Considerations for PoE deployments in switch closets

As companies supplant digital handsets with IP phones, they deploy Power-over-Ethernet switches to support them. PoE switches generate almost five times more heat than data-only switches, however, heat that has to be taken into account in switch-closet design.

There isn't much written about how to deal with heat in these small spaces, a fact that has resulted in a lot of ad hoc decision making. I've seen switch closets with three air conditioning systems for an otherwise small PoE switch deployment, and rooms stuffed with switches and no consideration given to heat.

According to the American Society of Heating, Refrigerating and Air-conditioning Engineers, the goal is to maintain switch-closet operating temperatures of 66 degrees to 77 degrees Fahrenheit while avoiding temperatures below 59 degrees or above 90 degrees. We'll ignore the recommendations about humidity levels, on the assumption that these rooms will be cooled either by ambient building air, which is always higher than 30% humidity, or an air conditioning unit that can control the humidity.

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As companies supplant digital handsets with IP phones, they deploy Power-over-Ethernet switches to support them. PoE switches generate almost five times more heat than data-only switches, however, heat that has to be taken into account in switch-closet design.

There isn't much written about how to deal with heat in these small spaces, a fact that has resulted in a lot of ad hoc decision making. I've seen switch closets with three air conditioning systems for an otherwise small PoE switch deployment, and rooms stuffed with switches and no consideration given to heat.

According to the American Society of Heating, Refrigerating and Air-conditioning Engineers, the goal is to maintain switch-closet operating temperatures of 66 degrees to 77 degrees Fahrenheit while avoiding temperatures below 59 degrees or above 90 degrees. We'll ignore the recommendations about humidity levels, on the assumption that these rooms will be cooled either by ambient building air, which is always higher than 30% humidity, or an air conditioning unit that can control the humidity.

Let's do the math

When adding up the heat output of the switch closet, it's important to note that the "watts into the closet = heat that needs to be removed" theory doesn't apply to PoE implementations because PoE switches deliver some of that power outside the closet.

Most switch manufacturers list the BTU output of their devices, but I would encourage you to check and validate those numbers or at least test-run the assumptions and validate the data before committing one way or the other.

Besides the switches, you'll need to calculate the heat output of all the other devices present, including light fixtures, UPS, routers, AC/DC converters and so forth. Accurately calculating the heat output allows you to choose the cooling solution that best fits your needs and alleviate future reengineering.

Next, find out the temperature settings for the building's environmental controls during and after business hours and weekends, because to conserve energy, most buildings' environments change when fewer people are present.

How to cool the closet

With all that done, you can figure out how to deal with the heat. First things first, however: Look to see whether the closet has a building vent. A cooling and heating vent can create havoc with the air environment, and may negate any remedy you put in place. Find out whether the vent is active and whether it is used for cooling and or heating the space. Incorporate the vent in your design or shut it off altogether.

Once you have dealt with that, consider these factors in your planning:

* Conduction. Heat flows through closet walls. On average, switch closets can dissipate close to 400W without needing any ventilation. Of course, this depends on three factors: room size and the material used for the walls; ambient temperature around the room; and ambient temperature during the weekend and after hours.

* Passive ventilation. Heat can flow into cooler air via a vent or grille, dissipating as much as 800W of heat.