"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.


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.


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.


Go get an electronic filter, if air efficiency is what you are after.
Honestly you are losing more in the ducting than you are in the filter. The
ducting of a typical house is not airtight, has very high friction surfaces
(dust, mice, insects, water, melted crayons, etc..), and the layout of the
ducting is less than optimal for airflow. In the distances you are talking
about only the friction really matters all that much.

So if you want to maximize your airflow, get a good air filter and keep your
ducts clean. Those cheapo filters I'm sure minimize air flow resistance,
but they are putting lots of dust into your furnace and wearing it out
faster, as well as putting lots of dust in your lungs.