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What Types of Dampers Are Used In HVAC Systems?

Most HVAC systems will use dampers in one form or another. Dampers control the direction and volume of air flow within a duct to maintain a balanced HVAC system. 

Dampers come in several configurations, operational designs, and functions to control air flow either manually or automatically. Dampers operate as a door that allows, redirects, diffuses, or stops airflow within a ducted HVAC system.

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How do dampers work and why do you need them in your HVAC system? What would happen if there were no dampers in your HVAC system?

Today, we will provide a brief description of the common damper types found in a typical HVAC system and how they help your system operate.

What Is The Purpose Of Dampers In an HVAC System?

Dampers allow an HVAC system designer to control airflow within a ducted HVAC system. Dampers operate similarly to a revolving door or a hinged door, depending on the damper. In most configurations, these dampers are adjustable to provide the installer with more precise control of the airflow.

Some dampers change the direction of airflow, while others simply reduce the volume of airflow by partially closing a duct. Other damper designs, such as a backdraft damper, do not change the direction, nor reduce the airflow, but allow it to flow in only one direction. Control and balancing dampers use rotating shutters to control air volume and speed within a climate zone.

What Types of Dampers Are Commonly Used In HVAC Systems?

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Most residential and light commercial HVAC systems will incorporate some combination of radial, rectangular, round-tube, collar tube, shutter, or blade style dampers to achieve a balanced system. Industrial applications may require custom designed dampers, but they generally perform the same function. 

All dampers do the same thing, which is to stop, limit, or redirect airflow. Most are made from galvanized steel, but other materials, like aluminum and stainless steel, are also used depending on the application. Each design offers specific benefits, like better sealing, one way operation, and precise airflow control.

Blade Style Dampers

Blade style dampers use either a single rotating blade, or several long blades that work together to control airflow. Most blade style dampers are of the round, square, or rectangular shape due to the shape of the ductwork. Sometimes referred to as a collar type damper, these dampers are usually located near a junction in the system to isolate a zone or area. 

Guillotine Style Dampers

Guillotine style dampers are often used when the system requires the best seal. Guillotine style dampers usually have a gasket that glides against the blades as it closes (like a guillotine) and creates an airtight seal. Guillotine dampers close very quickly and are useful in emergencies, like quickly preventing the spread of fire by denying it air.

Shutter Style Dampers

Backdraft dampers are a good example of a shutter style damper. In this design, shutters hang straight down via gravity, until a force (like air pushed by a fan), forces the shutters open. When the forced air is stopped, the shutters fall closed again. Because each shutter slightly overhangs the shutter below it, air will only flow in the direction of the force. 

Radial Style Dampers

Radial style dampers use more than one rotating blade to provide very precise air flow control. Radial style dampers open like a fanned out deck of playing cards, allowing the designer to precisely diffuse and control airflow. Radial style dampers have openings and notches in specific locations on the blades to maximize efficiency and effectiveness.

Rectangular Style Dampers

Rectangular style dampers usually use flat blades that pivot around a center hub, much like balancing dampers. In an HVAC system, you may find a rectangular damper in the form of a floor or wall register. Often, a slider is used to open or close the damper’s blade to control the airflow into a room.

Round Tube Style Dampers

Round tube dampers are usually longer than normal collar style dampers and are often a self-contained unit. Round tube style dampers will generally replace a short section of flexible ductwork with a short rigid section (less than 24”) and contain the damper and controls. Round tube style dampers are commonly inserted as needed after the system is operational.

How Are Dampers Operated?

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Dampers can be operated either manually or automatically, depending on the application and use of the system. For example, most residential systems will use manually operated blade style dampers. Blade style dampers are typically adjusted only once because the system requirements are not likely to change.

In most cases, the installer adjusts and sets the blade during the initial installation, although it can be changed in the future. 

Other HVAC systems require greater control and may include plenum spaces that require airflow restrictions. Often, these dampers are automatically adjusted based on temperature and volume presets and controlled by the system and a thermostat. Radial style dampers are common in this configuration due to their greater control capabilities.

Manually Operated HVAC Dampers

Most HVAC dampers will be the blade type, located inside a collar within the ductwork. The blade, which is slightly smaller than the diameter of the duct, spins like a revolving door. The locking adjustment handle extends outside the duct, allowing the installer to restrict or allow the airflow as needed. In most systems, if the handle is parallel to the duct, the damper is fully open.

Some dampers may need to be adjusted periodically, but not constantly. Often, these dampers are manually controlled via a cable system. Mechanical actuators that open and close the damper are controlled with these cable systems, which are very reliable and rarely need maintenance. 

