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QUESTION: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.
ANSWER: Mass in equal mass out always (that doesn't mean air speed equals air
speed). If your furnace is sucking in air at a certain amount per time it
is blowing it out at a certain amount per time - otherwise a leak would be
indicated. A long duct would increase frictional forces on the air and
reduce that amount per time.
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