Damper Showdown - Round Dampers vs. Transition Collars

Which works better for a spiral duct: a round damper or a square damper with a transition collar? Learn more about these options on the MCDLG Newsstand.

Spiral Ducts in HVAC

Spiral ducts are a versatile option for ductwork. Many HVAC systems use spiral ductwork to pass air through various parts of a building. Spiral ducts can bend to meet ductwork in any direction without a secondary angle. Exposed spiral ducts can even add a rustic flair to an interior space.

Whichever way you use spiral ducts, you will need control dampers that match their circular shape. You have two primary options to achieve this: use a rectangular control damper with a transitional sleeve or use a round damper. Each option has advantages and disadvantages. Which option would work best for your system?

Control damper with transition and round damper

From left to right: damper with transition collar and a round damper.

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Transition Collars

Attach transition collars to most rectangular and square dampers to facilitate mounting in non-rectangular ducts. Install collars on the damper's face and rear to allow connection to differently shaped ducts. Manufacturers will use fasteners to fix the collar to the damper’s frame, then caulk the perimeter to seal the seam between frame and collar.

Transition collars can be circles, ovals, and even squares, to fit a duct’s specific shape. You will need to specify the dimensions of the collar’s opening when ordering, including the size of the collar itself and the size of the transition opening. The transition collar should fit around the damper face and connect to the duct. The purpose of the transition collar is to make sure the damper fits the duct.

Transition collar installed on a control damper

Transition collars fit over the face of the damper.

Round Dampers

Round dampers are cylindrical. Their shapes, fully circular or ovular, allow them to fit any spiral duct. Manufacturers design round dampers to control airflow, balance air pressure, manage one-way backdraft flow, or control fire and smoke. These dampers typically have a single blade set in the middle of the cylindrical body. This blade is a round disc that fully covers the damper opening when the unit closes.

While open, the blade sits in the middle of the air stream. This single, flat blade profile provides more free area for air to pass through, which also minimizes pressure drop. Install round dampers in ducts or on faces. The difference between the damper’s inner and outer dimensions will be minor, but you should still use the correct dimensions when ordering.

MCDLG Blog 2022 - 5 Specialty - round

Round dampers will typically have a single damper blade that covers the opening when closed.

Which is better?

Each option has its advantages. With transition collars, you can install any damper into a spiral duct, if the collar fits. This can be helpful when choosing the right dampers. However, remember that the collar encompasses the damper’s frame. That means that the frame will be in the airway once installed, which will further reduce air performance.

Round dampers will typically have a single blade which makes a flat line when fully open. The round frame is also thin, making it unobtrusive when installed in duct. Round dampers are an excellent choice for spiral ducts, but they do not come with very many options. The maximum size of the damper will limit your options. For larger ducts, you will need to choose a different round damper or a damper with a transition collar. The size and single-blade design of round dampers cause externally mounted actuators.

While transition collars provide more options, they will also require a little extra maintenance. You will need to check the fasteners on the collar to ensure they are still tight. Caulk will deteriorate over time, so you will also need to apply new caulk to the seam. The collar will leak if not properly fastened to the damper. If the collar comes loose, it may obstruct the airway and damage your HVAC system. Round dampers have a continuous frame.

Each option has its advantages and disadvantages. Transition collars provide the most options for installation while round dampers provide the best air performance for the application. You may find a square damper that works well for the application. Or you may find a round damper that fits perfectly with your duct. Consider what you need the damper to do when making your selection.

Spiral ductwork in a factory setting