Duct Dynasty: 6-inch Duct Don't Always Deliver 100 cfm


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Duct Dynasty: 6-inch Duct Don't Always Deliver 100 cfm

6-inch Ducts Don't Always Deliver 100 cfm

by David Richardson


If you want to get a brawl started among HVAC contractors, start a discussion on duct sizing. This debate rivals those of Chevy® versus Ford®, and Coke® versus Pepsi® in the contentions it arouses. There is a passion among those in the HVAC industry for the methods they use for sizing ducts. These methods are typically traditions passed down over decades. If you question the duct sizing method, you question something that dad or even granddad taught them as the truth. You might as well spit in their face in many cases. After all, a 6” duct always delivers 100 CFM, right?
The unintended result of this tradition is HVAC systems that fail to perform at their anticipated levels. With this being such a large problem, how do you help someone understand that the duct design methods they have used for years might not be working like they assume? How can you help them understand a 6” might not always deliver 100 CFM? I had to deal with something similar to this problem years ago in regards to self-balancing duct systems.
The .10 Design Friction Rate
The .10 design friction rate is the most commonly passed down tradition of sizing ducts and the reason many assume a 6” duct delivers 100 CFM. The reason it is such a widely used assumption is because if you look at many slide duct calculators they have a recommended residential setting on them of .10” of water column. A lot of confusion sets in here as many believe this is the design pressure they have to use.
The recommended residential setting on a slide duct calculator is for 100 equivalent feet of duct. This is the amount of straight duct that the duct fitting is equivalent to. 100 equivalent feet of duct can easily be used up in one broad way 90 degree elbow with a square throat, round heel and no turning vanes. Think about that, a single duct elbow with a square throat, round heel and no turning vanes, would be the same as 100 feet of straight duct. Depending on the complexity of the duct system, you could easily end up with over 500 feet of equivalent duct before you know it. This adds up quickly and will often result in an undersized duct system that can’t handle the required airflow.
Often, you will end up with those who err on the side of caution and they go even lower than .10 as design criteria. The passion is the same among these individuals, but the design numbers used are all over the place. I’ve heard a variety of values used ranging from .08 on the supply and .06 on the return, to .10 on the supply and .05 on the return. Which design rate is the correct one and who is correct?
Measure to Remove Doubt
Sadly, the truth about how any of these duct system designs perform is completely unknown in most cases. The majority of contractors don’t measure the delivered airflow from their installed systems except with the palm of a hand. With this being common occurrence, is it any wonder there is so much division about which duct sizing method you should be using? How can you take a position in a duct sizing debate when you have no data or results to back up your position? Hint, you can’t. You don’t know how airflow a 6” duct is delivering unless you test it on start-up. The palm of your hand just isn’t going to get it done here.
If you want to prove that your method of duct design truly works, the way to do it is fairly simple, start measuring the final results of your installations. The test instruments and access to the skills required to measure delivered airflow have never been more readily available. Why would you procrastinate on doing this? Aren’t you the least bit curious to know if your duct sizing rules really work? Don’t you want to know how much airflow is really being delivered from that 6” duct?
Aren't you just the least bit curious how much airflow a 6 inch duct really puts out?
Some will see the opportunity in this, unfortunately many won’t. Until the measurement of delivered airflow begins, the debate over the invisible will continue. When the final installation is measured and verified, then you can remove all doubt about your sizing methods.
Balancing Hoods and Skeletons in Closets
For those that decide to measure their installations, there will be some skeletons that start falling out of the closet. These skeletons will make you ask some mean, nasty questions of yourself that are going to hurt really bad. This is a good thing, as it’s the beginning of improving your product that you provide to our customers.
There are installation practices that you are currently using that will need to be refined and improved. You’ll have to start looking at the capacity of the fans in the equipment you install. The impacts of the filters and coils you use are going to scare you to death. The past systems you have installed are going to be questioned and you’ll have a decision to make.
The good news is by measuring delivered airflow, you now have one more arrow in your quiver to solve these issues. Many won’t even know these issues exist as they don’t measure. If you have been running up on cracked heat exchangers, or bad compressors, you may need to start measuring airflow more than you suspect. Who knows, you might just see how much airflow a 6 inch duct really does deliver while you’re at it.
About the Author
David Richardson is a curriculum developer and instructor at the National Comfort Institiute (NCI). 
This article is used with the author's permission.
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