I will be using a Panasonic Whisper fan in a media closet ceiling, however, what should I do about the air intake from the crawlspace below? I imagine having it towards the back of the closet will work best. What can I do about a filter for this intake? What size should it be? Thanks.

If this is an in line fan get something at least the same diameter or bigger than the fan intake.

I have one of these hooked up to a fan tech in-line fan. It has a spring loaded backdraft protector. I'm not using any filter.

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If it is a ceiling mounted whisper fan the unit it self is the intake vent

The way I read your question you are asking about filtering the air..... is that correct? If so, I wouldn't worry about it assuming you are talking about just the equipment closet and not the theater itself.

I don't think I explained this properly. I'm talking about return air. Like a floor register, only it would be just a hole in the floor exposed to the crawlspace below. I imagine it's quite dirty down there, so I was thinking of putting a carbon filter there, but was wondering what others do.

I wouldn't be cutting a hole in the floor of my house to expose it to crawl space air. The only house I had with a crawl space it was dark, damp and had crawly things. Exhaust air out of the equipment stack area with your whisper fan either into another area of the house (preferred) or outside and use the air inside the house to provide your return air to the equipment stack area.

Just like a bathroom exhaust. Where do you think the air return comes from? Unless your house is super tight and fitted with an air exchange unit the return air comes from all the drafts and other leaks around the house.

Interesting, I would have thought it would be ideal to vent to the outside as that is code for bathroom fans, furnaces etc. During summertime I wouldn't want to vent it to another room since it will pump in hot air.

Currently the media closet (formerly known as the furnace closet) has an opening in the floor to the crawlspace, and the same goes for the water heater closet. I thought that was normal.

Interesting, I would have thought it would be ideal to vent to the outside as that is code for bathroom fans, furnaces etc. During summertime I wouldn't want to vent it to another room since it will pump in hot air.

Currently the media closet (formerly known as the furnace closet) has an opening in the floor to the crawlspace, and the same goes for the water heater closet. I thought that was normal.

OK I think we are mixing terms.

I thought you asked about an air intake not venting to the outside (as in exhaust vent to the outside). It's fine to vent outside just not energy efficient because it effectively pulls unconditioned air (think humid) into the house which must then be conditioned.

In my California crawl space home it too had a hole in the floor and to attic in the furnace closet. However the door was an exterior weather stripped door effective sealing the compartment from the inside air. The purpose was to provide a source of combustible air for the furnace, not air to cool electronics.

Now I'm confused again about the meaning of intake. The fan on the ceiling will be taking air and pushing it through a duct to the outside. It would not be good to push this air in the attic as it could encourage mold etc.

To make up this air, it could either take it from under the door, in which case it will have to be made up by having the hvac system work harder, or, it could take it from the register (hole) in the floor, (so unconditioned air), and that would minimize the use of conditioned air outside the closet. The closet is not airtight, but I would think it would be better than coming entirely from the conditioned air outside the closet. That was my thinking anyway.

What you are proposing is environmentally about the same as putting your electronics gear on a covered screened porch. If that works with your local weather conditions just put a critter screen over the intake and fire it up.