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Furnace Filters Robert Gammon 10-01-2006
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Posted by Robert Gammon on October 1, 2006, 4:11 pm
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.


Plumbing 468x60
Posted by Eigenvector on October 1, 2006, 7:26 pm

>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.



Posted by professorpaul on October 1, 2006, 8:27 pm
I went to my local HVAC guy and had him make me a washable filter for
$10. Some sort of rubber covered hair like stuff. I also got a spray
bottle of soluable oil to help pick up the dust. Every month I simply
give it a hose down in the shower, let drip dry, and spray again. This
is what I did many moons ago with the filter in an old Tektronic
oscilloscope.

The friction loss in the pipe/ducts is the main culprit. I have had an
electrostatic air filter and mechanical filter backup in one house I
owned. Had to turn a hose on it every 3-4 months, and also used the
filter oil on the screen, which became FILTHY!


Posted by wayne on October 1, 2006, 9:14 pm
if you have the space you could go with a 2" Filter it should give you
more surface are and more stiffness you can check the airflow coming
off the vents with the filter on and off

Robert Gammon wrote:

> 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.

Posted by Don Young on October 1, 2006, 11:19 pm

>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



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