Can I use standard 1" thick furnace filters in place of an old
accordion-type super thick furnace filter?

My new house has a furnace that accepts the OLD thick accordion-type
furnace filters. I hear these are very expensive. The space they
install into is very wide to fit them. The current homeowners have
inserted four regular filters instead (!), to try and save money.

I was thinking about epoxying some angled plastic or metal into the
filter space, to construct a slot that would fit ONE standard furnace
air filter.

Is this advisable? Will using a standard filter damage a furnace
designed to use thick filters?

"bryanska" writes:
Can I use standard 1" thick furnace filters in place of an old
accordion-type super thick furnace filter?

Probably not.

Using four 1" filters in place of one 4" filter, i.e., what
the current owners of your house are doing, is a very bad
idea. Here's what I said about this idea when someone asked
about it in this newsgroup in October 2003:

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).

Your idea, using a single 1" filter and installing plastic or
something to block off the remainder of the opening, isn't as
bad as using four 1" filters to fill the opening, but it
still isn't good, because, as noted above, the total surface
area through which air flows is reduced, which means that the
filter will get clogged up faster. Furthermore, it's
unlikely that the air flow would be what the furnace is
designed to have if you use the wrong kind of filter.