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Matt, Where Does the .1 Friction Rate Commonly Used to Size Ducts Come From?

In this edition of "HVAC Skill Builder," Matt discusses where the commonly used .1 friction rate for duct sizing comes from.

Q: Where Does the .1 Friction Rate Commonly Used to Size Ducts Come From? A: Well, the 0.1 unless people are doing the proper calculations it is in our manual D book and it's on a chart that actually falls in the center of a chart when you're doing your friction rate formula so a lot of people have just taken that .1 and has blanketed across every type of duct system that they use for sizing ducts and one thing that can get you in trouble with that is a lot of people are not measuring the TEL which is the total effective length and they're using this 0.1. So they have a duct system that runs really long, it's crushed, it's an accordion shape or anything like that. What happens is they use that 0.1 and then if for account for the other stuff in there that could affect that duct performance and it usually leads to a lot of bad things happening. So the best thing to do is do the friction rate formula and that way you can get your proper design friction rate when designing ducts and do not use a 0.1 for every single duct system because they're all going to be unique, they're all going to render different results but when you're sizing stuff, you want to make sure you have all the calculations to ensure that you put in the duct system correctly and it's going to perform to yours and the customers' expectations.

Matt Akins is the Manager of HVACR Technical Education at ACCA.

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Posted In: HVAC Skill Builder, Technical Tips

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