# Duct CFM Chart | Round & Flex Duct | Calculation Guide

Many people want to have a quick check on the duct size for a given CFM without going through complex calculations. Hence, I thought providing some duct CFM charts would be helpful.

Generally, the size of a duct is determined by the CFM and the velocity limits for the supply and return sides of the duct system. However, the duct must not result in a total friction loss greater than the available static pressure of the blower.

The ANSI/ACCA Manual D is the Air Conditioning Contractors of America procedure for sizing residential duct systems. Let’s quickly go through the result and I’ll explain the calculation process later.

## Duct CFM Charts

Round metal ducts and round flexible ducts are the two most common types of ducts used in American houses. The following provides the appropriate duct sizes based on different airflow rates and applications.

### Duct Size for 50-800 CFM

From 50 CFM and 200 CFM to 600 CFM and 800 CFM, these are the most common CFMs found in typical houses with a central HVAC system. The central HVAC system typically uses round metal ducts as the main duct and round flexible ducts as the branch duct for air distribution.

The below ducts are sized based on the friction loss chart provided in the ACCA Manual D with a friction loss of not more than 0.1 in.WG and a velocity not exceeding 900 FPM (guided for supply air ducts):

### Duct Size for 850-1800 CFM

High airflow rates such as 1200 CFM and 1600 CFM are common for 3-ton to 4-ton furnaces. These CFMs require larger duct sizes in order to reduce the air velocity for noise control.

The below ducts are sized based on the friction loss chart provided in the ACCA Manual D with a friction loss of not more than 0.1 in.WG and a velocity not exceeding 900 FPM (guided for supply air ducts):

### Duct Size for 2000-4500 CFM

Airflow rates up to 2400 CFM are still common in residential applications. The below ducts are sized based on the friction loss chart provided in the ACCA Manual D with a friction loss of not more than 0.1 in.WG and a velocity not exceeding 900 FPM (guided for supply air ducts):

### Range Hood Duct Size

Kitchen range hoods typically don’t come with a duct. However, most range hood manufacturers provide guidelines on the suitable duct size. The type of duct used for range hoods is typically round rigid metal ducts. For range hoods, the duct can have a maximum air velocity of 1200 FPM.

The below duct sizes are based on the given CFM and are subject to a maximum duct length of 30 ft, and a friction loss of not more than 0.1 in.WG and a velocity not exceeding 1200 FPM:

### Exhaust Fan Duct Size

It is also common to have exhaust fans in houses. It could be a bathroom exhaust fan. Typically, round metal ducts or round flexible ducts are used to connect the exhaust fan. The duct velocity for exhaust fans can go up to 1200 FPM.

The below ducts are sized based on the friction loss chart provided in the ACCA Manual D with a friction loss of not more than 0.1 in.WG and a velocity not exceeding 1200 FPM:

## How to Calculate Duct Size?

The ACCA Manual D has a comprehensive guideline on duct sizing. Usually, we design a duct system using the equal friction method, meaning we fix a friction loss value, size the duct and then select a suitable blower.

However, in most residential applications, the blower is selected first. Hence, the duct can be sized based on the external static pressure minus the pressure drop of components such as the cooling coil, air filter and humidifier.

As long as the ducts are sized such that the total friction loss in the duct system is about the same as the blower at a given CFM based on the data provided by the manufacturer, then the blower is able to deliver the required airflow.

But, it is not that simple for non-technical people to calculate the total friction loss in a duct system. Nonetheless, if you want to install a small section of the duct, the duct size friction loss chart can be used to determine the suitable size.

The above friction loss chart for round metal ducts is extracted from the ACCA Manual D. Generally, most ducts are sized based on a friction loss of 0.1 in.WG. Then, as per the ACCA Manual D recommendation for noise control, the maximum velocity of supply and return air ducts should not exceed 900 FPM and 700 FPM respectively.

So, to use the friction loss chart to size your ducts, fix the friction loss at 0.1 in.WG (red) and the velocity at 900 FPM (pink) first as shown in the above chart. Then, pull the airflow rate to find a suitable duct size that stays within the design criteria.

For example, if you want to size a duct for 200 CFM, pull the airflow line (green) to match the smallest duct size line (blue). By using an 8″ duct for 200 CFM, the resulting friction loss and velocity are about 0.07 in.WG and 580 FPM respectively which are well within our design criteria.

Let’s say to save cost, you choose to use a 6″ duct for 200 CFM. Hence, the resulting friction loss and velocity are 0.3 in.WG and 1000 FPM respectively which are beyond our design criteria.

However, it doesn’t mean that a 6″ duct can’t be used for 200 CFM. It just means that the friction loss and velocity will be higher. From 900 FPM to 1000 FPM, the velocity increase is not much. Hence, the noise produced by the airflow in the duct might not be significant.

Subsequently, the friction loss is 3 times when a 6″ duct is used instead of an 8″ duct for 200 CFM. The resulting friction loss with a 6″ duct is 0.3 in.WG. It means that for every 100 ft, the friction loss is 0.3 in.WG. If your duct length is not 100 ft, the friction loss is reduced proportionally.

For instance, if the longest duct (from the blower to the farthest vent) is 30 ft, a 6″ duct carrying 200 CFM will result in a friction loss of 30 ÷ 100 x 0.3 = 0.09 in.WG.

The friction loss due to the duct alone is one of the elements in determining the total friction loss in the duct system. Other elements that contribute to friction loss are the vent, duct elbow, duct collar, damper and etc.

For example, if the 30 ft duct has one vent only and the friction loss of the vent is assumed to be 0.1 in.WG (provided by the vent manufacturer), the total friction loss of the duct system is 0.09 + 0.1 = 0.19 in.WG.

Assume that you have a blower with a performance of 200 CFM at 0.2 in.WG, you can be sure that with a total friction loss of 0.19 in.WG present in the duct system, your blower is able to deliver the 200 CFM of airflow. Similarly, if the total friction loss in the duct system is more than the static pressure of the blower, the blower will deliver less airflow.

## Conclusion

Ducts can be sized before the blower selection or after the blower selection depending on local practices and applications. Nonetheless, the maximum allowable velocity in the duct is a critical factor to consider for noise control. As for the friction loss, it depends on the duct length and the friction loss of other components in the duct system.

If you need me to help with your HVAC system, consider my Email Consultation service.

This article was originally published on aircondlounge.com. Actions will be taken for unauthorised republication of this article.

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