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.
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.
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.
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
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!
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:
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.
There were some good suggestions here. Thanks for the ideas.
Electrostatic filters do a good job of cleaning, but they are lossy
filters. A significant amount of dirt escapes and must pass thru the
filter a second or third time before it gets captured. By that time,
it probably been trapped on the AC coil, in a duct somewhere, settled
onto a surface (ANY surface) in the home, or in an occupant's lungs.
Electrostatic filters get a bad rap in the HVAC industry when they are
used as the primary filter. Interior surfaces of the furnace get plated
up with dust. They work better as a stand alone filter like the one
that Sharper Image sells (Richard Thalheimer on TV with a baby in the
most recent commercial)
The V-bank is a good idea too. Used in commercial air handlers, it
doubles the air filter area, and halves the air flow rate thru the
filters. We may be able to buy such a rack and install it, or build it
Most of the data I have seen on the 2" filters indicates that the filter
mfgs are using more efficient filter media in them so the friction loss
is higher. 0.20 clean pressure drop are the figures I have seen for
MERV 8 - MERV 11 filters.
As static pressure increases (duct resistance plus, AC coil resistance
plus, filter resistance) velocity falls. With zero external resistance,
my furnace can deliver 1328 CFM. With 0.5 in wg of external resistance,
air flow drops to about 1150 CFM.
In this house, the furnace is centrally located. roughly 90% of the
airflow is thru ducts that are less than 10 feet in length.
Unfortunately, most of these ideas fail to address an issue important to
my health. I am mold spore sensitive. To keep my chest clear, I need a
filtration system that captures most of the particles 1 micron in size
and larger. The low efficiency filters that the furnace mfgs want us to
use are only effective on the 10 micron and larger particles, the stuff
we can see on surfaces, the junk that covers the surface of the air filter.
One solution is a separately powered HEPA filter. Has its own fan so it
does not load the air handler,and it can be set to run continuously,
circulating air thru the entire house. Unfortunately, I can't install
it in this very compact house. It attaches to the return air plenum,
processes some (or all in some configurations) of the air and returns it
to the plenum.
Another possible solution is another commercial/industrial filter
design. Synthetic media bag filters are available in sizes to fit my
filter frame. As per ASHRAE 52.2 ratings MERV 12 is what they get
spec'd at. These filters capture about 70% of the 1 micron and larger
particles, over 90% of the 3micron and larger particles and near 99% of
the 10 micron and larger particles.
Ok what are the issues with this filter?
1. Price - not too bad, about $30 to $40 each
2. Lifetime of the filter - excellent, change once or twice a year
3. Pressure drop -Excellent, I can pick models that approach the
pressure drop of MERV3-4 filters,i.e. 0.07-0.08 in wg at my furnace's
4. Available sizes - BIG problem in some cases 12x24, 20x20, 20x24, and
24x24 are the ONLY sizes available. You can pair these up to create
larger filters if you provide gaskets between adjacent filters, or you
give each one its own frame.
5. Mounting - Good, The headers on the filter are compatible with 1"
6. Strength - bending. Excellent, these filters have Steel frames. They
are designed for use with systems that can achieve 500fpm across the
filter face vs the 300 fpm that disposable filters can tolerate.
7.Availability - any HVAC company that services commercial buildings
knows where to get these and there are several suppliers on Internet
that will accept orders by phone too.
There are still other mechanical options for these filters. Some come
in steel or wood boxes. Some come in rigid plastic frames (several V
pattern filter faces). Most of these other options have at least one or
1.Pressure drop - the unsupported bag filters have the lowest pressure
drop of any available MERV 11 or MERV 12 filter
2. Mounting - many of these other options do not include header we need
to mount the filter in a residential filter frame.
One other thing you could do is to put a filter on the return ducts?
How many do you have would that work? You could also see how dirty the
filters were. As for the 2" filters where I am they make them all to
order as there are so many odd sizes. You could just have them make
the filter out of whatever paper you want. If you have room for the V
idea that would be a good solution also. You could even go with a 4"
Robert Gammon wrote:
I have ONE return duct. It sits right under the furnace. I put the
filter here, there is no other place for it.
2" and 4" filters, even MERV 8 versions, put a serious burden on the air
I have seen the specs for Filtrete, PrecisionAire, Purolator,
Accumulair and several others in a 2" deep size. They all put too much
pressure on the fan.
I change the filters on a regular basis, usually about every two months
with a pleated MERV 8 filter.
The message from the furnace manufacturers is CLEAR, we care about the
equipment, we don't give a damn about you.
Except for some ground source heat pump manufacturers,who now include
very large 28x30 (and larger) MERV11 2 inch filters with the unit, AND
they have increased the fan HP from 1/3 to 1/2 HP AND they offer a fan
upgrade to 1 HP on the most popular 3 ton and 4 ton models. These units
come with a 2 inch filter rack installed and there is an option to
install a 4 inch filter rack.
My fan is a 1/3 HP fan like almost everyone has in a 3 ton unit these days.
Built in 1982. 1200 sq ft.
Return air comes thru ONE single point source, a plenum of plywood and
timber underneath the furnace (up flow model).
Extremely common building practice in the late 70s and early 80s in most
parts of the USA that use forced air heating and cooling.
Houses in the 90s started the return duct per room idea ( at least for
rooms with size)
Supply ducts to laundry room, master bedroom, master dressing area and
master bath are under 6 feet long. Kitchen and living room ducts are
under 10 feet long
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.