Drafty vents, moldy walls, and windmilling exhaust or inlet fans all have a common cause—faulty ventilation. But we can narrow this down even further and say that one of the first things you should check is the backdraft damper.
This guide will bring you up to speed with everything you need to know about backdraft dampers—what they are, how they work, how to choose and use them, where to place them, and most importantly, why you should care.
Backdraft dampers are one of many kinds of dampers that can be installed in HVAC and exhaust systems. They ensure the unidirectional flow of air. They are required by code in some systems and highly recommended in all systems.
What Is a Damper?
Dampers are devices for controlling the direction, volume, and/or flow of air through a building’s ducting, intakes, and exhaust vents.
Looking at this in a little more detail, we can say that dampers are used to control:
- The inward flow of air at intakes, which is important for maintaining air pressure balances and indoor air quality among other things.
- The outward flow of air through exhausts, which is important to ensure that damaging or harmful air pollutants and moisture are directed to the outside where they cannot cause such damage and harm.
- Air temperature and humidity by managing how stale or conditioned air mixes with external air.
- Differences in air pressure across various rooms, ducting zones, or between the interior and exterior of a building.
Dampers are composed of a frame in which is set one or more manually or automatically adjustable blades (or flaps).
In the passive position, these blades cover the opening of the frame. Fully closed, they’re meant to form a tight seal with the neighboring blade or with the frame, restricting the airflow.
The blades have the ability to be opened (by different mechanisms and forces). When fully open, blades allow the maximum amount of air to flow through the damper.
There are also a few bladeless dampers options. We will cover how they function in a later section.
Damper Construction and Types
A home may benefit from or even officially require one or more dampers if it has any type of ventilation devices, including passive vents, range hoods, intake or exhaust fans, and HVAC systems.
Dampers are commonly made of steel, aluminum, fiberglass, or plastic, and the frames can be rectangular, square, oval, or round to suit the different shapes of ducts and openings. Round or cylindrical frames are also called sleeves.
The blades open and close on a one-way hinge and can be operated in a variety of ways, representing one way that dampers can be categorized. They can also be categorized according to their application.
Let’s start with the applications.
Damper Types Based on Application
- Backdraft dampers: These are intended to ensure unidirectional airflow through ducts and vents, while preventing air from flowing in the opposite direction. Most often used for bathroom fans and range hoods.
- Balancing dampers: These are for levelling out imbalances in air pressure between connected rooms in buildings with centralized ventilation. Balancing dampers prevent air pressure changes in one room or space from significantly affecting those in the adjoining room or space.
- Control dampers: These actively regulate airflow in ventilation systems. Control dampers are similar to backdraft dampers in that they can open and close. However, the blades are not designed to open when air is forced against them from any direction. They have to be opened and closed.
Note that control dampers are also sometimes used for the purpose of preventing backdraft, such as with motorized dampers that are wired to work in sync with bathroom exhaust fans.
- Safety dampers: These are specialized dampers designed to shut in the event of fire and smoke. These are safety devices that need to be certified by safety certification bodies such as UL.
Dampers Based on Operating Mechanism
Manual dampers have a lever or chain that must be manually operated by the user in order to open and close the blades.
They are commonly used as balancing dampers as those need to be adjusted very rarely.
Manual dampers are not suited for use as backdraft dampers, which require frequent changes in blade position. Think about having to turn on the fan and open the damper every time you shower—we all struggle to remember to turn the fan on as it is!
Gravity dampers rely on gravity and air pressure to open or close. Often used as backdraft dampers, they open when air flows in one direction, and close when there is no air flow or when the flow direction is reversed.
While cheap, they can be noisy when it’s windy outside.
Similar to gravity dampers, these generally rely on air pressure to open the damper but use spring tension rather than gravity to drive the closing action. Butterfly dampers often have springs and are frequently used as backdraft dampers.
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A solenoid or electric motor controls the opening/closing of electronic dampers. These are most popular as control dampers in HVAC systems but are sometimes used as backdraft dampers too.
Caped Dampers or Bladeless Dampers
A relatively recent innovation is the caped damper, which consists of a windsock-type fabric housed within the sleeve or frame. The fabric sock replaces conventional damper blades, allowing airflow only in one direction.
