Ductwork & Venting Components Glossary

Ductwork & Venting Components Glossary

By: First Supply

A functional HVAC system includes a furnace, thermostat, refrigerant lines, an evaporator coil, automated zone dampers, actuators that control the dampers and a condensing unit. The system also needs a functional network of ductwork. Built with sheet metal duct sections, ductwork connects the air handler to air diffusers and vents throughout a building. In this article, we give a short description of many of the key parts and air flow fittings used in a ductwork system.

Air Diffuser

This is a distribution point where conditioned air is mixed with room air. Typically, covered with some type of grille and located in the floor or ceiling.
See Air Diffusers

Air Handler Unit (AHU)

An air handler unit attaches to ductwork. This large metal box contains a blower, heating and cooling elements, filters, and dampers. AHU’s come in both indoor and outdoor designs.
See Air Handler Unit

Angle Boot

The most common style of boot in a traditional HVAC system with layouts that include a trunk duct running perpendicular to the floor joists. This allows the trunk to be easily supported and allows for takeoffs from the top of the trunk duct.
See Angle Boot

Appliance Adapter

Connects an appliance to a venting system.
See Appliance Adapter

Automatic Dampers

A physical plate that controls airflow through a duct or air diffuser.
See Automatic Damper


Delivers conditioned air into living space or commercial structure.
See Boot

Butterfly Damper

Restricts the flow of materials in ducting systems where flow control and low leakage isolation are a priority. They are especially useful in applications such as incinerators, balancing systems, control air systems, stack isolation, and scrubber systems.
See Butterfly Damper

By-Pass Damper

A pressure relief valve placed between the supply and return ducts of the forced air duct work. As Zone dampers start closing, a bypass damper will open and divert some of the supply air to the return. This prevents a pressure buildup in the supply duct which can cause fan cavitation, excessive air velocities, and excessive zone damper blow-by.
See By-Pass Damper

Ceiling Diffuser

This air flow device is installed in the ceiling to discharge air in a specific pattern, path, or direction.
See Ceiling Diffuser


Join two vents or pipe sections. A coupling is most often used to join similar-sized sections, as opposed to adapters, reducers, and increasers, which are used to connect different-sized duct sections.
See Coupling

Damper Motor

An electro-mechanical device that opens and closes a damper to prevent heat from escaping.
See Damper Motor


A physical plate that controls airflow through a duct or air diffuser. Dampers simply regulate air flow. There are dampers each with specific functions. Volume control dampers allow the adjustment of the amount of air flow, while smoke or fire dampers seal off a duct when they detect smoke.
See Damper

Draft Hood

Attached between a flue outlet and vent pipe to draw air through the chimney as necessary to create a constant flow. A draft hood is critical to the operation of a gas water heater or furnace.
See Draft Hood

Drain Pan

A pan located below the evaporator coil that catches the excess moisture that drops from conditioned air. Also known as, a Condensate Pan.
See Drain Pan

Dryer Box

Protects a dryer exhaust hose and allows a dryer to be pushed flush with a wall without restricting airflow through the hose. Provides a cleaner finish and protects against fire spread.
See Dryer Box

Dryer Hood

Fits snugly on a back draft damper to prevent cold air, rain, or snow from entering a structure. May include a removable pest screen that allows removal of debris and lint.
See Dryer Hood


The main channels that connect to the air handler and provide a conduit for warmed or cooled air to be blown throughout a structure. The duct trunk is the large main trunk, while the duct pipes connect the trunk to various rooms.
See Ducts

Duct Fittings

There are various names for duct fittings, the most common being reducers. Duct fitting reducers allow a change from one size duct to another. These fittings also balance airflow and equalize pressure. A vent cap fitting helps the performance by protecting the open end of a ductwork vent stack. Includes ells, tees, and reducers, are responsible for equalizing the duct pressure and balancing the airflow. For example, a reducer is a fitting that’s used when a change from one size duct to another size is needed.
See Duct Fittings

Duct Liners

Provide insulation to reduce noise and reduce energy use by preventing heat/cooling loss.
See Duct Liners

Duct Strap

Used to hang flexible and sheet metal duct. Duct strap is sold in rolls and can be cut to size with sheet metal snips.
See Duct Strap

Duct Vent Saddle

Used to branch a round duct run off another duct run at a 45° angle for efficiency and improved air flow.
See Duct Vent Saddle


