Tag: air flow

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

When you start talking airflow, it can get pretty in-depth pretty quick. There is a big gap between what is useful for the average tech to apply every day and the whole story so let’s start with the simplest part to understand, Static Pressure.

Static pressure is simply the force exerted in all directions within any contained substance, or in this case air. This means it’s not the directional force of air moving or blowing (that is called velocity pressure), it is simply to force pushing out on the positive side of the air system and pulling in on the negative side.

Measuring static pressure helps a tech know whether or not the system has excessive resistance to air flow overall or at a particular point.

Static pressure is measured in inches of water column (“WC) and is the amount of pressure needed to displace one inch of water in a water manometer.


A Magnehelic is a brand name for a high-quality Dwyer analog pressure gauge that comes in many different scales. Many techs will already have a high-quality digital differential manometer (like the Testo 510) for reading gas pressure, which makes getting a separate Magnehelic largely unnecessary.

When using a manometer or a Magnehelic, you will first zero it out to room pressure (for a Magnehelic make sure it is level). Next place the negative side probe in the return side of the unit after the filter but before the blower and place the positive probe in the supply duct. Keep the negative side probe away from the side of the blower and insert the probes in as straight and square as possible. It is advised to use a static pressure tip like the one shown below to prevent air velocity pressure or air currents from interfering with the static pressure reading.

With a static pressure tip point the tip against the direction of airflow (points opposite the airflow) in both the return and supply. DO NOT confuse a static pressure tip with a pitot tube tip. A pitot tube tip is designed to measure velocity pressure or total pressure (velocity + static = total)  NOT static pressure, and it will have an open end.

Total external static pressure is return plus supply, positive plus negative and in general, you would like to see it be 0.5″ or less…

If you see 0.9″ or higher that is when you start to see trouble on most residential systems, but as always, each piece of equipment is different depending mostly on motor design. Whenever possible design your equipment / duct system so the result is 0.4″ – 0.6″ of total static (Once again talking general residential / light commercial here).

If you do find it to be high, then read the return and supply separately to see which is higher which is just a matter of removing the hoses to your manometer or Magnehelic alternately. Whichever reads higher is the greater cause of the issue.

I could keep going on this, but instead, I will just link to some more in-depth articles if you want to do more reading.

— Bryan

Epic airflow write up from Dwyer 

Measuring Airflow from TruTech

Troubleshooting Ductwork by ACHR News



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