There's nothing more basic than this: You must have a filter, and it must be at
your air mover

Air conditioning manufacturers require air filters be provided, to protect cooling
coils from becoming plugged with dirt. That has nothing to do with the quality of
the air you breathe (more on that later); it’s all about minimizing the risk of
compressor and heat exchanger failure due to insufficient airflow. It’s further
required that the filter be installed either at the equipment inlet or inside the unit.
These are basic requirements, common to all the manufacturers whose
Installation Instructions we’ve read.

¶605.1 of the 2006 IMC requires all central heating and air conditioning systems
to have filters installed upstream of the heat exchanger or coil. ¶605.3 further
requires ducts “be constructed to allow an even distribution of air over the entire
filter.” Those requirements have been in the Code since at least 1998.

IMPORTANT NOTE: New systems not provided with filters can void equipment
manufacturer’s warranty, and do violate Code.

It’s permitted to use “building cavities” (stud and joist spaces) as ducts to
transport return air. We don’t like that because of the risk for mold growth should
the organic materials become wet; and there’s always construction debris
present, that can present a fire hazard.

True Story: We’ve seen several instances of H&AC contractors providing “filter
grilles” at walls and ceilings, and then transporting the return air back to the unit
through construction debris laden building cavities, and omitting the required
filters at the units; their intent was to make it easier for the homeowner to change
filters. Younger viewers say “DUH” right now.

Fact: Such installations violate equipment manufacturers’ Installation Instructions.

Now let’s discuss
Indoor Air Quality (“IAQ”), an extremely hot topic in today’s

IAQ Basics

Everyone knows you need a better filter to improve IAQ, but first things first: The
supply ductwork must be clean, and must be kept clean forever (what good does it
do to  upgrade your air filter if there’s construction debris downstream of it?). So
often we’ve seen this simple truth overlooked, even by the best builders and
H&AC contractors.

Another basic: Your supply ductwork must be clean, or the filter will be
worthless in terms of improving your "IAQ" (Indoor Air Quality).

True Story: A plant operations superintendent with whom we worked had spent
some time as a carpet installer on new construction in his early years; he swore it
was routine for them to sweep construction debris into floor openings (to include
air outlets) before laying carpet pad. We'd told him of a recent inspection where
we found handfuls of construction debris below floor diffusers; he then told us his
story, and we laughed about it at the time. Sad but true.

Many builders demand the H&AC guy run the heating and air conditioning during
construction; that’s prohibited by many equipment manufacturers, because of the
risk of equipment failure due to buildup of construction debris inside the unit. It’s
quite common to find construction debris and/or fine white dust in the return
ductwork, blower compartment and adhering to the blower wheel and cooling coil.

IMPORTANT NOTE: The first thing to do when setting out to improve IAQ is to hire a
pro to clean the return ductwork, blower compartment, blower wheel and housing,
cooling coil and supply ductwork.

Air Filters – Advanced

The filter must not unduly restrict the airflow; to do so is to reduce airflow and risk
compressor and/or heat exchanger failure. Here we’re concerned with “pressure
drop” (“PD”), and that’s a function of filter size and type and “air velocity” (the speed
at which air travels through the filter – expressed in “feet per minute” or “FPM”);
typical PD’s range from about 0.02” to 0.07” “WC” (“water column”) or so for
standard filters of the type equipment manufacturers require. Better filters have
higher PD’s; it’s not uncommon to see PD’s of 0.20” or more, which can be more
than ten times the PD of a basic filter. That’s important to know because of the
risks associated with substandard airflow.

IMPORTANT NOTE: Think twice before buying one of those expensive pleated
filters at your local home improvement store; you’ll be reducing your system’s
airflow (those filters have a much higher PD), and that could destroy your
compressor and/or heat exchanger.

IMPORTANT NOTE: Upgrading the filter should be done during the duct design
process, so that the designer can provide for the increased PD.

That’s not to say you can’t retrofit better filters, only that you must do so very
carefully; you must be especially careful with four and five ton systems (a lot of the
large systems we see are already borderline in terms of air delivery, and adding
resistance to flow results in substandard flow).

In the “Advanced” section of  "
Metal Ductwork", we learned that some fitting
designs have higher PD’s than others, so it might be improving fitting designs will
offset the higher PD of that exotic filter we’d like to retrofit.

How exotic should we get? Well, it’s all about basics here; a 1” thick pleated filter
improves filtration efficiency significantly (just the standard variety, please), and
you can buy replacements at your local home improvement store. Another type
that’s good is the 4” cartridge filter offered by many manufacturers, but whose
replacements are harder for homeowners to find, as well as more expensive to


Common Mistake #1: There's a real concern with IAQ these days (and rightly so),
and we often find sophisticated filters installed in duct systems containing huge
amounts of construction debris on the supply side (Younger viewers: Here's an
appropriate place to say "DUH").

Common Mistake #2: Buying an expensive pleated filter at your local home store
to replace that flimsy throwaway filter your air mover came with. Pleated filters,
especially those proclaiming higher efficiencies have HUGE pressure drops and
can restrict airflow to substandard levels; cooling compressor damage can result.

Common Mistake #3: Retrofitting a sophisticated filter without considering its
effect on system flow.

One Contractor's Successful Approach to Filtration

At the left is a three ton system (1,200 CFM); the installer provided a 16x25x2
throwaway filter. We determined the filter was unduly restricting flow:
       16x25 = 400 sq. in.
       400 sq. in. / 144 sq. in. per SF = 2.78 SF
       1,200 CFM / 2.78 SF = 432 FPM
300 FPM is the maximum for this type of filter.

Furthermore, the installation allowed bypass and inleakage from the furnace
room (The latter is a serious Code violation).

We asked the contractor we were working with if he could come up with some sort
of a "V" bank arrangement to increase the filter surface area; he consulted a local
sheet metal shop, who came up with the arrangement at the right. Here's the
math on that:
       2 @ 18x24 = 864 sq. in.
       864 sq. in. / 144 sq. in. per SF = 6.00 SF
       1,200 CFM / 6.00 SF = 200 FPM

The shop is to be congratulated for producing a locally-made product that looks
like a factory-made item; the contractor is to be congratulated for upgrading the
filtration (Pleated types are provided) and reducing resistance to flow (The pleated
would normally have a higher resistance, but because on the lower velocity, more
than 0.10" of resistance was saved).

Notice the gasketing to minimize inleakage. Did we mention the piano hinge and
the latches to secure the cover?

This homeowner
upgraded to MERV
11. Unfortunately,
the contractor didn't
provide the required
transition between
the filter and the
furnace, and a huge
restriction to flow
was created.

See below.
Filter "Bypass"

If you look carefully
along the left edge of
this filter, you can see
into the return duct

Air is literally flowing
from the duct
AROUND the filter
and into the furnace.

As it turns out, the
filter itself also
created a huge
restriction to flow:
System capacity is
three tons, or 1,200
CFM (cubic feet per
minute); PD is about
0.25" at that flow.

We felt there was no
way the system
could deliver that
much air without
major duct
improvements to
We don't like using
building cavities for air
return. This joist bay
is being used as a
return duct as
permitted by Code; it's
"legal". However, this
home employed "filter
grilles" in walls in lieu
of a filter at the unit
(That violated Code
and Manufacturer's
Instructions), meaning
 this space is
downstream of the

Click on the image to
see what these
homeowners were
The installation must
not permit any
bypass” (unfiltered
air traveling around,
instead of through,
the filter); to this end,
there must be tight-
fitting channels or
frames (known as
"safing") surrounding
the filter.