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Help. I have a custom built in home theater/high end audio
installation. The equipment is behind closed wooden doors. There are
two 5ft racks full of blister generating equipment (4 power conditions,
5 mono block amps, satelite box plus the head gear that's getting
baked). There is an 10" gap between the back of the builtin unit and
the wall. In the wall behind the unit are 4 120mm fans, two at the
bottom, two at the top (bottom blows in, top blows out from/into the
next room).

Problem: Still baking equipment. There is also plenty of bottom air
flow but it seems that the equpment, being densly packed and close to
the front doors prevents the air flow from circulating from the
bottom-back, to the front-top and out the top/back. It appears that
the open space behind the rack is getting the circulation but not the
equipment. I have engieered the built-ins such that the audio
equipment is directly under the HVAC air return in the ceiling (drawing
air up into the HVAC unit). This draws additional air through the
builtin but I'm still concerned about the heat. This is a bit complex
sounding and I have a nice PDF drawing of the current installation that
is worth a thousand words if you'd like to take a look. Photos
avaailable as well. As many of you already posted, the solution MUST
be quiet yet effective. Perhaps 200lbs of dry ice on top?

Without seeing it of course it's hard to know.
The problem may be that you have conflicting airflow within the spaces.
and while a lot of air is potentially being blown in or sucked out, in fact
it may just be circulating internally.
If each unit has it's own fans internally or air vents then you need to make
sure that your fans are assisting the air to move in the same direction.

Also if it's possible to re-order your units in the rack, try putting the
hottest units at the bottom, giving them the coolest air, rather than adding
to their heat generation by giving them the air heated by all the others
items.

And take a look at the heat sink air vents in each item & try & make sure
that air can flow freely through them.. this may mean even rotating items
sideways in the rack, if it's not possible to leave enough space.

If it's a home built cabinet, drill some big holes through each supporting
shelf to allow upward airflow .

I hope this helps

Cheers,
Ric