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.