Flex And Round Duct Sizing Charts CFM (4,6,8,10,12-Inch CFM Duct Charts) (2024)

FAQs

How many cfm does a 4 inch flex duct have? ›

Reviewing the chart, a 4” pipe will convey 395 CFM at 4500 FPM. This means that a 4” pick-up on a machine will use 395 CFM from the filtering system; or, if you know that a machine will require approximately 400 CFM to remove the waste, you should use a 4” pipe for that purpose.

Generally, a 12' x 12' room requires a 4” x 8” duct. The main factor in determining this size is airflow volume, measured in cubic feet per minute, or CFM. To adjust the CFM, you can change the speed of the blower or alter the ductwork size.

Since the size of your ductwork can increase or lower this measurement, you'll have to find the necessary CFM for each room before you can get the right duct size for each space. Cubic Feet Per Minute = (HVAC Unit Tons x 400) / total square footage of home. Calculate for each individual room.

This reversible 14-inch Flex-Wave fan moves 1,900 cfm of airflow. More than having just a different look, the Flex-Wave electric-fan blade has several aerodynamic advantages for air movement: The leading edge of the fan blade is no longer a single, solid line.

Sometimes the best route for a flexible duct run is up and over an obstruction. But without proper support, ducts can develops sags or kinks that reduce airflow. To prevent problems, use saddles or straps to support at regular intervals.

However, the IAPMO passed a ruling stating that flexible piping should be no longer than five feet for residential properties. After review, it was extended to six feet, but it could still cause issues with a project if you can't use flexible ducts.

The air now has to re-pressurize to regain its turbulent flow so it can continue to push its way down the duct. It requires about 24′ or two feet to do this. This is the 'Two-Foot Rule?. Ideally, you should allow at least 24′ between takeoffs and at the end of any duct run.

The 6" pipe will need 885 CFM at the same velocity (See chart).

One of the most used methods is the HVAC duct sizing rule of thumb. In fact, even professional AC contractors might use this method to size the AC ducts in your home. According to this rule, you need to install an air conditioner of one-tonne capacity for every 500 square feet of the floor area of your residence.

Excessive sagging is a bad practice for flex duct installation. By code, ducts should not have more than a half-inch of sag for every foot between supports. Sometimes, ducts can't just travel along an attic floor. Some flex ducts need proper supports and strapping, especially in commercial spaces.

How to select a flexible duct? ›

One simply needs a ruler or measuring tape along with the end of the hose or outlet. Keep in mind that you have to begin the measurements from the interior wall of the circle. The distance to the opposing point of the circle is the measure of the flex duct sizing for the inner diameter.

If your hood is between 601 and 900 CFM, you need a minimum duct size of 7”. If your hood is between 901 and 1200 CFM, then you should go for a minimum duct size of 8”, and if you have a 1200 CFM hood, consider going for a minimum duct size of 10”.

Flex duct is typically available in insulation values of R-4, R-6, and R-8. For residential HVAC systems, insulated flex duct typically comes in diameters of 4 inches through 10 inches; above 10 inches, it comes in even sizes of 12, 14, 16, etc., up to 22 inches measured at the radius of the metal helix.

The most typical register size is the 4x10. To give an idea of CFM's, if you are feeling a strong blast of air out of that size of register then you're feeling about 160 CFM.

In this instance, the 800 cfm divided by 4,000 ft/min calls for a duct area of 0.2 sq feet.

The most typical register size is the 4x10. To give an idea of CFM's, if you are feeling a strong blast of air out of that size of register then you're feeling about 160 CFM.

If your hood is between 601 and 900 CFM, you need a minimum duct size of 7”. If your hood is between 901 and 1200 CFM, then you should go for a minimum duct size of 8”, and if you have a 1200 CFM hood, consider going for a minimum duct size of 10”.

You would need a single 20 X 20 filter grille for a two-ton unit rated to move 800 cfm (400 sq. in. x 2 cfm = 800 cfm).