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So now we are finally ready to size the main plenum. We use the equal friction method to size the furnace. The equal friction method is the easiest and most accurate way to determine duct sizing for non-complex duct systems. To size a duct we use what we call an air duct calculator. You will need this for sizing your duct system.

At the top of the slide chart you will see Friction Per 100' of Duct. We will use .10 to size the supply plenum. In the slide area slide the 1200 cfm mark under the .10 friction per 100' of duct. Next read under EQUIVALENT RECTANGULAR DUCT. There you will see all the different sizes you can use for rectangular duct. For example in this situation you can select 7" x 30", 16" x 12", and 14" x 14". Try to size closest to the size you need for the clearance you have to work with in your application and relative to the opening in the discharge of the air handler or furnace. If we were to use round duct than you would look in the ROUND DUCT DIAMETER (IN.) area on the duct calculator and you would see that a 15" round duct would be the size to handle 1200 cfm.

If you have a large duct run then you would use the VELOCITY-REDUCTION METHOD. We are not addressing the velocity reduction method in this article. The Velocity Reduction method is used on large duct systems and applications that are more complex.  

After this we selected 12" X 16" as our supply plenum size. This is the clear inside dimension of the duct. Keep in mind that if your clearances are tight you will need to add an additional 2" to the size to allow for the thickness of the duct board and if you're using the 1-1/2" thick duct board which is a code requirement in certain areas such as Florida, then you would use 3".

I determine that if I have 16 supply diffusers and the system requires 1200 cfm then each diffuser must be capable of delivering 1200 cfm divided by 16 or 75 cfm. No! This is a very common mistake made in duct design.

I want to have more air than the minimum. Why? Because in the summer I'll need more air to the South of the house for air conditioning and in the winter I'll need more air toward the North part of the house for heating. Each Main Trunk must be capable of supplying the amount of 100 cfm per diffuser that the trunk is feeding. So we take Main Trunk "A" and see that it is supplying 8 diffusers or a total air volume of 800 cfm. So we look at the duct calculator and put 800 cfm at the .10 friction loss and the size I chose was 12" x 12". I only have enough clearance on the house for 15" height so I chose the 12 x 16 which when fabricated will be 14" x 14" because of allowing for the 1" thickness of the duct board. Then on Secondary Supply Trunk "A1" the system is supplying 5 diffusers at 100 cfm or a total of 500 cfm. Again I used the duct calculator to select 500 cfm at .10 static pressure loss for 500 cfm and I selected 8 x 12". On the Secondary Trunk "A2" we are supplying 3 diffusers at 100 cfm for a total of 300 cfm. Again referencing the duct calculator, selecting .10 friction loss, I chose 6 X 12" as the size. For Main Trunk "B" the system is supplying 8 diffusers at 100 cfm or 800 cfm total. Referencing the duct calculator at .10 friction loss and 800 cfm I chose 12 x 16" as the size. And this process continues until I've sized all the main trunks.

Each Take-off is the same 100 CFM and as a rule of thumb to keep things easier, I try to stick with 6" flex or 6" round for the take off size. So as you see by the drawing all the take offs are at 6' flex meaning 6" flexible duct and 100 CFM.

If you're with me so far you're doing great.

Now the next item is to size the return duct system. Here we will use .08 friction loss making the return duct larger than the supply duct. The reason is simple. It is much more difficult to pull air than it is to push air so the fan needs less resistance on the return air.

I've only used 4 return air ducts because space clearance won't allow me to run any more ducting. That means that each return diffuser will be returning 300 cfm each. 1200 total cfm divided by 4 diffusers gives 300 cfm per diffuser for the return air.

On the main return trunk I referenced the duct calculator at .08 friction loss and with 1200 cfm I chose 12" x 16". Then since I'm past 2 diffusers I only need to select a duct size for 2 remaining diffusers at 300 cfm each or a total of 600 cfm. So again using the duct calculator I reference .08 friction loss at 600 cfm and I chose 10" x 12" as my size. Then I have one more Return Trunk that is rated at one diffuser of 300 cfm. Again referencing the duct calculator at .08 friction loss I chose 6" x 12" as my size.

If you've followed me this far you should be capable of designing an HVAC duct system for your house.

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