Stay away from using the return air as a method of cooling your equipment. Dust would be the #1 problem and some of the others could be condensation on equipment or if enough heat is pulled into the return you could cause your heating equipment to cut out on high limit.

If your equipment is in a small closet like room I would suggest you use wall grills 1 high and 1 low. This would allow the hot air to escape from the top and let cooler air into the bottom. Its a natural effect that should work.

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