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We typically use 85-88* as our tipping point. Below that it should be ok, above and it needs attention. With a rack it can often be as easy as adding doors and side panels and creating a chimney that will cool passively using convection.What do you consider really warm.
My walk in closet that houses the av gear averages 80-85f.
89f at the top of the rack
Do I need cooling?
I agree with you, at the bottom and exit at top.Any fan blowing out is going to suck dust in wherever it can. Any fans blowing in are going to pressurize the cabinet to some degree which helps keep dust out. These fans should have a filter.
If possible put the fans at the bottom and exit vents at the top.