How to Install Balancing Dampers

Knoji reviews products and up-and-coming brands we think you'll love. In certain cases, we may receive a commission from brands mentioned in our guides. Learn more.
How to install a balancing damper in your ductwork to control airflow to hot or cold rooms.

If you have ever had the experience where one room in your house is either too hot or too cold, you may need to adjust your balancing dampers in the branch ductwork in your home’s HVAC system. You may find that you don’t have any balancing dampers installed, which is all too common.

Many contractors will simply use the floor or wall registers to adjust the amount of supply air entering a room. This is not a good practice for several reasons;

1) the dampers on floor and wall registers can completely seal off any airflow which can lead to the branch duct being pressurized and splitting that will lead to energy being wasted,

2) the dampers on floor and wall registers can be very noisy which can be annoying since they are in the room they serve,

3) the dampers can be easily moved by children or when cleaning and change the airflow rate.

Balancing dampers are designed to have a gap around the edges to prevent pressure building up in the ductwork.


In forced air system the cooling load drives the size of the ductwork since the temperature difference is lower. If the outside temperature is 95 DF and the supply air temperature is 55 F, the temperature difference is 40 degrees F. In the winter, the outside temperature may be 25 F and the supply air temperature can be 110 F for a temperature difference of 85 F!

You can see that you would need less than half the amount of air to heat a space as you would to cool it, but since the difference between the indoor and outdoor temperature is also greater in the winter, you need more warm air because the house loses heat faster. It also depends on the type of construction, insulation, and number or windows, but still, the cooling load is greater than the heating load, unless you live in Northern climates.

CFM per Room – Estimates

Bedroom – 70 CFM – 5” round duct

Large Bedroom – 125 CFM – 7” round duct

Living Room - 150 CFM – 8” round duct

Family Room – 180 CFM – 8” round duct

To check the size of your ductwork, you can use this online calculator to see what size ductwork should be for various airflow rates.

Many times contractors will use the same size flexible duct for all their branch runs, usually 6 or 7 inches in diameter. This can cause some zones to be overcooled which is another reason why balancing dampers should be installed.

Installing a Balancing Damper

Balancing or volume dampers can be purchased at some home centers and also online. If you have rectangular sheet metal ductwork for your branch lines, you may need to go to an HVAC supply house or order them online.

Balancing dampers for round ducts are simple sheet metal disks that can be turned with a small handle and locked in place with a wing nut. They can be hard to find on flexible duct work since the insulation jacket is often left long and pulled over the handle. The damper should be located near the takeoff of the main duct trunk. You can buy balancing dampers inside a metal sleeve that can be connected to the branch line and then to the flex run.

With fiber board or sheet metal ductwork, there should be a round takeoff coming off of the main trunk. Flexible ductwork is then attached to the end of a rigid sheet metal duct. To install a balancing damper, cut the metal or plastic duct band that holds the flex duct to the branch line. You can attach a balancing damper and collar directly to the end of the metal duct and then reattach the flex duct to the other end of the collar. Use self-tapping sheet metal screws to secure the balancing collar to the metal duct and then a new plastic band for securing the flex. The plastic duct bands look like large wire ties.

Balancing Damper shown with Collar and Handle (Ruskin, Inc.)

For installing a new collar into fiber board ductwork, you will need to install a fiber board duct collar and then screw the balancing damper into that.

Rectangular Balancing Damper

The balancing damper should be at least 3 duct diameters away from the connection to the main trunk. For example: if the branch duct is 7 inches in diameter, the damper should be no closer than 21 inches from the takeoff connection on the main trunk. This reduces noise and provides better control of the airflow.

Fiberboard Takeoff Connection shown with Damper

If you are working with sheet metal ducts and are unable to remove a section of ductwork you can cut a slit in the side of the branch line with aviation shears and then drill a hole in the other side of the duct for the pin to set in place. Then drop the damper in place into the hole and secure with a washer and nut. For the side with the handle, cut a piece of sheet metal to cover the slit, drill a hole for the other pin and secure the cover with sheet metal screws. Attach the handle with the included wing nut.

Damper Hardware (Ventfabrics, Inc.)

Unless you have special measuring equipment you won’t know exactly how much air is flowing through the duct. The airflow is not linear, so closing the damper halfway will not cut the airflow in half. It will be a process of trial and air, sorry error, until you get the desired effect. The damper should always be open at least a third of the way.

Remember that without accurately measuring the airflow in your system’s ductwork you will be subjectively controlling the airflow in a room. You may have to readjust the position in the winter and summer, so it is a good idea to make a mark on the collar to so the correct position.

Additional Resources:

Ventfabrics, Inc. (Ventlok)

Ruskin, Inc.

1 comment

Pamella Taylor
Posted on Jan 8, 2013