Sometimes design requirements place ductwork in less than ideal locations, so that adjusting dampers requires moving or uninstalling other components. In these cases, mechanical actuators are extremely useful because they save the time and effort required to simply access a switch. These mechanical actuators are very reliable and rarely need maintenance.

Automatically Operated HVAC Dampers

As mentioned previously, some dampers are controlled automatically by electrical switches and actuators. Automatic dampers tend to be used in complex HVAC designs where greater control within separate zones is required for a balanced system. Automatic dampers are also common in locations that are difficult to manually access, like in a very low attic space.

Some newer HVAC damper designs incorporate wireless technology. As an answer to manual dampers that need periodic adjustment, these dampers are controlled remotely. Each damper is fitted with a wireless receiver and transmitter that receives commands from a central terminal. Wireless dampers can be retrofitted to replace manual dampers that are a hassle to access.

What Are Dampers Used For In an HVAC System?

Control Dampers

Control dampers are used to stop, allow, or mix airflow by adjusting the rotating blades within the damper. Control dampers are often controlled by a thermostat or remote control to stop airflow, fully allow airflow, or mix separate airflows in various ratios. In most configurations, a control damper is designed to open and close automatically to control the air volume and pressure.

Balancing Dampers

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Balancing dampers are similar to control dampers, in that they both open and close multiple blades to restrict airflow. Balancing dampers are usually used between two adjoining spaces where one is receiving more air pressure than the other. Balancing dampers are typically set by the installer and permanently locked into position, but they can be readjusted.

What Is the Difference Between a Balancing Damper and a Control Damper?

Control dampers and balancing dampers are easily confused for each other because both look similar and perform a similar function. Knowing the difference is important, because they are not necessarily interchangeable.

Balancing Dampers Have Parallel Blades

Balancing dampers are designed to impede airflow on one side of the damper and increase it on the other. Balancing dampers typically have three or four parallel blades that all move in the same direction at the same time. Most balancing damper blades rotate around a center pin and can be rotated almost 180 degrees in either direction.

Control Dampers Have Opposed Blades

Control dampers, however, use opposing blades that seal against each other, as opposed to parallel. Each blade rotates in the opposite direction of the blade next to it, which applies pressure against the seal from both directions. In contrast to balancing dampers, control dampers will either be fully open or fully closed, not partially open.

Backdraft Dampers

Backdraft dampers are common in a residential home, not just in the HVAC system. Backdraft dampers allow air travel in one direction only, so they are used in bathroom vent fans, range hoods, and dryer vents as well. Backdraft dampers allow exhaust air to exit the home, but outside air pressure prevents a backdraft damper from opening in reverse. 

Backdraft dampers employ shutters that use gravity to hang in the closed position until air pressure from the house side pushes them. Bathroom vent fans, range hood fans, and dryer exhaust fans force these shutters open, allowing the air to escape. When the air pressure is removed, gravity automatically closes the backdraft damper blades shut.

Industrial Dampers

Butterfly Damper
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Butterfly Damper

Industrial HVAC systems sometimes require elaborate airflow control because the structures are so large. For example, heated air traveling long distances through ductwork may cool off before it reaches its destination. Butterfly dampers are commonly used in industrial applications to provide more control over air leakage and flow to solve this problem.

Butterfly dampers tend to work best in round ducts, as they use a flat, circular metal blade. The blade spins around a hub, like hands on a watch, to adjust the airflow. The blades include notches which resemble a butterfly’s wings, hence the name. Butterfly dampers are commonly used not just for airflow control, but other gases as well.

Inlet Vane Damper

Inlet vane dampers are commonly used in industrial applications, usually near the air intake. In most applications, inlet vane dampers are used to precisely control airflow and air pressure from the inlet side of the blower fan. Inlet vane dampers are unique in that the angle of attack from the blades is adjustable.

Just like the blades on a helicopter, the blades on an inlet vane damper can be pivoted in place to allow more or less airflow. Inlet vane dampers are usually round and are available in both residential and industrial versions.

Multi-Zone Dampers

Multi-zone dampers are electrically controlled dampers that can be either programmed in a particular state, or they can be adjusted as needed by the system. Many HVAC systems are divided into control zones, which usually means the rooms that have dedicated ductwork. Each room will have its own ductwork, which means it can be considered a zone.

Similar to a security system, each HVAC zone will be controllable and programmable, providing more consistent air flow and pressure. Multi-zone HVAC dampers can be used in conjunction with other dampers, like backdraft and control dampers to establish the most efficient airflow path. Multi-zone dampers are often controlled continuously by the system and the thermostat. 

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