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What do drafty vents, moldy walls, and windmilling exhaust or inlet fans all have in common? If you answered, “faulty ventilation”, you already know more about residential ventilation than I did when I first got started. But what most folks don’t know is that one of the most common bugbears in home ventilation — and among the simplest to fix — is the backdraft damper.
Whether you’re wondering what a backdraft damper is, or already grappling with your home’s ducting to get a damper working properly, you’ve come to the right place.
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. (Hint: effective backdraft dampers can save you $$$, and do a whole lot more).
Let’s dive in!
What Is a Damper and Do You Need One?
Dampers are devices for controlling the direction, volume, and flow of air through a building’s ducting, intakes, or exhaust vents. Commonly made of steel, aluminum, fiberglass, or plastic, dampers typically contain one or more manually or automatically adjustable blades set in a frame.
Dampers are used to control:
- inward flow at intakes
- outward flow through exhausts
- 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 building interior and exterior
A backdraft damper is a type of damper intended specifically to ensure one-way airflow through a duct or vent while preventing air from flowing in the reverse direction.
A home may 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.
In the following sections, we’ll discuss the different types of dampers, and how to choose the right damper for your home.
Damper Construction and Types
To understand the different types of ventilation dampers, we first need a quick overview of their construction. Dampers typically consist of a frame and one or more adjustable blades. There are a few ‘bladeless’ dampers, but we will discuss these later.
Damper frames can be rectangular, square, oval, or round, to suit the different shapes of ducts and openings. Rectangular and square frames have at least four elements: the top and bottom frame members, and the side members, called jambs. Round or cylindrical frames are called sleeves.
The frame houses one or more damper blades, where each blade can change its angle and position to control the flow of air through the damper. When fully open, blades allow the maximum amount of air to flow through the damper. Fully closed, they’re meant to form a tight seal with the neighboring blade or with the frame, restricting the airflow.
Each blade changes its angle by moving around the axle on which it is mounted. ‘Blade action’ — the manner in which blades move — varies across models, with different actions suited to different applications.
Dampers are classified into categories based on application, actuator mechanism, and shape.
These categories are not necessarily mutually exclusive, as explained below.
Dampers Based on Application
- Backdraft dampers: Intended to channel unidirectional airflow through ducts and vents, preventing air from flowing in the opposite direction. Most often used for bathroom fans and range hoods.
- Balancing dampers: For levelling out imbalances in air pressure between connected rooms in buildings with centralized ventilation.
- Control dampers: For actively regulating airflow through ducts and vents in ventilation systems. These dampers are often driven by manual, pneumatic, or electric mechanisms.
- Safety dampers: Specially designed to shut off in the event of fire and smoke. These are safety devices that need to be certified by safety certification bodies such as UL.
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.
Dampers Based on Actuator Mechanism
These must be opened and closed manually, via a control lever or chain. 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.
These 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 the damper. These are most popular as control dampers in HVAC systems but are sometimes used as backdraft dampers too.
Owing to their simplicity, both, gravity dampers and spring dampers, are often integrated into wall and roof vent caps. Wall shutters are generally spring or gravity-type dampers.
Dampers can also be driven pneumatically, but these are usually only used with large, centralized HVAC systems. Pneumatic actuators are becoming less popular due to their need for frequent maintenance and part replacement.
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:
1. Function: In addition to the primary function of preventing backdraft, 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.
Air pressure for dampers is usually expressed in ‘inches of water gauge’ (iwg). It can be measured with a manometer, taking readings at the point in the ductwork where the damper is to be installed. Maximum pressure should be measured while all upstream appliances connected to that duct, like exhaust fans or range hoods, are running. (Amazon has a great selection of affordable digital pressure gauges to do this.)
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 is calculated by the formula,
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)
= 160 / (3.14 x (0.33)2)
= 160 / 0.342
= 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: Air flowing through ducting will experience a drop in static pressure when it encounters any resistance, such as with a damper or other object. 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.
6. Leakage: Dampers aren’t usually airtight, even when their blades are fully closed. Air that seeps through a closed damper is its ‘leakage’. Large amounts of leakage may cause upstream fans to have to work harder, and also lead to temperature loss to 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.
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.
7. Material: 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 the air in your ducting lose less temperature to the outside environment.
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-return 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 (eg: 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 outperform 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 Placement and Maintenance
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 actuators, 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.
- Actuators should power on and drive the damper correctly. Keep in mind that pneumatic actuators 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.
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 actuate backdraft dampers, unlike motorized dampers. However, motorized dampers can function as backdraft dampers, such as on exhaust fans with motorized dampers.
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