Tag: CFM

This article is written by one of the smartest guys I know online, Neil Comparetto. Neil is a little nervous about writing a tech tip so make sure to give him lots of positive affirmation on this one. 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

As a service technician, we are often expected to understand a bit about design to fully diagnose a problem. Duct velocity has many ramifications in a system including

  • High air velocity at supply registers and return grilles resulting in air noise
  • Low velocity in certain ducts resulting in unnecessary gains and losses
  • Low velocity at supply registers resulting in poor “throw” and therefore room temperature control
  • High air velocity inside fan coils and over cased coils resulting in higher bypass factor and lower latent heat removal
  • High TESP (Total External Static Pressure) due to high duct velocity

Duct FPM can be measured using a pitot tube and a sensitive manometer, in duct vane anemometers like the Testo 416  or a hot wire anemometer like the Testo 425. Measuring grille/register face velocity is much easier and can be done with any quality vane anemometer, with my favorite being the Testo 417 large vane anemometer

First, you must realize that residential, commercial and industrial spaces tend to run very different design duct velocities. If you have ever sat in a theater, mall or auditorium and been hit in the face with an airstream from a vent 20 feet away you have experienced HIGH designed velocity. When spaces are large, high face velocities are required to throw across greater distances and circulate the air properly.

In residential applications, you will want to see 700 to 900 FPM velocity in duct trunks and 600 to 700 FPM in branch ducts to maintain a good balance of low static pressure and good flow, preventing unneeded duct gains and losses.

Return grilles themselves should be sized as large as possible to reduce face velocity to 500 FPM or lower. This helps greatly reduce total system static pressure as well as return grille noise.

Supply grilles and diffusers should be sized for the appropriate CFM and throw based on the manufacturer’s grille specs like the ones from Hart & Cooley shown above. Keep in mind that the higher the FPM the further the air will throw but also the noisier the grille will be.

— Bryan


When tightening down a blower wheel or a fan blade on a motor shaft ONLY tighten it on the flat of the shaft.

If you have more than one screw, but only one flat surface on the shaft then only tighten the one set screw.


Refrain from overtightening set screws, they need to bite into the shaft but you don’t need to mangle the poor thing. Both setting on the curve and overtightening of these conditions can make it hard or impossible to remove the blade or wheel later. Simple, but important.

— Bryan

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