Tag: duct

This tech tip is written by one of the best techs I know. Neil Comparetto.

I think that we all can agree that duct leakage is not ideal. Our job is to condition the space. If we can’t control the air, that becomes difficult. On top of that anytime you are losing already paid for conditioned air. But really, how bad could it be?

I’m in Richmond Virginia, so we’ll use that as our example location. According to ACCA Manual J summer design conditions our outdoor design temperature is 92° Fahrenheit, with a moisture content of 106 grains per pound. (grains is a measurement of absolute moisture). Let’s use the indoor conditions 75° F and 50% relative humidity, which converts to 65 grains of moisture.

Our example system will be a 3 ton air conditioner moving 1200 CFM with ducts in a vented attic. For this exercise we won’t get into duct sensible heat gain that even a 100% tight duct system will have to overcome.

This system will have a modest 10% supply duct leakage into the attic (Energy Star estimates that the typical duct system has 20-30% duct leakage). Assume 0% return leakage (which is unlikely). So we already know that 10% of our capacity is gone, never to return again into the attic. On a 3 ton air conditioner that will be roughly 3,600 btuh. We are now delivering 1080 CFM of supply air to the living space, and returning 1200 CFM. Where does the additional 120 CFM of return air come from? You guessed it, outside. The supply duct leakage into the attic, outside of our thermal and pressure boundary, has now brought the living space into a negative pressure. No big deal, it’s only 120 CFM… but have you ever done the math!?

Stick with me, it’s not as bad as it looks. Here are the formulas for the sensible and latent heat required to bring the infiltration air back to indoor conditions (75°/ 50%RH).

Sensible BTUH = 1.08 x CFM x (Outdoor temp – indoor temp) Latent BTUH = 0.68 x CFM x (Outdoor grains – Indoor grains)

Let’s use 92° F as our outdoor air temperature number. In all likelihood, considering that the attic floor/ceiling plane is one of the leakiest parts of the house, and the attic is typically > 120° F, that in real life it will be higher than whatever outdoor temperature is.

Our example will look like this:

1.08 x 120 CFM x (92°-75°) = 2,203 btuh of sensible heat

.68 x 120 CFM x (106 grains – 65 grains) = 3,346 btuh of latent heat

2,203 + 3,346= 5,549 btuh of total heat.

That is an additional 5,549 btuh of total heat. The 3,346 btuh of latent heat is the more difficult number to deal with. Next time you are bored flip through your favorite air conditioner’s product data and see what it can produce, you may be surprised. Don’t forget about the 3,600 btuh that’s up in the attic somewhere. And just think, this is from only 10% supply duct leakage, considerably more is very possible.

As you can imagine in the heating season this problem doesn’t go away. Typically outside air is much drier than indoor air, and duct leakage will dry out the indoor space. If the heating system is a heat pump the capacity loss is corrected by electric strip heat, which is bad. That means when you seal the ducts auxiliary heat is reduced, which is good.

Leaky ducts can contribute to many more issues than just energy loss and comfort. Did you know that a one square inch hole in the duct system is equal to thirty inch hole in the building envelope? The potential to create pressure imbalances in the building is tremendous. Pressure imbalances can cause many issues, like flues backdrafting, excess dust and allergens, uneven temperatures, and moisture issues to name a few.

Something as simple as sealing ducts can solve many issues, hopefully you include it in your scope of work.

— Neil

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

In this video we cover the basics of using the Testo 510i with a pitot tube to do a duct traverse and easily calculate Velocity in FPM and volume in CFM on a small 8″ duct. Using this method is handy because you can use the reliable, accurate and inexpensive 510i to perform the measurement without any other equipment other than tubes and a pitot tube.

As stated in the video, a pitot tube is best (most accurately) used in the following conditions –

  • Medium to High Air Velocities
  • With 4 -8 feet of hose
  • In low turbulence air at least 8.5 diameters downstream of any turns, fittings or diffusers (I was less than this in the video resulting in lower accuracy)
  • In a duct at least 30 times larger than the pitot tube diameter (It was less than this in the video resulting in lower accuracy)


For more information see the following links –

Dwyer Guidelines

TruTech Tools Traverse Quick Chart

TruTech Measuring with a pitot tube

Testo 510i specs

Video on the performance of a rectangular time average traverse

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