When air attempts to flow in the opposite direction, the fabric folds over itself, forming a seal to prevent reverse flow.
How to Choose the Right Backdraft Damper for Your Home
Here are the seven key factors to consider while choosing a backdraft damper for your home (alternatively, you can jump to the comparison table).
In addition to the primary function of preventing backdrafting, are there any other functional requirements, such as temperature insulation, noise reduction, or insect deterrence?
2. Size and Shape
What are the dimensions and shape requirements for the damper? These depend mainly on the ductwork or opening where the damper needs to be installed.
3. Maximum Pressure
What is the maximum air pressure in the ductwork or at the opening where the damper will be installed? Every damper model is rated to a certain maximum pressure, beyond which it may not function properly and may get damaged.
4. Duct Velocity
Every damper is rated to a certain maximum airflow velocity, usually expressed in feet per minute (FPM).
Dampers may not function properly, or may get damaged, if the airflow velocity exceeds their maximum rated limit.
Duct velocity = 4005 x √dynamic pressure (See point #3 for how to calculate dynamic pressure.)
Alternatively, if your damper has just one blower upstream, simply find out the blower’s manufacturer-specified output, expressed in cubic feet per minute (CFM), and divide this by the duct’s cross-sectional area.
Duct velocity = Blower airflow / Duct cross-sectional area
For instance, if your range hood fan rated 160 CFM is ducting through an 8-inch circular tube, the typical velocity you can expect is:
Duct velocity = 160 / (πr2)*
Duct velocity = 160 / (3.14 x (0.33)2)
Duct velocity = 160 / 0.342
Duct velocity = 467.84 FPM
*πr2 is the formula for the cross-sectional area of the circular tube.
The 8-inch diameter of the tube is first converted to feet (0.66 feet) and then divided in half to get the tube’s radius of 0.33 feet.
5. Pressure Drops
Dampers decrease velocity pressure by disrupting laminar flow and increasing resistance. Any upstream blowers will have to work harder to overcome this resistance, so excessive pressure drops lead to higher energy consumption and may damage fans or shorten fan life.
Every damper causes some amount of pressure drop, however small.
If you have blowers that are running a lot of the time, such as those in HVAC systems, choosing a backdraft damper that causes the least pressure drop can result in significant energy savings in the long run.
Even what we would consider airtight dampers are not truly airtight and they experience leakage (air seeping though a closed damper).
The Air Movement and Control Association (AMCA) measures and certifies the leakage efficiency of dampers under various conditions.
Dampers with AMCA ratings are tested for leak performance to American National Standards Institute (ANSI) standards.
The Class 1A rating represents the best leak performance, with progressively more leakage through classes 1, 2 and 3.
Large amounts of leakage may cause upstream fans to have to work harder, and also lead to thermal exchange with the outside environment. Both these factors lead to poor energy efficiency.
Not only will the air leak waste energy, but it will also create an uncomfortable cold draft.
Dampers for residential applications are usually made of either metal or plastic. Plastic is lighter than metal, and also has the advantage of being a better thermal insulator, helping to prevent the energy-wasting thermal exchange discussed above.
Modern plastic dampers are also a lot more durable than their yesteryear counterparts, thanks to plastics with special weather-resistant additives.
That said, metal dampers are available in a far wider array of sizes and shapes, so it’s much easier to find one suited to your specific needs in metal rather than plastic.
Ventilation Damper Comparison Table
The table below offers a comparison of the performance of different types of dampers across all of these parameters.
