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QUESTION:My newer gas furnace uses a pleated type filter that is about four
inches thick and it appears I can only get a replacement from the
furnace company and they are expensive. Is there any harm in using
four one inch filters of the type most hardware stores sell?
ANSWER: You need to be *very* careful about doing something like this.
The purpose of the pleats is to increase the surface area of
the membrane through which the forced air flows. The
increased surface area is necessary because the materials used
in moden filters are much less porous, so much less air can
flow through per unit area (as compared to, e.g., old,
"sponge" filters).
Another reason for the increased surface area is that since a
constant amount of dirt will collect on the entire filter as
it gets old, the larger its surface area, the longer it'll
take for the filter to get clogged up enough to reduce the
efficiency of the furnace and require replacement (because
the dirt will be spread out over a larger area).
If you replace a single filter with four-inch-wide pleats with
four with one-inch-wide pleats, you're not only reducing the
surface area in each of the four filters, but the air now has
to go through four layers instead of one, so the fan will
need to work more to drive the same amount of air through the
system.
Furthermore, most of the dirt will collect on the first of
the four filters, which means that it'll get clooged up and
need to be replaced sooner.
In short, if you do what you suggested, you'll reduce the
efficiency of your furnace, wear out filters more often, and
possibly wear out the fan and/or heat exchanger prematurely
(the fan because it'll be working harder, and the heat
exchanger because the generated heat won't be dissipated as
quickly as designed because the air won't be flowing as
quickly).
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