# Tag: duct design

## How to Determine the Friction Rate for Residential Duct Design

This article is written by one of the smartest guys I know online, Neil Comparetto. Thanks Neil!

Recently I posted a question in the HVAC School Group on Facebook, “when designing a residential duct system what friction rate do you use?”. As of writing this, only one answer was correct according to ACCA’s Manual D.

I feel there is some confusion on what friction rate is and what friction rate to use with a duct calculator. Hopefully, after reading this tech tip you will have a better understanding.

So, what is friction rate?

Friction rate (FR) is the pressure drop between two points in a duct system that are separated by a specific distance. Duct calculators use 100′ as a reference distance. So, if you were to set the friction rate at .1″ on your duct calculator for a specific CFM the duct calculator will give you choices on what size of duct to use. Expect a pressure drop of .1″ w.c. over 100′ of straight duct at that CFM and duct size / type.

Determining the Friction Rate

First, you need to know what the external static pressure (ESP) rating for the selected air handling equipment is. ( external static pressure means external to that piece of equipment. For an air handler, everything that came in the box is accounted for, including the coil and typically the throwaway filter. For a furnace the indoor coil is external and counts against the available static pressure)

Next you have to subtract the pressure losses (CPL) of the air-side components (coil, filter, supply and return registers/grilles, balancing dampers, etc.). Now you will have the remaining available static pressure (ASP). ASP = (ESP – CPL)

Now it’s time to calculate the total effective length (TEL) of the duct system. In the Manual D each type of duct fitting has been assigned an equivalent length value in feet. This is done with an equation converting pressure drop across the fitting to length in feet (there is a reference velocity and a reference friction rate in the equation). Add up both the supply and return duct system in feet. It is important to note that this is not a sum of the whole distribution system. The most restrictive run, from the air handling apparatus to the boot is used. Supply TEL + Return TEL = TEL

The formula for calculating the friction rate is FR= (ASP x 100) / TEL
This formula will give you the friction rate to size the ducts for this specific duct system. If you test static pressure undersized duct systems are very common, almost expected. This is because a “rule of thumb” was used when designing the ducts.

This is just an introduction to the duct design process. I encourage you to familiarize yourself with ACCA’s Manual D and go build a great system!

— Neil Comparetto

## Flex Ducts – Best Practices

Some techs and contractors swear that flex ducts are an evil invention and should never be used in ANY circumstance. I agree with what duct design expert Jack Rise said on the podcast when I asked him about flex ducts he said:

“There’s a lot of problems with flex duct, there really is and it’s a good product but we abuse it…. It’s a good product, it’s just poorly handled”

While the proper sealing of ductwork in unconditioned spaces is nearly universally recognized as important, it is rare that a flex system get’s installed properly in these other important areas.

Fully Extend The Flex

Some guidelines suggest pulling a 25′ piece of flex fully extended for 1 full minute before attempting to install it. This reduces the compression and the depth the of the corrugation (the accordion spiral inside the duct). The more compressed the duct is when it’s installed the greater the air resistance of the duct will be. The air duct council states that 30% of compression can result in 4 TIMES the air resistance. This means that fully extending the flex is a big deal and may be one of the most overlooked aspects of flex system installations. Cutting off that 2′ – 6′ of extra flex on the end instead of just “using the whole bag” can mean the difference between a good and a poor duct system in many cases.

Strap and Support the Flex

Jack Rise spoke about how he tested a duct and measured a .2″ wc change in static when he altered a duct from sagging to properly strapped. In retrofit applications, many companies focus on “sealing” connections but they often don’t truly address sagging ducts with proper strapping. the allowable amount of sag is only 1/2″ per 4′ of length which isn’t much. Don’t ONLY rely on the code required strapping in your jurisdiction, just because a system passes inspection doesn’t mean it’s installed correctly.

Keep the Curves to a Minimum

When designing a duct system you must calculate TEL (Total Effective Length) not just length. In a flex system each curve has a HUGE impact on the TEL and when a field install doesn’t match the design it can throw the whole system out of whack both from an air balance standpoint as well as a system performance by increasing the TESP (Total External Static Pressure). Every bend and angle matters so keep it extended, properly routed and well supported and all will be well so long as the design is correct.

For more info go to the ADC (American Duct Council) website at flexibleduct.org or download their excellent guide HERE

— Bryan

## Common Duct Design Mistakes w/ Jack Rise (Podcast)

In this episode Jack Rise talks about ACCA manual D, friction rate, face velocity, duct velocity and what is ACTUALLY wrong with flex duct.

If you have an iPhone subscribe to the podcast HERE and if you have an Android phone subscribe HERE

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