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"
filter
Robert Gammon wrote:
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 as suggested.
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
flow rate 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" filter frames.
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 two problems.
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.