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.

Keeping the CROSS SECTIONAL area (duct size) constant while increasing
the area (what you've called "SURFACE AREA")of filter material in
contact with the fluid (i.e., AIR) moving through the duct increases the
air resistance (in this case, the VISCOUS drag). QED