Tag: duct


This article is written by my good friend Neil Comparetto, one of the all-around best dudes in the industry and a guy who practices what he preaches on duct design. Thanks Neil!

These are some fundamentals for designing and installing duct systems that I’ve learned over the years. Included are links to some great resources if you choose to dig a little deeper.

#1. Lower the air velocity and static pressure will follow. 

The total equivalent lengths (TEL) for fittings in Manual D is based on 900 feet per minute air velocity. When the velocity is lowered, friction is lowered, hence TEL is lowered. This means you can “get away” with some lower-performing fittings if the velocity is low.

In ACCA’s Manual D appendix 15 in the conclusion it states “There are scores of things to worry about when designing and installing a comfort system. Low velocity through a duct system is not one of them.”

John Semmelhack, owner of the building science firm Think Little in Charlottesville, VA, proved this to me when I installed one of his designs. We conditioned a large area with a small low static ducted mini-split without any “high performance” fittings. Basically, he increased the duct sizes to above what’s “normal” to make sure velocity doesn’t go above a set value. (We’re not talking about increasing the duct sizes a lot, one or two sizes bigger than you’re used to.) The total external static pressure was .17” WC. When we commissioned the system every register delivered design airflow.

This article by Allison Bailes goes into way more detail on the topic. https:// www.energyvanguard.com/blog/is-low-velocity-bad-air-flow-ducts

#2. The largest “duct” in the house is the house itself. 

What connects the air coming out of the supply registers to the air going into the return grille? The house. It’s a big duct. Another reason why it’s difficult to provide comfort solutions without looking at the house as a system. In a battle between HVAC and leaky poorly insulated house, the house always wins. (And the people lose.)

This article by David Richardson is one of the reasons I started to take the “house is a system” approach. https://www.achrnews.com/articles/126814-the-building-side-of-the-ductsyste

#3. The closer the duct is to the blower the more important it is. 

This is where pressures in the duct system are the highest. Low-performance fittings create higher pressure drops (compared to lower pressure parts of the duct system), “system effect” can come into play, and any duct leakage will be intensified.

It’s important to have larger, high-performance fittings, with as much of a straight section as possible entering and leaving the equipment. I touch on this again in rule #13.

#4. Return location doesn’t matter as long as pressure imbalances aren’t created. 

“Add a return” is the go-to move for some to solve comfort issues. But if the equipment is moving the design airflow and the room does not have a pressure imbalance adding a return will not change anything. Supply registers condition air and create room air currents, not returns. What’s important is that return air has a free path back to the return grille, not the grille’s location.  

#5. Air is a fluid, ducts should not leak. 

Duct leaks in unconditioned spaces will cause outside air to enter into the conditioned space. This is an energy and IAQ penalty, I’ve heard Nate Adams call it the “double whammy”, basically you pay for it twice. (Once to condition the air, then again to re-condition the outside air.) More on duct leakage in this tech tip https://www.hvacrschool.com/duct-leakage-canbe-costly/)

Even if the ducts are in conditioned space they still shouldn’t leak. It is not efficient or high performance having an air leak in unintended spaces. You will not be able match room-by-room design airflow if x amount of air is leaking behind walls.

This study by Comfort Institute (owned by Aeroseal) was conducted to prove leaky ducts in conditioned space matter. https://comfortinstitute.org/blog/healthy-home/research-ductsealing-whole-house-leakage/

#6. Install balancing dampers to regulate airflow. 

Without balancing dampers it’s difficult to balance airflow… Some rely on design software to create a “self-balancing” system. Personally, I haven’t seen this work other than accidentally.

Our preference is to use registers with opposable blade dampers. This makes balancing a lot faster and easier. On top of that, they are always accessible, and they don’t leak air like typical takeoff manual dampers do.  

#7. Increase filter surface area to lower pressure drop. 

When designing to a static pressure of 0.50 inches water column (in.w.c.), a good rule of thumb is to keep the pressure drop across the air filter at or below 0.10in.w.c. This will not happen with a standard 1” filter if you plan on using anything other than a see-through “rock-catcher”. If you want a high-performance filter (you should) the 4” media type have low-pressure drops, but you might have to use two for anything above a 3 ton. Our preference is 2” filter grilles with MERV 13 filters. Easy to accommodate on new construction, and retrofits with multiple returns.

