the cooling unit from the hot side of the racks, not from the cold side.
• Keep the room free of stored boxes that block airflow.

These are the same principles used in larger facilities, described in airflow management.

Monitoring and alarms

Even a well-designed room should have independent temperature monitoring at the front of the racks, sending alarms by email or text, ideally through the building automation system. Many smart PDUs and UPS units include environmental sensors at low cost. Add a water leak sensor under any cooling unit with a condensate pump.

Review alarms quarterly and test them. A temperature alarm that goes to an employee who left two years ago does not protect anything.

Frequently asked questions

Can I just leave the door open?

An open door may lower temperature temporarily but undermines security, fire separation and the cooling of the surrounding space. It also confirms that the room needs dedicated cooling.

Should the room have a fire damper or sprinklers?

That depends on the building code requirements for the room's fire separation and the building's sprinkler design. Ducts penetrating rated walls need fire dampers. Discuss suppression options with the fire protection engineer.

Key takeaways

• Size cooling from measured IT power plus UPS losses and envelope gains, compared on sensible capacity.
• Do not rely on a building system scheduled for occupancy; small IT rooms need 24/7 dedicated cooling.
• Outdoor units must be rated for low-ambient cooling to run through Canadian winters.
• Match redundancy to how critical the room is, and put cooling on generator power where IT is backed up.
• Independent temperature alarms at the rack inlet catch problems before equipment fails.