The second half of proper airflow in a residential HVAC system is the ductwork. If, in our previous article, we have a 3 ton AC system designed to deliver 1270 CFM at .2 IWC, the goal is to size the ducts for .2 IWC of air flow resistance. To do this you have to understand how to use a duct slide rule or ductulator and understand the difference between pressure drop and friction rate.

Pressure Drop –

Pressure drop in a duct system is the loss in pressure between any two points in a duct system, regardless of the distance between the two points.

Friction Rate –

Friction rate is the loss in pressure between two points in a duct system separated by a specific distance.

Ductulators and friction charts use 100 ft. for their reference. So, before calculating the friction rate for a duct system and sizing the ductwork, the system pressure drop must be converted to the equivalent of a hundred feet of duct. The formula for calculating the friction rate for a specific length of duct is:

FR = (PD x 100) / TEL
Where
FR – Friction Rate in IWC
PD – Pressure Drop
TEL – Total Effective Length

Here’s an example: The output of the blower at 0.20 IWC is 1000 CFM. Using a ductulator, what size should the duct would you use?

FR = (0.20 x 100) / 300
FR = 0.067 IWC

Now that you have calculated the friction rate, use your ductulator to calculate the size of the duct. Rotate or slide the the CFM dial and line it up with 0.067 IWC and the ductulator gives you the size for the ductwork… 15″. Fairly simple when you have a straight duct run, but ductwork systems are never straight. There are lengths of straight sections, fittings, and in the case of flexible ductwork, bends. So, the total pressure drop in a ductwork system is the sum of all of the lengths and fittings. Fittings, can add a pressure drop equivalent to a very long run of ductwork. If a fitting has the same pressure drop as a 40 ft. length of duct, the fitting has an equivalent length of 40 ft. ACCA Manual D Appendix 3 has nearly all of the fittings diagrammed with their equivalent length.

Examples:

• Square 90 elbow EL (equivalent length) = 80 ft.
• Flex duct 90 EL(equivalent length) = 10 ft.

Using the equivalent length of a fitting makes it easy. By adding the fitting length values to the straight run lengths you come up with the TEL or Total Effective Length. You then use the formula above to calculate the friction rate. Here is an example using the same blower as before: Remember, a square 90 elbow EL (equivalent length) = 80 ft.

TEL = 460 ft.

FR = (0.20 x 100) /460
FR = .044 IWC

###### Use your ductulator to figure out the duct size.

Did you come up with 16″? 