|Parameter||Lowest performance||Highest performance|
|Backdraft prevention||Cape backdraft dampers generally don’t fare very well in conditions with light but steady reverse airflow, as the sock does not seal well in such conditions.||Electric and spring-operated backdraft dampers close with the highest torque, keeping out reverse flow most effectively.|
|Thermal insulation||Butterfly dampers made of aluminum or steel generally offer the lowest insulation as a result of high conductivity and lack of a tight seal.||Thanks to low conductivity, non-metal dampers offer superior insulation, provided that the blades seal correctly, which is rare on cheap models.|
|Noise reduction||Gravity dampers can be quite noisy, especially in metal, as outside turbulent air can cause the blades to flutter.||Cape dampers and electric dampers are often reported as making the least amount of noise.|
|Max pressure||Varies based on model||Varies based on model|
|Max velocity||Varies based on model||Varies based on model|
|Pressure drop||Backdraft dampers with slats (e.g., wall shutters) and hinged, two-blade butterfly dampers can have higher pressure drops than other dampers of comparable cross-sectional area.||Metal dampers with a smooth surface finish and minimal rivets in the airflow out-perform plastic dampers. Single-blade dampers often cause the least pressure drop.|
|Selection of sizes and shapes||Plastic and cape backdraft dampers have the least choice in dimensions and shapes.||Aluminum and steel backdraft dampers are available in an array of sizes and shapes, and can be fabricated to custom specifications.|
Backdraft Damper Orientation
Depending on the model, backdraft dampers need to be installed in either a horizontal or vertical orientation.
Some dampers can be installed in both vertical and horizontal planes, but such use must be explicitly approved by the manufacturer.
When installed for airflow in the horizontal plane, backdraft dampers with slats must always be positioned with the blades (slats) oriented horizontally.
In addition to damper orientation, the direction of airflow is also an essential consideration for dampers installed in vertical airflows. For example, most gravity dampers suited for vertical airflows will only work with airflow up, and will not function correctly with airflow down.
Before installation, always consult the manufacturer’s instructions for model-specific guidelines and cautions.
Dampers require regular maintenance and should be inspected at least every six months, as a rule of thumb.
Here are a few tips on what to watch out for during a damper maintenance inspection:
- Smooth and free movement of blades throughout the blade’s full range of movement. Verify with the manufacturer’s instructions whether any electric/pneumatic/spring operations, if present, can be safely disconnected to test the blades by hand.
- Chips, cracks, and signs of corrosion on the blades or frame. Check the frame and joints for grit, grime, or other debris that might impede movement.
- Operating mechanisms should power on and drive the damper correctly. Keep in mind that pneumatic mechanisms require periodic replacement.
- Check seals for cracks, position, and wear. If possible, test the tightness of seals with the damper fully closed.
- Lubricate moving parts as needed. Important: Only use lubricants approved by the damper manufacturer. Do not use unapproved or standard, oil-based lubricants, as these can damage seals, and attract dust and grit, causing parts to wear out or jam over time.
Do You Need One?
In some cases, such as bathroom exhaust fans, backdraft dampers are not required by the building codes. In other cases, such as for range hoods, the building codes specifically require the installation of the dampers.
However, any system that is designed to channel air in a specific direction and for which air traveling in the opposite direction would cause issues will benefit from the presence of a backdraft damper.
- stop warm or cold drafts from blowing through the ducts and into the house, negating all the hard work done by your HVAC system, increasing your electricity bill and carbon footprint, and making you uncomfortable.
- stop the loss of heated air during winter, which also wastes electricity.
- prevent kitchen fumes or bathroom steam from being blown back into the house causing all the problems that you were trying to prevent in the first place (e.g., mold, grease buildup).
- ensure the efficiency of air movement from one place to another because it does not have to fight against air moving in the opposite direction.
- prevent condensation from dripping from fans and vents.
- prevent bathroom odors and other smells, like cigarette smoke from coming into you house through the vents (more likely to be a problem in apartments).
Where do you put a backdraft damper?
Ducts or openings where conditioned, filtered, or clean air meets unconditioned, unfiltered, or dirty air are ideal locations for backdraft damper placement or installation. Typical examples are wall vents, roof vents, and exhaust ducts from bathrooms and range hoods.
Do I need more than one backdraft damper?
Some exhausts like range hoods and bathroom fans have inbuilt backdraft dampers. In such cases, an extra backdraft damper where the device’s ducts meet external air can improve backdraft prevention and fan life. Ensure the extra dampers do not cause excessive pressure drops, which can damage fans.
What is the difference between louvers, backdraft dampers, and motorized dampers?
Louvers keep dirt and debris out of ducts, while backdraft dampers ensure unidirectional airflow.
Springs, gravity, or airflow usually control backdraft dampers, unlike motorized dampers. However, motorized dampers can function as backdraft dampers, such as on exhaust fans with motorized dampers.