Allow tight corners in a duct system to maintain the best airflow for maximum efficiency in an HVAC system. Elbows are made of 26-gauge galvanized sheet metal up to 12” or 24-gauge galvanized sheet metal from 14” to 24”.
See Elbows

Flex Ducts

Spiral wire-reinforced inner air conduit is wrapped with resilient fibrous glass and covered with flexible vapor retarder of reinforced foil or plastic film.
See Flex Duct

Flexible Blanket

Designed to wrap around and insulate exterior of commercial and residential ducts and plenums. Used in temperatures between 40 deg F and 250 deg F.
See Flexible Blanket


Gasses are vented outdoors through a flue. Generally found in HVAC systems where noxious fumes are created through the heating or cooling process.
See Flue

Gear Clamp

Secures a flex hose onto a pipe and various ducting components together. Also known as a worm drive clamp or a worm gear clamp.
See Gear Clamp

Insulated Pipe Sleeves

Pipe sleeves are constructed with several layers to reduce heat and condensation build-up. Sleeves will have durable outer layer made of PVC or aluminum foil that protects the inner insulating layers.
See Insulated Pipe Sleeves


There are many types of outlets including diffusers, grilles, and registers. They all divide airflow in a ductwork design.
See Outlets


The main component of any HVAC ductwork system is an air distribution box, called a plenum. This device distributes the air collection and flow of an HVAC system. The supply plenum sends air from the centralized unit to the rooms with air vents while the return plenum carries air from your return intakes back to a central air handler.
See Plenum


Usually three or four supply branches are connected to main trunk before a reducer changes duct size. These can be rectangular, square, or round.
See Reducer

Refrigerant Lines

Refrigerant lines deliver refrigerant to compressors in an HVAC system. This refrigerant allows your HVAC system to cool and heat air in a home.
See Refrigerant Lines

Register Boots

The last stop before heated or cooled air enters the conditioned area of a building, duct boots are responsible for delivering air from the supply runs into a room. Without a duct boot heated or cooled air would have no way of reaching its destination. Fastened to other ducts, including ceiling and wall units. Airtight gaskets prevent leakage. Available in Straight Register, End Register and 90 degree register styles.
See Register Boots

Register Boxes

Also known as floor boxes, register boxes have a galvanized steel grille with moving parts that can be opened and closed to direct airflow. The placement and size of registers is critical to HVAC efficiency.
See Register Box

Return Air Boxes

A plenum for attaching a return duct to a return grille in ceiling or floor.
See Return Air Box

Return Air Filter Grill

Controls the direction, diffusion, and intensity of returned air. Located in the walls or floors, depending on the HVAC system they help keep pet hair, dust, and debris out of the HVAC system.
See Return Filter

Roof Flashings

Direct water away from pipes vented through the roof of a building.
See Roof Flashing

Round Dampers

Regulate the volume of air moving through duct systems. They can be used as positive shutoffs and can also be used for automatic control in low-leakage HVAC systems, or systems requiring modulation airflow. Control dampers are typically installed inside ductwork of low to medium airflow velocity and static pressure capabilities.
See Round Damper

Stack Head

Vertical branch ducts that allow vertical airflow in walls. Used for supply air, return air, and exhaust air.
See Stack Head

Start Collar

Connects a flat surface of ductwork to a round duct pipe. These include tabs on one end that help hold the collar in place when bent in an alternating (in/out) pattern.
See Start Collar


Ties a flexible into a main duct system. These may include a damper which can be set to increase or decrease the airflow.
See Takeoff

Tee Cap

Terminates one end of a tee duct attached to a flexible line.
See Tee Cap

Tee Duct

Connects a flexible line to the back of a stove or can used to pass through a chimney thimble.
See Tee Duct

Vanes & Rails

Made for HVAC duct systems that require efficient airflow and quiet operation. Airfoil design also reduces generated sound power and relative pressure loss to their lowest attainable levels.
See Vanes & Rails

Vent Caps

Provides protection for the open end of a ductwork vent stack. Each fitting and configuration helps the overall commercial ductwork performance.
See Vent Caps

Vent Pipe

Replaces stale indoor air with fresh outdoor air to remove dangerous combustion gases and allow a furnace to operate safely.
See Vent Pipes


Provides a way to branch off an existing ductwork trunk line.
See Wye

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