On Thu, 23 Oct 2008 09:39:35 -0700, Smitty Two
wrote:

In article ,
Erma1ina wrote:

Bob F wrote:

"dpb" wrote in message ...
james wrote:
My furnace was installed with a 16x20x1 pleated filter.

It looks like there is enough space to fit a 16x20x2 filter. This should
last
longer and reduce air resistance.

Is there any reason not to?

How can doubling the thickness _reduce_ resistance?

It doubles the surface area.

It's the CROSS SECTIONAL area that is important in fluid dynamics.
Doubling the amount of material in that CROSS SECTIONAL area will
RESTRICT the flow.

THANK YOU!
I don't think so. Imagine a 1 foot x 1 foot duct. Now imagine a it
has a 1 foot by 1 foot filter in it. Now take a second case of the
same duct, but you have a 1 foot by 2 foot filter... same material
but double the area. If you put it in that same 1 x1 duct by putting
it in at a angle (60 degrees from perpendicular), air flow will be
better. You have double the cross section because you have
essentially created one big pleat. If you made a filter by
zig-zagging that 1 x 2 filter into a 1 x 1 holder, you have what the
OP was asking about.