Airtight ductwork is one of the keys to a quality heating and
cooling system. It helps to make a home more healthful, more
comfortable, and more energy efficient. By reducing the load on
the equipment, it also reduces wear and tear.
There is only one way to make sure that the ductwork has been
thoroughly sealed: Test it. A simple pressure test can measure
the airtightness of the air distribution system and helps you
hold your installers to a high standard of quality.
Of course, airtight ductwork is only one of the keys to
quality hvac. A Manual J load analysis and Manual
D duct design, proper refrigerant charge, and correct
airflow over the evaporative coil are all just as vital. But
poorly sealed ductwork can negate good work in all those other
areas: You can properly size the unit, lay out the distribution
system correctly, tune the compressor perfectly, and set the
fan just right, but if leaky ductwork blows most of your
conditioned air into the attic, or pulls most of the system's
return air from the crawlspace, you'll have a system that costs
too much to run and doesn't do its job.
Pressure testing is not a new concept in home construction.
Plumbers have their work pressure tested on every job, and the
test forces them to get it right -- meaning no leaks, period.
But when plumbing leaks, there is an immediate consequence:
Things get wet. Air leakage, on the other hand, is invisible
and seldom causes immediate trouble. Instead, homeowners
gradually become aware of comfort problems, high utility bills,
and decreasing indoor air quality over time. The test is the
only way to identify the defects in advance.
In ten years of testing ductwork before drywall is hung, I've
seen some excellent ductwork installations and some atrocious
ones. I've learned that if you want consistently good results,
you have to test consistently. All tradespeople do better work
when they know they face inspections. As one local mechanical
contractor's lead man put it to me, "There are three kinds of
air distribution systems: regular, sealed, and sealed and
tested." If you test your installer's work, it will get better.
And realistically, until you start testing, and showing your
installers the results, they won't even know that their system
leaks. They certainly won't know where it leaks. The test makes
them face reality, but it also helps them learn.
The duct airtightness testing system I use is called
Minneapolis Duct Blaster, from the Energy Conservatory
(612/827-1117, www.energyconservatory.com). It consists of
a calibrated variable-speed fan, a fan speed control, and a
digital pressure-reading gauge called a digital manometer (see
Figure 1.The Minneapolis Duct Blaster is a
calibrated variable-speed fan that's tied into the heating or
cooling air distribution system at a supply plenum, return
grille, or air handler compartment (top). The digital pressure
gauge (bottom) calculates air leakage rates from pressure and
fan speed data.
The test itself is pretty simple: We attach the Duct Blaster
fan to the air distribution system at a return grille, a supply
plenum, or the blower compartment on the air handling unit. We
temporarily seal off all the registers and grilles. Then we
turn on the Duct Blaster fan and apply pressure. The Duct
Blaster system measures the airflow needed to create a test
pressure of 25 pascals (a 0.10-inch water column) in the duct
system. This airflow rate is our duct leakage measurement. We
compare the duct leakage reading with a recognized standard,
and we give the system either a pass or a fail. The whole
process takes less than two hours (mostly for setup and
takedown) and typically costs around $220.
Over the years, my company has headed off a lot of potential
problems by testing ductwork before the drywall is hung. When
the drywall is up, we can still gain access to the ducts, but
the problems are harder to locate and assess.
Why Airtight Ductwork
Good duct sealing makes a major contribution to the
healthfulness, safety, comfort, and efficiency of a new home.
Sealing the ducts is even more important than sealing the
building envelope, because when the air handler is running, the
pressures in the air distribution system are much greater than
in the building or the outside air. Pressure differences are
what drive infiltration; a hole or crack in the building
envelope is not an air leakage point unless it sees a pressure.
Ducts always operate under pressure, so a hole in the ductwork
is always a leak.
To clarify the importance of pressure, let's look at some
numbers. Wind, stack effect, exhaust fans, and the like
generate air pressures across the building shell that range
from 0.5 to 10 pascals. On average, a house is usually in the
low end of this range. (A pascal is a very small metric unit of
pressure. There are 25 pascals in 1/10 inch of water column. If
you were to put a straw into a glass of water and suck the
water up the straw 1 inch, you'd create an inch of water column
pressure, or 250 pascals.)
Pressures created in air distribution systems when the air
handler is running range from 10 to 125 pascals, tens of times
greater than the ordinary pressures acting on the house
envelope. And these pressures, unlike the wind, are continuous
when the air handler is running. That is why house infiltration
rates can double or triple when the equipment is active.
