"Robert Gammon" wrote in message
news
I got to reading up on this topic as research fora new house that I want to
start building soon.

I read that most furnace fans are only capable of driving about 0.5 in
water of pressure. AC coils take 0.2 to 0.3 in of water pressure drop,
ducts take another 0.1 to 0.2 in of pressure and then nothing is left for
the furnace filter. That's why the cheapest of all filters are generally
included on furnaces.

To top it off, furnace installers often do not read the specs for the
units that they are installing.

We had our HVAC replaced about 3 years ago with a 12 SEER,80% gas combo.
At the time, I insisted that the existing 20x20x1 furnace filter be
replaced with a 20x25x1 model ($50 part,$50 filter(20x25x5 Honeywell),
$200 labor).

As I have done this research, I have looked at the face of the $6 filter I
have installed in the unit presently. It is heavily bowed when the furnace
is operating (but makes less noise than the old furnace with a 20x20x1
filter in it). So much bow is there that I worry about the filter
collapsing. So far, so good, none have collapsed.

However, reading the specs NOW, its clear that the HVAC installers were
not reading the manual. In the mfg's manual, it clearly says for this size
fan, operating at roughly 1200 CFM (can operate as high as 1328 with no
load),the unit needs not 400sq inches that I had, not 480 sq inches that I
now have, but 576 inches (24x24). This extra back pressure on the fan is
ROBBING performance, and costing me money. It is at least a 10% drop in
air flow.

Cheap solution is to revert to 99 cent fiberglass and change them every
few weeks. I want to think of something better than this. BTW
increasing the size of the filter to 20x30 is not do able as the
structural supports for the furnace are in the way. Any ideas are
welcome.

If you have room to do so, you can install a larger filter slanted instead
of flat or install two filters in a V configuration to get more filter area.
A little work with pieces of metal angle (the suspended ceiling type is
cheap and light weight), a drill, and screws or pop rivets will make a
suitable holder. You can put some heavy welding rods behind the filters to
keep them from bowing, if needed.

Don Young