How Ductwork Functions

How does new duct work distribute the air and heat throughout your home or business?


In order to understand how a heating and cooling system works, it’s easiest to think of it in terms of being a circulatory system, rather than a respiratory system. Air circulates from the HVAC unit, through the ductwork to the rooms, and then is returned to the HVAC unit to be heated or cooled again.

Delivery system

Ductwork, whether for a home or business, is comprised of steel, aluminum, polyurethane or fiberglass panels or sheets, or flexible coils which deliver air from the air handler to the registers.

Ductwork with damper valves.

Ductwork with damper valves.

Through the branches

The supply air, in the supply plenum, is pushed by blowers through a supply duct trunk, where it is connected to smaller branch ducts by fittings called takeoffs. If or when the air has to move in a vertical direction, stacks, which are wide, thin, vertical ductwork, can be required to move the air through walls. They are connected to the main horizontal ductwork with a stack boot. At the top, a stack head connects the stack to more horizontal ductwork which reaches to the end of the house.

Turning vanes

Turning vanes are used to guide air smoothly when it’s necessary to change the direction of the air flow (up to 90 degrees) unless flexible ductwork is used. If flexible ductwork is used, it’s imperative that the duct tubing isn’t damaged, crushed or bent at more than a 90-degree angle.

Register heads

Register heads connect the ductwork to wall-mounted (or sometimes floor-mounted) air registers. Each room in the house should have at least one air register, although they often have more than one in a bigger room. Volume control dampers at the register (or even within the ducts themselves) control the amount of air flow.

Air return register

An air return register sends the air through a return plenum, which takes the air from the return register back to the central air handler. In a residence, this is usually where the air filter is located and is known as an air filter return. In a commercial distribution, a diffuser or grille is used as a return register.

Balance of the system with the area

Duct design determines the flow of conditioned air or heat throughout the building. The size of the HVAC unit, ductwork and registers depend on the size of the area to be heated and cooled. If the HVAC unit is too small for the ductwork, it will blow without creating enough pressure to deliver air to each of the registers, If it’s too large, it’s cost prohibitive, since HVAC units vary greatly in price based on the type and the number of BTU’s they can deliver. So, it’s important that the system is designed for the space that it heats and cools.

Other factors which affect ductwork installation include the type of HVAC, the layout of the house, the number of registers, and the number of temperature-controlled zones. A badly designed or improperly installed ductwork system can decrease the efficiency of an HVAC unit be as much as 40%.

If you have Phoenix heating and cooling needs, want to know more about ductwork or are having problems with your HVAC system, give us a call! 911 Heating, Cooling and Plumbing can service all commercial and residential HVAC and plumbing needs. Call 602-795-2900 today!