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I have central air and heat at my appartment I moved into about 6 months ago. Today I decided it was probably long past time to figure out how to change filters in the furnace so I poke around until I find the slot where the filter is.

However, when I pull it out, it looks like it was installed yesterday. It's perfectly clean, doesn't have a speck of dust on it. I've been using it for the past six months so I highly doubt it should look this way.

It's a really cheap fiberglass filter (Ace hardware brand), could it be defective? Could the landlord have placed it in the furnace incorrectly? Could something be wrong with the furnace? Or is this expected?

My last apartment had a permanent filter that had to be pulled out and washed every couple weeks or else it got filthy. Please help me figure out what's going on here.

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up vote 7 down vote accepted

The thin fiberglass filters (usually blue in color) are referred to as "rock catchers" by some of my fellow construction friends. They don't do much to catch dust, but they'll stop anything large from damaging the blower. It's entirely possible that it was changed before you moved in and just hasn't had enough time to catch the little dust that it will catch.

If you really want to filter the air, you'll want to upgrade this to the pleated filters. Just remember that these do require changing on a more regular basis.

The other possibility is that you may have filters at your return vents doing a majority of the air filtering, and the filter at the blower is simply redundant.

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As far as I can tell, the return vents do not filter. – Malfist Jan 2 '12 at 22:19

That is very unusual, is there a chance your landloard changed it for you recently? Mine seems to get dirty almost instantly.

Some things I'd check:

  1. The airflow direction of the filter is correct. You can verify this by feeling the air flow and comparing it to the arrow that is usually printed on the filter.
  2. Is the filter the correct size? If it's not then the air will just go around it. The filter should fill the space completely.
  3. Is the fan running often?
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