When homeowners buy a new air conditioner, they usually obsess over the SEER efficiency rating and the brand name on the box. However, the most expensive AC unit in the world will perform terribly if it is connected to undersized ductwork.
Think of your ductwork like the plumbing in your house. If you connect a massive, high-pressure water pump to a tiny garden hose, the pump is going to overwork and eventually burn out. The exact same principle applies to your HVAC system. Below, we explain how to calculate the airflow your system needs and provide standard duct sizing charts used by professionals.
What is CFM?
In the HVAC industry, airflow is measured in CFM, which stands for Cubic Feet per Minute. It is a measurement of the volume of air flowing through your ducts.
To determine how much CFM your home requires, you must first know the size of your air conditioner. AC systems are sized by "Tonnage" (which is a measurement of BTUs, not physical weight). The golden rule of HVAC design is that a standard system requires approximately 400 CFM of airflow per 1 Ton of cooling capacity.
- 2.0 Ton AC = 800 CFM required
- 3.0 Ton AC = 1,200 CFM required
- 4.0 Ton AC = 1,600 CFM required
- 5.0 Ton AC = 2,000 CFM required
If you install a 3-ton AC unit, the supply plenum and the main duct trunks must be physically large enough to handle 1,200 CFM of air at an acceptable static pressure (usually 0.1 friction rate).
Round Duct Sizing Chart
Round ducts are typically made of galvanized sheet metal or flexible insulated tubing ("flex duct"). They are most commonly used for the branch lines that run from the main trunk to the individual room vents. (Note: The CFM values below are based on standard residential friction rates of 0.1 in. w.c.)
| Round Duct Diameter | Max CFM (Metal) | Max CFM (Flex Duct) |
|---|---|---|
| 4 inches | 40 CFM | 30 CFM |
| 6 inches | 100 CFM | 75 CFM |
| 8 inches | 200 CFM | 160 CFM |
| 10 inches | 400 CFM | 300 CFM |
| 12 inches | 600 CFM | 480 CFM |
| 14 inches | 900 CFM | 700 CFM |
| 16 inches | 1,300 CFM | 1,000 CFM |
Metal vs. Flex Duct: Notice that flex duct carries less air than smooth metal pipe of the exact same diameter. Because the inside of a flex duct is ribbed and corrugated, it creates more friction, which slows the air down.
Rectangular Duct Sizing Chart
Rectangular ducts are typically used for the main "trunk" lines that run directly out of the air handler or furnace across the length of the attic or basement. They can carry massive volumes of air.
| Rectangular Dimensions | Equivalent Round Size | Max CFM |
|---|---|---|
| 8" x 8" | 8.7 inches | 250 CFM |
| 10" x 8" | 9.8 inches | 400 CFM |
| 12" x 8" | 10.7 inches | 480 CFM |
| 14" x 8" | 11.5 inches | 600 CFM |
| 16" x 8" | 12.5 inches | 700 CFM |
| 20" x 8" | 14.0 inches | 900 CFM |
| 24" x 8" | 15.4 inches | 1,200 CFM |
Signs Your Ducts are Undersized
A very common mistake is upgrading the size of the air conditioner (e.g., going from a 3-ton to a 4-ton) without upgrading the ductwork to match. If your ducts are too small for your equipment, you will experience several severe issues:
Loud Whistling Noises
If your system is trying to pull 1,600 CFM through a return duct designed for 1,000 CFM, the air velocity increases drastically. This creates a loud "whooshing" or whistling noise at the return grilles.
Uneven Temperatures
High static pressure from undersized ducts prevents air from reaching the furthest rooms in the house. This results in severe hot and cold spots, particularly in upstairs bedrooms.
Frozen Evaporator Coils
If the ductwork restricts airflow so much that not enough warm home air blows over the indoor AC coil, the coil will drop below freezing and turn into a solid block of ice.
Premature Motor Burnout
Blower motors are designed to push against a specific amount of resistance. Forcing the motor to fight against undersized, restrictive ductwork will cause it to overheat and fail years before it should.
