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Makeup Air Damper | What Is It and Why Is It Used

Makeup air is important to replace air exhausted by appliances such as range hoods. A makeup unit is designed to ensure that this happens in the most effective and efficient manner. The makeup air damper plays an important role in achieving these goals.

Dampers, of which there are different types, can be thought of as one-way valves. They exist in exhaust systems themselves but are also required in the makeup air units that counter the action of exhaust fans. Without dampers, a number of issues would plague your makeup air system.

Makeup air dampers can be operated electrically or by air pressure and gravity. They are one-way valves that allow air to enter the house but not exit it. They also only open when the exhaust system is on.

What Is a Makeup Air Damper?

A damper is a type of valve that regulates the flow of air in a piece of equipment. In a makeup air unit, this damper controls airflow into and out of the system.

When open, the makeup air damper allows air to flow into the system. This air then enters the room or space requiring makeup air.

Illustration of round and rectangular makeup air damper

The valve only opens one way, so it cannot be inverted to allow air to flow out.

The only other position that the damper can have is closed, which halts the airflow completely.

Makeup airflow needs to be thus controlled because it’s only needed when the exhaust systems are operating, and it wouldn’t help the situation if air was allowed to escape in the opposite direction.

The damper itself may be in the shape of a cylinder with moving elements, such as blades or flaps that control airflow by changing position.  

Some dampers may come instead in rectangular shapes but with a similar design.

The different shapes are to ensure that you can find an appropriate makeup air damper if you have round or rectangular makeup air ducts.

How Do They Work?

Gravity Makeup Air Dampers

A gravity damper may also be known as a backdraft damper.

This type of damper is usually not motorized. Instead, as suggested by the name, airflow is regulated by gravity or the movement of air through the makeup air unit.

As fresh air is blown in from the outside through the makeup unit, the velocity of the air generates pressure that pushes the blades or flaps in the damper open.

These flaps are angled and attached in such a way that they open inward in the same direction as the air and when they are pushed open, they are opened against the force of gravity. They cannot open in the opposite direction because of the nature of the hinge used to affix the blades to the rest of the damper.

When the pressure eases, gravity causes the flap to close again.

Spring tensions or counterbalance weights may be used in some circumstances to adjust the pressure of airflow needed to push open the blades in gravity/backdraft dampers.

Similarly, if air starts to flow backward, the blades close, so that air cannot pass back into the makeup air unit to exit the house.  

Electrically Operated Damper

An electrically operated damper may otherwise be known as a motorized damper and it operates automatically.

Suncourt ZoneMaster Fully Adjustable Motorized Airflow Control Zone Damper Air Duct Fan (12 inch), Usually Open, 24 Volt Thermostat Compatibility, 24VAC/20VA Plug-in Transformer Included, Silver

These types of dampers operate on a spring that opens or closes the blade or flap when stimulated by an actuator.

The actuator in turn is stimulated by an electrical signal.

The damper may be signaled to close or open depending on air exhaustion from other appliances, or it may respond to signals from pressure switches.

For example, an electrically operated damper may be signaled to open when an exhaust appliance, such as a range hood, is turned on.

In this way, makeup air is provided as air is being exhausted from a room to prevent the negative effects caused by depressurization.

Some dampers are closed as the default position and are signaled to open, when necessary, while others are open as the default and close in response to a signal.

Since these dampers are not reliant on gravity, they can be installed vertically or horizontally.

Where Is the Damper Located?

A makeup air unit is usually comprised of multiple components.

These include a vent leading to the outside to provide fresh air, a bank of air filters, a fan or blower, a heating/cooling system to regulate the entering air’s condition, and of course, the damper.

Make-up air diagram inside the house, location of motorized damper for makeup air unit

The damper is typically located at or near the vent leading to the outside since the air has to be allowed in before it can be filtered or conditioned.

An insect screen may precede the damper at the opening of the vent or be located just next to the damper.

Consequences of Missing or Broken Dampers

Since a damper is responsible for allowing airflow only in one direction, broken or missing dampers result in issues with the effective and efficient provision of makeup air, which can present as a number of problems.

If a damper is broken, there might be unnecessary noise as air is allowed to move back and forth instead of only in one direction.

There may also be issues with the temperature regulation in your home.

Uncontrolled movement of air in and out of a house will lead to the loss of conditioned air or the inflow of unconditioned air. Both situation results in increased work on the part of your HVAC system, which is trying to maintain the temperature set on your thermostat.

The natural flow of air down the temperature gradient means that the air will flow in the direction you do not want it to go.

If it’s summer, hot air will not leave the house. It will come in because it’s typically cooler in a modern house than outside in summer. If it’s winter, then cold air will not leave the house, it will come in for similar reasons.

In cases where a damper is broken such that it remains closed and does not open to let fresh air in, there may not be enough air to replace air exhausted by appliances in the room.

This would result in reduced function of such appliances and in may also lead to the buildup of odors.

Without sufficient makeup air due to a damper that does not open, backdrafting may also occur.

Backdrafting occurs when air is pulled from drains and vents to fill negative pressure created when makeup air is not available to replace exhausted air.

This can also result in toxic gases such as carbon monoxide being pulled back into a room where combustion appliances are operating.

Hence, you can see why it is important to have a functional damper in a makeup air unit.


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