Years ago, when we were in Lake Forest, CA, our server closet was under
the stairs. We called it the "sweat-box," and even though we kept the
door wide open 24/7, we lost a hard drive or two before I outfitted it
with a makeshift plenum (made out of a cheap blue tarp) in the ceiling
cavity, and a pair of 24" box fans exhausting into the stairwell.
When we moved to Costa Mesa, I designed a new server closet with an
industrial roof-mount exhaust fan and a perforated-steel security screen
door. We call it "the cage."
Both places, we keep an ordinary red-alcohol bulb thermometer on the
wall; nowadays, if the cage temperature starts to go up above 80
degrees, we crank up the speed control on the fan.