I’ve Got Friends…In Hot Places

Typical IT Closet packed with IT Equipment

The goal of this article is to provide several options that you, the specifying engineer, can use to better combat the awkward heat loads that small rooms filled with IT equipment can present.

The design of data centers and large computer rooms always includes a sophisticated cooling system. Across our territory we rarely encounter situations that require datacenter-level cooling strategies. It’s more common to encounter IT devices located in closets, small rooms, and other locations that were never designed with provisions for cooling IT equipment. The power density of IT equipment has increased over time and the result is that distributed IT equipment such as routers, switches, and servers often overheat or fail prematurely due to inadequate cooling in these spaces.

An appropriate specification for cooling small IT closets should assure compatibility with future loads, provide sufficient air flow, allow for ease of installation of cooling equipment, prevent oversizing, maximize electrical efficiency, and be flexible enough to work in various shapes and sizes of small rooms.

The first step in determining the appropriate cooling solution for an IT closet is to identify proper temperature and humidity. Equipment vendors usually provide a maximum temperature under which their devices are designed to operate. Keep in mind that this is the maximum temperature at which the vendor is able to guarantee performance and reliability for the stated warranty period, but that by operating the equipment at a lower temperature you will be able to significantly prolong its usable life. For example: a user can expect a typical UPS battery operating at 104°F to last 1.5 years or less as compared to 3 – 5 years if operated within AHRAE TC 9.9′s recommended range of 68 – 77F for IT equipment. Maintaining ambient relative humidity levels between 45% and 55% is recommended for optimal performance and reliability.

So now that we know our IT closet ambient conditions, what do we do next? There are four main approaches that you can take:


Ventilation can be a practical method for IT closet cooling for very low heat loads. Venting can be passive using appropriately placed vents, or it can be fan-assisted. For IT equipment power levels below 700 Watts, passive ventilation can be effective for IT closets. For equipment between 700 Watts and 2,000 Watts, fan-assisted ventilation can work. For reference, 2,000 Watts represents a little over half a ton of required cooling capacity.

Verdit: Not at a great solution, but OK as a temporary bandaid or if the budget demands it.


While pulling off of a building’s main comfort cooling system may sound cost effective, problems such as insufficient airflow, system cycling, ill-placed thermostats, and night setbacks can wreak havoc on IT closets with heat loads over half a ton. In order to use a building comfort cooling system to cool an IT closet, the closet in question would have to be made into a dedicated zone with its own properly sized supply and return ducts, terminal units (i.e. fan coil unit, VAV box) and controls (i.e. thermostats). This is oftentimes not practical and not cost-effective.

Verdit: Better than ventilation only, but not by much. The negatives of this approach far outweigh the positives.


The most effective way to gain control of IT closet temperatures is by installing dedicated air conditioning equipment. When the power level in a closet exceeds approximately 2,000 Watts, dedicated air conditioning equipment is recommended. From my experience ductless minisplits with wall-mounted air handlers tend to be the most often specified solution across my territory for IT closet cooling.

Ductless equipment can work well due to its load-matching capability from its inverter-driven condensing unit, but lack of sufficient directed airflow can be a problem. Other problems include lower sensible heat ratios, ease of humidistat integration, lack of integral humidifiers (sometimes required on computer room equipment), and cost. Almost all ductless minisplit equipment is provided in heat-pump configurations, which means you are paying for something that you don’t need (i.e. heating). In my experience a dedicated computer room air conditioner doesn’t cost much more than an appropriately sized and configured Computer Room Air Conditioner. If you are set on specifying a ductless minisplit please contact us or visit for more information.

Verdit: This approach works most of the time, but is not the best application of ductless equipment.


Despite the size of the load, if you have an application that requires cooling of IT equipment I encourage you to always specify a Computer Room Air Conditioner, also known as a CRAC unit. CRAC units are also known as CAHU and CACU’s, but CRAC is definitely the most common. The beauty of a CRAC unit is its almost infinite component configuration possibilities (to combat the almost limitless design problems that IT equipment can impose in small places). They can integrate with a wide variety of heat rejection technologies, including chilled water, air-cooled direct expansion, water-cooled direct expansion, and glycol dry coolers.

Common options for CRAC units include, but are not limited to: low ambient controls, electric, hot water, steam or hot gas reheat/heat, steam humidifier, BMS compatible microprocessor controls, hot gas bypass systems, condensate pumps, main power disconnects, smoke detectors, firestats, and water detectors.

For innovative CRAC solutions for IT closets I encourage you to look at AboveAir Technologies’ ceiling-mounted CRAC units for heat loads over half a ton. Their equipment comes in packaged and split configurations, and even fits in a lay-in ceiling.


AboveAir Technologies' CRAC Unit for Lay-In Ceilings

Verdit: Almost always the best solution to IT closet conditioning. Can be extremely cost competitive with similarly sized ductless minisplit.

McNellage & Associates are absolutely passionate about innovative data center HVAC equipment, and have more experience than anyone else when it comes to IT closet cooling. We would love to work with you and your team on your next project! Click here to contact us and learn how to make your next CRAC project accomodate your design requirements AND budget.

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