What Are HVAC Dampers and How Do They Work?

As long as your HVAC system runs smoothly, you may not think too much about what a damper is or how it works. However, if you wonder why your house is so much warmer upstairs or why some rooms feel colder than others, this often overlooked part may be the reason. Let’s look at how HVAC dampers work, where to find them, and how they can make your home comfier.

What Is an HVAC Damper?

The purpose of HVAC dampers is to control airflow to specific parts of your home to regulate cooling in different zones and prevent your HVAC system from running constantly to maintain temperature.

You can open and close HVAC dampers to control the amount of airflow coming through the ducts. By choosing specific parts of your duct system to open or close at certain times, you can divide your home into zones and direct air to the rooms where you need it most.

Dampers help you avoid overheating or overcooling certain rooms, giving you more options to control your family’s comfort and optimize energy usage.

How Do HVAC Dampers Work?

HVAC dampers are essentially valves for airflow. When closed, they act like a river dam, blocking airflow in a home’s ductwork like a dam stops water from flowing in a river. A closed damper will shut airflow to a specific zone that doesn’t need conditioning. Meanwhile, open dampers allow air to flow freely through their connected ducts. Unlike vents, they’re closer to the air source—your air handler or blower motor—for more effective airflow control.

By closing some of your system’s dampers—either manually or via automation from a thermostat in an HVAC zone—you can conserve energy while directing airflow toward any ducts with open dampers. This determines the amount of cooled or heated air that flows to different parts of your home. If you want a particular room or set of rooms to be cooled or heated, dampers can help.

Since cold air naturally sinks and hot air rises, dampers can allow you to use physics to your advantage. During the winter, you can direct warm air to lower levels where it gets cooler. In the summer, you can direct cold air to upstairs areas where heat gathers.

How to Locate HVAC Dampers

If your home has HVAC dampers, you’ll most likely find them between the main trunk line (aka, the large middle or “body” part of your unit) and your supply ducts. The trunk line leads directly out of the furnace or air handler to deliver conditioned air to the entire home, while supply ducts are small, round branch lines that blow air into each room.

However, every home is different. Some homes may have dampers at other points closer to supply ducts, while others without dedicated heating and cooling zones may not have any dampers. You’ll often be able to spot dampers on exposed ductwork in your basement or crawlspace. Designs vary between manufacturers, but most have a handle on the outside of a duct that you can turn to open or close the damper.

If one of your dampers isn’t functioning, an HVAC damper replacement costs about $350. Call a local duct installer for an estimate.

Types of HVAC Dampers

HVAC dampers can either be automatic or manual. Automatic dampers use a motor to open and close valves automatically, so you don’t have to adjust them as the seasons change. These are usually controlled by individual thermostats in your HVAC zones. Manual dampers are managed via valves outside your air ducts that you can manually open and close.

Within these categories, you’ll find various damper types for different types of ductwork that come in numerous shapes and sizes. There are many types of HVAC dampers used in homes that differ in their design and operation, some of which can either be manual or automatic.

• Volume control dampers: These dampers are manual and let you control airflow at each HVAC supply. They consist of a lever that opens and closes the vent to restrict or promote airflow.

• Zone dampers: Zone dampers are always automatic and adjust based on thermostats in your zones. These appear in hidden ductwork most of the time, usually close to your air handler or furnace.

• Backdraft dampers: Backdraft dampers only allow for airflow in one direction, so they prevent air loss when your HVAC system isn’t active. These are popular in systems that have fresh air intake systems. A blower door test can often easily detect issues with backdraft dampers or the need to install one.

• Fire dampers: Fire dampers are more common in multi-family homes, but they can appear in larger single-family homes as well. These shut automatically in the case of a fire to slow the spread of smoke and flames.

• Opposed blade dampers: Opposed blade dampers have multiple blades that can move independently. These are usually automatic dampers that offer directional airflow control. They can also restrict airflow to a particular section of your home with the blades closed.

• Parallel blade dampers: These work similarly to opposed blade dampers, but the blades are parallel and offer less control over directional airflow.

• Butterfly flat dish dampers: As the name suggests, the hinges for these dampers look like little butterflies that sit on top of ducts. The dampers within consist of a round blade and a seal around the circumference that meets the blade’s edges, making them best for round ducts. When in line with the duct, the blade controls airflow in one direction. They’re very easy to install and take up virtually no space.

• Louver dampers: Louver dampers start automatically closing when there is higher pressure in an area of your HVAC system. They are powerful air blockers that can match any duct size, making them the most common type of damper in homes.

Adjusting Your HVAC Dampers

If your home has manual HVAC dampers, you'll need to adjust them yourself as the seasons change or as rooms get heated and cooled. You can control your HVAC dampers by opening or closing the damper flap or valve for each damper, and you can find these valves in the ductwork throughout your HVAC system or on your supply vents.

Start by turning off your AC and heater. If your ducts are accessible, you'll likely see a damper access panel on the outside. Each damper should be labeled to indicate which room in your HVAC zoning system it controls.

Loosen or tighten each damper lever to the desired position. In the summer, you'll want to close the AC dampers on your lower floors so cold air flows upstairs and flows down naturally. In the winter, you'll want to close upstairs dampers so warm air flows into lower levels and naturally rises. Depending on your needs, you can adjust dampers so they’re only halfway open.

If you have automatic dampers, you only need to set the thermostats in each zone to “heat” or “cool,” depending on the season, and then set your desired temperature for each zone.