Filtering Out
Updated 8/19/10
If you're like a lot of people, you use the 57 cent fiberglass air filters in your furnace or air conditioning unit. They do a half-way decent job, and the price is right.

But I know a lot about a lot, and if something don't look right, it catches my attention.

Most of these filters boast what they call "Variable Density" construction. What this mean is that, the side of the filter where the air first contacts it, has the fibers spaced farther apart, or more coarse. As the air continues though the filter, the fibers get closer and closer together, being a tight mesh at the point the air exits from the filter.

The idea is to trap the larger dirt particles in the coarse fibers, without quickly plugging the filter, and having the medium-sized particles trapped partway through the filter, and the smallest particles trapped just before the air leaves the filter. This is sometimes called "depth loading", meaning the entire thickness of the filter will be utilized.

When looking at this type of filter, it is quite obvious to the eye which side of the filter is the coarse, or "air in" side, and the finer "air out" side.

Now for the B.S... For many years, many different manufacturers had the airflow arrow on the filter backwards. The result when installed, was that the air first contacts the fine side, plugging it up very quickly, while leaving the remaining thickness of the filter unused. I'm sure that this was just an accident, hey? I doubt that it really caused people to buy more filters or anything like that. And so.

UPDATE 8/19/10 The "Blue" brand filters are MORE DECEPTIVE THAN EVER! They have a single arrow on one edge. Next to that arrow is a sales pitch for you upgrade to one of their better filters, with the words "more info" or something similar, in fine print, by that arrow, meaning that the arrow is really pointing to more info printed on the face of the filter. How convenient that that arrow just happens to be the wrong direction for airflow, which most people would assume the arrow is for.

I buy some 2 inch thick fiberglass filters for some commercial equipment I take care of. They have the "Variable Density" feature also, and they were kind enough to put a layer of the fine stuff on both sides, so no matter how you install it, it's wrong and will plug up right away! So I have to whip out my pocket knife, and cut through the fine mesh all the way around the "air in" side, and carefully peel it off. Talk about beating the system!

So if you're using the fiberglass filters, make sure that the air is entering the filter on the coarser side, no matter what the airflow arrow indicates.

And for the record, the filter on most furnaces is located at the point where the air is going IN to the furnace, so the finer side of the filter should be towards the furnace, or the furnace blower.

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