I have my server and networking equipment on the top shelf of a typical bedroom closet. The heat is stuck in the top of the closet and the components just cook. For about two years I've just left it as-is because even though it was hot, it never caused a problem. However, I've added a bunch of drives and upgraded my server recently and decided I need to deal with the heat before the warm weather kicks in. My goal isn't to have the closet at the standard room temperature, but just to have a way to remove excess heat build-up by getting some sort of air flow going.
I read several threads in other forums on AVS and elsewhere. Some involved cutting a hole and using a fan, either a bathroom vent fan or PC fans rigged up to external power. The most basic is to use a vent fan blowing into the attic. The problem with that is you are also blowing your cooler air into the attic where your return air system cannot reuse it. Plus you are running a fan 24/7 and that fan is either cheap and loud or expensive and quiet.
The next solution seems to be cutting a hole into the closet and using fans to blow the air into an adjacent room while having enough cool air flow into the closet either under the doors or through a secondary hole/vent. This keeps the air in your home and lets the return air system in that area deal with sucking up the heat. I almost went with this approach and even bought some fans, molex adapters, and an external molex PCI slot connector. But now I need to return that because I went with the HVAC approach.
I decided to use my "high efficiency" HVAC unit to my advantage since it runs its fan 24/7 cycling the air at a lower speed until it speeds up for heating/cooling. I went with a 4" solution and bought insulated flex duct, starter collar, diffuser (used in reverse), 2x band clamps, foil tape, and a 4" dia sheet metal (bi-metal) hole saw.
I cut a 4" hole in my closet ceiling and inserted the diffuser
Then I connected the 4" insulated flex duct
using foil tape and band clamps.
On the other end I drilled the 4" hole into my return air box near the three other 10" return ducts. Make sure you have a sheet metal cutter
or you won't get far and have to go back to the store the next day...
I then inserted the 4" Starting Collar
and connected the other end of the duct with foil tape and band clamps.
As a final touch I drilled a 4" hole through the lower part of the closet and inserted a 4" Eave Vent
to provide enough intake of room temperature air. I bought two of these and just used the cap from the second. It is the perfect length to go between a wall the thickness of a 2x4 and two pieces of Sheetrock. These could probably be used with one of the other solutions as well.
All that is left is to seal the ducts with duct sealant, but I'm waiting to make sure I'm happy with everything as-is. It is a big enough PIA to remove foil tape, I don't want to be removing duct sealant as well.
From my testing, using my ThermaPen, the top of my closet is now staying at 85F and my HDDs are at 30C (86F). That isn't bad IMO. Stuff is warm, but not as bad as it was. In the end I spent around $65 or so on the entire solution, excluding tools. It would have been cheaper if I didn't have to buy 25' of ducting since I only needed about 10'.
I'm sure some HVAC contractor would have a fit, but I tried to do everything I could properly.