Temperatures. Remember, too,
that leaks in supply ductwork involve conditioned air, not room
air. The air that escapes from supply ductwork has been heated
to 140°F or cooled to 58°F, and it is not getting
to the room it was meant for. If most of the ductwork is in the
attic or crawlspace (which is often the case), all this heated
or chilled air is now leaking straight to the outdoors. The
resulting energy loss is much greater than the loss that occurs
when 70°F room air leaks out around a window, for
example. That's why simple duct tightening may cut heating and
cooling costs by 15% to 30% in many homes.
Duct leaks also typically affect comfort, because the system
doesn't deliver the intended amount of conditioned air to
satisfy the design load for a room. Rooms at the end of long
duct runs suffer the most: The farther away from the fan a room
is, the greater the likelihood that air will find its way out
of a leak instead of going where it's meant to.
On the return side of the air distribution system, leaks pull
ambient air or even hot attic air into the system, further
compromising efficiency. Leaks also threaten indoor air
quality: Air from a crawlspace may carry pesticides, moisture,
radon, and mold spores, all of which can get sucked into the
return leak, then sent through the supply side to every room in
Energy penalty. How much do
these leaks affect energy consumption? A 1999 summary report of
19 separate studies from around the country
(www.aceee.org/pubs/a992.htm) suggests that
the average annual energy savings potential in a typical house
from sealing the ductwork is around 17%. These studies include
both heating and cooling climates. Anywhere ductwork can leak
outside of the house, there is the potential for a large energy
penalty. In one study, researchers at the Florida Solar Energy
Center found that sealing ductwork in existing homes cut
cooling bills by about a third.
Locating the Leaks
Where are these leaks happening? Basically, leaks are possible
at any joint or seam in the system, from the air handlers
supplied by manufacturers, right on through to the sheet-metal
supply boots your installer brings to the job. If you look at
all the joints, you'll find the leaks.
There are leaks on sheet-metal trunk ducts every 48 inches,
where the trunk sections connect. Every collar attached to the
trunk, where flex duct branches take off, represents a possible
leak. So does every adjustable elbow. We also find leaks where
flex duct connects to supply boots, in the boots themselves,
and around the plaster grounds. I'm often surprised by how
leaky some of the manufacturers' mechanical units are. I've
tested units that leaked 40 to 45 cfm right out of the
I used to commonly find flex ducts that either had never been
connected or had been accidentally disconnected by another
trade. During the early to mid 1990s, most framed return
cavities were major leakage sites. Disconnected ducts and
framed cavities still tend to be the biggest culprits, but
practices have improved significantly.
Stud and joist bays used as returns are still common in my
part of the country and often present a lot of problems. All
framed cavities should be lined with OSB, plywood, duct board,
drywall, or sheet metal and then sealed with mastic at every
joint. Hvac contractors sometimes blame the framers for not
lining the cavities, but we don't accept that excuse -- if it
leaks, you fail.
The most frustrating leaks are where the hvac installer tried
to seal the system and failed -- where someone did the sealing
but didn't get it right. For example, it hasn't yet dawned on
everyone that each joint and seam have to be sealed on four
sides, not just three. If a crew hasn't had the benefit of some
training, and has never seen a system tested and fogged, it's a
good bet the system won't pass. Installers just can't picture
the consequences of an average duct sealing job.
Running the Test
A Duct Blaster test on a single air distribution system takes
about 45 minutes to 1 1/2 hours to perform. Most of that time
is spent sealing off registers and grilles and setting up the
equipment. Once a system is prepared, it takes only about 2
minutes to perform the test.
The first step is to attach the Duct Blaster fan to the air
distribution system (Figure 2). We can attach it to the blower
compartment on the air handler or to a return grille. If the
air handler is not set when I arrive to test, I connect the
Duct Blaster directly to the trunk lines in the mechanical room
where the air handler will eventually be set. But we prefer to
test with the air handler installed, because of the leakage
we've seen in even brand-new units.
Figure 2.The author tapes a plastic transition
fitting to a piece of cardboard cut to fit the opening in the
air distribution system (top), then places the assembly in the
opening (left center) and connects the fan to the opening with
a length of flex duct (right center). It's best to test the
system with the air handler installed, because the air handlers
are often a leakage point. If the air handler has not been set,
the author hooks up the Duct Blaster to the trunk line where
the air handler will go. The equipment can also be attached to
a return grille (bottom).