This room’s too hot, that one’s too cold, the second floor’s hotter than the first, etc.
We’ve all experienced those kinds of things, and they indicate a need for “air
balancing”. Here’s what you need to know for "DIY Balancing".


“Dampers” are used to balance airflows, and there are three types:
1. “Volume dampers” provided in the branch ducts, typically at the connections to
the main ducts.
2. “Opposed blade dampers” integral with air outlets; this type is characterized by
pairs of damper blades that move in opposite directions.
3. "Single leaf dampers” integral with air outlets, wherein all blades move in the
same direction.

IMPORTANT NOTE: Opposed blade dampers are only permitted as the sole
means of balancing where it is physically impossible to provide a volume damper;
that only occurs when an outlet is connected directly to a main, without a branch

IMPORTANT NOTE: Single leaf dampers are not permitted as the sole means of
balancing under any circumstances.

IMPORTANT NOTE: Volume dampers are required in most installations

“SMACNA” (Sheet Metal and Air Conditioning Contractors National Association,
Inc.) publishes a wide range of Industry Standards. ¶7.3.3 of their “
Comfort System Installation Standards Manual
” Seventh Edition 1998 provides
the basis for our statements about opposed blade and single leaf dampers.

“ACCA” (Air Conditioning Contractors of America) also publishes a wide range of
Industry  Standards. ¶8-2 of their Manual D (“
Residential Duct Systems” 2006) is
aptly titled “
Balancing Dampers Are Required”; it says “… a balancing damper
must be installed in each runout duct.” (That’s the same as “branch” duct).

Finally, balancing at outlets is problematic at best: The settings can’t be locked,
and objectionable air noise usually develops; balancing with volume dampers
solves both problems.

Seems unnecessary to say this, but dampers must be accessible; it’s not
uncommon to have to cut drywall to access dampers in finished basements.
Another problem we see is dampers being covered with duct insulation; and it’s
not uncommon to see flex duct clamps (think big “wire ties” here) run over damper

IMPORTANT NOTE: Locating your dampers and rendering them accessible may
require some creativity.


Balancing can be a DIY project, BUT before you attempt balancing, make sure
flex ducts are acceptable: It's no use trying to balance if you have excess
resistance to flow because of poor workmanship on the flex.

IMPORTANT NOTE: Poor workmanship on your flex may be the primary cause
of the imbalance; it can certainly be one of the causes.

OK, your flex is good, you have dampers, they’re accessible and you’re ready to
balance. The first step is to make sure all dampers are fully open (damper arm
parallel with direction of airflow). Some installers like to bend the arms 90º so
they’re perpendicular to the ducts, to make them easier to find; if that was done,
it’ll be necessary to bend them back before proceeding. If any are partially closed,
open them and live with the system a few days before making further changes.

After you’re satisfied all dampers are open, observe which rooms are too cool and
which are too warm (We're writing this in June, so the following pertains to cooling
– reverse things for heating – the next instruction becomes “…the warm rooms…”
for example). Throttle dampers supplying the cool rooms about one third of the
way (dampers are fully closed after a 90° traverse; you want about one third of that
now, or about a 30 degree traverse). Then live with the system a few more days.
Repeat as necessary. This is DIY for a lot of people, but if you don't feel
comfortable getting into it, call a pro.

IMPORTANT NOTE: The most important dampers are those supplying the room
with the thermostat (or the room nearest the thermostat if yours is in the hall); if
that room’s receiving too much air, the thermostat will turn off the cooling before
the other rooms come down to temperature.

IMPORTANT NOTE: You may find some of your summer settings have to be
changed in the winter, and vice versa. Deal with it.

You may find it necessary to fully close dampers supplying small rooms; that’s
usually the case. You’ve probably seen closing a damper doesn’t totally stop the
airflow; they’re not tight closing. We've seen cases where undersized dampers
were provided (6” damper in a 7” duct – it’s all that was on the truck that day), or
where dampers were literally folded in half before installing (you have to wonder if
alcohol and/or drugs were involved there). If you find an outlet that’s still blasting
with the damper closed, first double check that you’ve correctly identified which
branch duct supplies that outlet; then it may be necessary to disassemble the
duct to see what’s going on.

If you find it necessary to fully close a lot of dampers to cool that hard-to-cool
room, then you need duct improvements and it may be time to call a contractor
(depending on how much DIY you want to attempt): The duct(s) supplying the
room is (are) undersized and/or restricted; you need to increase the size,
eliminate the restriction or add a duct.

That’s it. You should be able to have all rooms within a degree or two of each
other; that includes two story homes supplied by a single air system. Fred
corrected horrible imbalance in his two story condo, but it took some doing.

Here's the scary part: Every time you throttle a damper, you're reducing system
airflow a little (by making the air travel farther down the duct system and
adding resistance to flow in the process); you need to be aware of that fact,
and you need to check that airflow is still within acceptable limits once you're

IMPORTANT NOTE: Balancing a system reduces total airflow; it’s important to
maintain adequate airflow through the equipment, as explained in “BASICS” on
home page.

IMPORTANT NOTE: Do not fully close all the dampers (in fact, do not close a high
percentage of them); to do so is to increase the risk of compressor and/or heat
exchanger failure.

IMPORTANT NOTE: Recheck your TD’s and compare with the ones you recorded
after reading “BASICS” once balancing is complete; call a professional if the TD’s
have changed more than 10%.
We've all seen
registers" before;
this type is not
permitted as a
balancing device,
because it relies on
a "single leaf
damper" (wherein
the blades all move
in the same
Most registers used
in homes are
"single leaf" types,
meaning most
registers in homes
can't be used for

Floor Diffusers
Registers are
not "vents"
: That
term is most often
used to describe
what conducts
products to the

So in the unlikely
event you have
"VENTS" in your
rooms, leave the
doors and
windows open
and hope for the

From the Top

Damper arms are
sometimes bent
90 degrees by
installers, to
facilitate locating
them; those arms
must be bent back
to make adjustments.

Open damper (arm
parallel with flow).

Closed damper (arm
perpendicular to flow.
Note the gap
between the blade
and the duct -
dampers don't close

Drilling a 3/16" hole
where it can't hurt
anything inside, dial
type pocket
thermometer in
supply and return.

The "TD" is a very
important number.

A satisfied DIY
client just told us

"Volume Dampers
Work, Registers

(For Balancing)

These are "Floor
". While
they do have
opposed blade
dampers, they are
not permitted as
balancing devices
because a balancing
damper ( Technical
term is
Volume Damper")
can easily be
provided in the
branch duct
supplying the floor
contractors always
provide those as a
standard part of
their installations