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A leaky attic duct system has the potential to lower cooling system performance and increase indoor relative humidity.

Two Unwelcome Results of Attic Supply Duct Leakage in Cooling Operation

Oct. 3, 2023
If you install and service HVAC systems where the ducts are in attics, it's important to understand how supply duct leakage affects your customer's comfort and HVAC system performance.

Not too long ago, someone asked me an interesting question. "Do supply leaks in attic duct systems pick up and transport hot attic air into the living area?" I must admit, I never looked at attic supply duct leakage in the cooling mode this way. However, it brought up some great discussion about what happens when it exists.

If you install and service HVAC systems where the ducts are in attics, it's important to understand how supply duct leakage affects your customer's comfort and HVAC system performance. Let's look at what happens when cool supply air leaks into a hot attic. It may lead you to solutions for some of your long-standing problems.

Loss of Conditioned Air

The first issue to consider is what happens to air lost into the attic through supply duct leaks. It helps to review two airflow principles that illustrate what happens as air circulates through a supply duct system. 

1.   Air takes the path of least resistance - it's easier for air to pass through a large duct leak (or multiple small leaks) near the air-handling equipment than to travel to a supply register.

2.    One cfm in = One cfm out – our industry measures airflow in cfm (cubic feet per minute). Ideally, for each cfm of air that goes into a supply duct system, the same amount should also come out.

Let's say you have a three-ton cooling system and determine the supply duct has 1050 cfm blowing into it. You should also have 1050 cfm blowing out from supply registers in a leak-free duct system. However, if the supply ducts leak, air will follow the path of least resistance, and you'll no longer have one cfm in and one cfm out. 

In this scenario, less conditioned supply air is available from the registers to cool the building. As a result, equipment often can't maintain comfortable conditions,
and you end up with hot and muggy rooms. This is when the complaints begin. You'll notice these issues are from a lack of delivered airflow, not attic air pushing into the living space. 

Also, supply air that leaks from an externally insulated duct system can lead to condensation between the duct and insulation. If you ever deal with a moisture-damaged ceiling, and the cause isn't apparent, supply duct leakage might be the reason. 

Addition of Unconditioned Air

While supply duct leaks in an attic do not directly pick up and transport attic air into the living space, they can create a negative pressure (vacuum) situation that pulls attic air and outside air into the living space. This leads to extra heat and moisture that the HVAC system tries to remove. Unfortunately, the equipment can't handle the excess load and capacity plummets. 

Supply air that leaks from an externally insulated duct system can lead to condensation between the duct and insulation.

Remember the principle of one cfm in = one cfm out. The blower tries to move the same amount of air. It doesn't care where it comes from. Let's say you have 200 cfm of supply duct leakage, but all the return duct is inside the living space and sealed airtight. 

Now you have a difference in airflow amounts. Too much return air is pulled from the living space, and insufficient supply air is blown into it. If you measured airflow from this system, you would see 850 cfm total from the supply registers and 1050 cfm through the return grille.  Imagine how your customers will feel when the extra 200 cfm of return air comes in from the attic and outdoors. It doesn't take long to overwhelm a system's capacity when you mix 140°F attic air and 72°F dew point outside air. You're left with a system that runs non-stop and doesn't remove enough humidity or lower the space temperature.

Next Steps

If you suspect a supply duct leakage problem, begin with a visual inspection of the duct system. You may notice your problem is something simple, like a disconnected supply branch that the cable guy kicked loose. However, if there is a deeper problem, consider additional testing beyond this article. 

If you suspect a supply duct leakage problem, begin with a visual inspection of the duct system. You may notice your problem is something simple, like a disconnected supply branch that the cable guy kicked loose. However, if there is a deeper problem, consider additional testing beyond this article. 

One test is to perform a building pressure test with a micromanometer. It's a quick test that can give you a peak at attic duct system integrity. At a high level, you turn the air handler fan to its highest operating speed and watch what happens to pressure inside the living space. It could reveal supply duct leakage if it goes into a vacuum (negative pressure). Remember, systems with a central return and/or closing interior doors can amplify this effect. 

If the results are inconclusive, it's time to measure delivered airflow with a quality balancing hood. You should charge an additional fee for performance testing like this–don't do it for free. If you find supply duct leakage, add a quick total external static pressure test and fan airflow measurement to ensure the duct system can handle sealing or determine if you need to recommend additional duct upgrades. 

David Richardson serves the HVAC industry as Director of Training for the National Comfort Institute, Inc. (NCI). NCI specializes in training focusing on improving, measuring, and verifying HVAC and Building Performance.

If you're an HVAC contractor or technician interested in measuring duct system performance, contact David at ncilink.com/ContactMe. NCI's website, www.nationalcomfortinstitute.com is full of free information to help you improve your professionalism and strengthen your company.

About the Author

David Richardson | Director, technical curriculum

David Richardson serves the HVAC industry as director of technical curriculum at National Comfort Institute, Inc. (NCI), Avon, Ohio. NCI specializes in training that focuses on improving, measuring, and verifying HVAC and Building Performance.