Slightly OT: Calculating HVAC requirements for server rooms

Jack Bates jbates at
Sat May 2 06:52:32 CDT 2009

William Herrin wrote:
> You shouldn't even *have* a drop ceiling in a modern computer room.
> You want the room to be as tall as practical so that the air from the
> hot aisles has somewhere to go on its way back to the HVAC, other than
> back through and around the cabinets.

I love my 30' ceiling. Even with all the things that are wrong with our 
HVAC setup, the servers survive due to that ceiling.

> Personally, I've found that there's a pretty wide disparity between
> HVAC professionals that are capable of hooking up a CRAC and turning
> it on versus HVAC professionals that understand the holistic picture
> including hot aisles, cold aisles, humidity control and flow. I
> wouldn't want to call in a professional without first understanding
> the problem well enough to assess whether I was getting a competent
> answer.

I had this issue. They hooked up the redundant systems, but didn't 
bother with much else. The return feed is below the units pulling 
ambient air, and the cold air is injected 15+ feet above the isle behind 
the servers, intermixing with the hot air as it rises up the wall.

At least it works, but it could be better and changes will need to be 
made before I can reach 50% capacity in the racks.


More information about the NANOG mailing list