Keep in mind that the MERV rating of filters is typically at a low velocity, say 300 FPM. Performance of the filter drops when velocity is increased.

Allison Bailes (yes, him again) of Energy Vanguard recently blogged on this topic. https:// www.energyvanguard.com/blog/path-low-pressure-drop-across-high-merv-filter

#8. Avoid installing ducts in unconditioned spaces. 

Vented attics are the worst possible place for the ducts. It’s the hottest part of the house in the summer, and the coldest in the winter.  Ducts in vented attics come with a large energy penalty. Vented crawl spaces do not have as heavy of an energy penalty, but because of high humidity, present other challenges.

Supply duct leakage will cause outside air infiltration due to depressurizing the living space. Any return leakage will directly bring in attic or crawl space air.

Here’s a great article from one of the best building science resources. https:// www.buildingscience.com/documents/insights/bsi-074-duct-dynasty

#9. Select registers with enough throw to provide adequate air mixing. 

This helps with providing even room temperatures, prevents stratification, and reduces the number of registers needed per square foot.

We use a lot curved blade ceiling registers located close to an interior wall, pointed toward the exterior. In addition to good air mixing interior high wall or ceiling register placement also uses less materials when compared to an exterior location. Simple, small duct systems cost less, leak less, and have less thermal transfer.

This is one of my favorite free duct design resources. https://www.nrel.gov/docs/fy12osti/


#10. Test your duct system to verify design. 

Feedback may be the single most important thing when it comes to your process and improving it. Whether it’s from a duct leakage test (even when it’s not required), or measuring airflow, feedback accelerates the learning process and allows you to make adjustments to your design on the fly.  

#11. Flex duct is high performance if installed straight and tight. 

Don’t believe me? Try it. Because of rule #10, I know this to be true.

Yes, I probably could have linked Energy Vanguard to every rule. https:// www.energyvanguard.com/blog/57709/How-to-Install-Flex-Duct-Properly

#12. Don’t blow air on people. 

In my opinion this is why heat pumps get a bad rap. It’s not comfortable having 85° air blowing on you. At the same time that 85° air may be enough to maintain a 70° room temperature. Keep this in mind when selecting register type and location.  

#13. Use elbows with radius throats or turning vanes. 

In ACCA’s Manual D, Appendix 3, you will see how much of an impact radius throats and turning vanes have in comparison to square throats, and elbows without turning vanes. You will also see that radius heel vs. square heel makes little difference.

Piggybacking on rule #3, high-efficiency fittings have a greater impact the closer they are to the equipment. A lot of times when doing a retrofit, I will install turning vanes in the existing fittings that are close to the equipment. This is a good way to make measurable improvements without reinventing the wheel.

#14. Design and build quiet duct systems. 

Noise can come from several sources, vibration, turbulence, high velocity, and the equipment.

Depending on the type of equipment and its orientation, there are several ways to isolate it from the structure to reduce vibrations. I always use canvas connectors as close to the equipment as possible so ducts can be supported while being decoupled from the equipment. Vibration pads under the equipment help if it is installed on a platform or stand. Hanging equipment using spring or neoprene isolators provides better isolation, but is more time consuming and costly.

Air noise from high velocity and turbulence can be reduced by fittings with sweeping turns and lowering the velocity. In addition to that, silencers (such as Fantech’s LD line) are effective, as well as flexible duct. Some use an acoustical liner (which works) but we choose not to because of IAQ concerns. (Potential deterioration and releasing fiberglass into the airstream, also tends to promote mold growth on the supply side).

One technique for addressing equipment noise is reducing line-of-site. Most of the time this can be done with a few turns in the ducts between the equipment and any registers and grilles.

Neil Comparetto,
Co-owner of Comparetto Comfort Solutions in Virginia

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

This tech tip is written by one of the best all-around HVAC minds out there. 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

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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