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Measure and Quantify Live Return Duct Leakage

This summer, attic temperatures in many parts of the country will exceed 130F. You can measure the effects of return duct leaks and calculate how they reduce air conditioning system capacity.

This summer, attic temperatures in many parts of the country will exceed 130°. Return duct leaks from an attic often reduce system delivered capacity by 50% or more on the warmest summer afternoons. Let’s take a look at how you can measure the effects of return duct leaks and calculate how they reduce air conditioning system capacity. 

Symptoms of Return Duct Leaks
If you were to record your customers’ conversations when they unknowingly request a service call for a return duct leak in the attic, here are the top three complaints you may hear.

Notice, none of these complaints mention anything about the return ducts. The symptoms they describe are only recognizable as return duct leaks by a trained technician or customer service representative who can “see” the problem for what it is.   

“My air conditioner runs, and works fine in the mornings, but seems to stop working on hot afternoons.”

 Because return ducts are attached to the suction side of the fan, any leaks will pull hot attic air into the air conditioning system.. That attic air adds heat to the return air in the duct which must be removed by the air conditioner before it can cool the building.

On a very hot day, a large return duct leak can pull in enough hot air to overpower the equipment’s cooling capacity.


On a very hot day, a large return duct leak can pull in enough hot air to overpower the equipment’s cooling capacity. The equipment may be operating perfectly, but the system isn’t. You’ll have to pull your head out of the box to find and solve this problem.

“Our utility bills skyrocket more than they should during summer. It seems our air conditioning system never turns off.”

Hot air pulled in through return duct leakage is one reason for high utility bills. The air conditioner runs continually, unable to remove enough heat to cool the building.

The other cause for high utility bills is heat from the attic attacking cooler air in the return duct, right through the insulation. Code requires a minimum of R-8 duct insulation in most areas of the country. If the attic is 130ᵒ and the return air is 75ᵒ the air temperature difference between the two is 55ᵒ. Is R-8 insulation enough?

Many contractors diagnosing this problem prescribe and install up to R-30 duct installation in hotter areas of the country.

“When our air conditioner runs in the summer, the house gets extra dusty.”

Attics can be pretty nasty places when it comes to cleanliness. A return duct leak pulling in cellulose insulation can assure a dusty home for a decade or more. Fix the return duct leaks, and the dust problem goes away.

Inspect the Return Duct System
Walking or crawling ducts in an attic isn’t easy, but it pays big dividends to you, your customers, and your company. Return duct repairs can command very profitable prices. Make sure the job is worth it to you and your customers. Strap on a breathing mask and get up there.

Return leaks closest to the equipment pull the most attic air because pressure is up to 50 times higher than at the grilles. However, at the end of the duct, you may find one duct disconnected from a return air can, so keep crawling. Mark the leaks in an obvious way, so the installers can easily find and repair them. Of course, check the supply duct system while you’re up there.

Diagnostic Testing and Calculations
Temperature diagnostics -- The first test to find a return duct leak is to measure several air temperatures. This simple test only takes a few minutes and requires minimal test instruments.

Measure air temperature entering the farthest return grille. Let’s say it’s 75°. Ideally, the air entering the equipment should also be 75°. Measure the air temperature entering the equipment. Let’s say it’s 85°. Subtract the return grille temperature of 75° from the equipment entering air temperature of 85°, to find the return air temperature increased 10°.

That’s way too much! To put this into perspective, suppose the temperature drop over the air conditioning equipment is 20ᵒ. Divide the 10ᵒ return duct temperature increase, into the equipment temperature drop of 20ᵒ (10ᵒ ÷ 20ᵒ = .50) to find 50% of the air conditioning equipment’s cooling lost to return duct leakage and poor duct insulation. In other words, the system is only working at 50% of rated capacity.

Remember, this problem is invisible if you only test at the equipment. A maximum amount of return duct temperature loss is 5% or as little as your customer will pay to have you fix.

Airflow Diagnostics -- The second test requires more time and test equipment, but it is the best way to quantify live return duct leakage in an operating HVAC system. 

Use an air balancing hood to measure airflow into each return grille and then add them together. Let’s say it’s 770 cfm (cubic feet per minute).

Next, measure total external static pressure (TESP) and verify the fan speed setting. Use these two points to plot fan airflow on the manufacturer’s fan table. If you’d like a detailed procedure on plotting fan airflow, email Doc. Let’s say plotted fan airflow is 1200 cfm.

Subtract 870 cfm from 1200 cfm (1200 cfm - 870 cfm) to find 430 cfm of live return duct leakage.

To further help quantify return duct leak percentage, divide 430 cfm of leakage by 1200 cfm of fan airflow. (430 cfm ÷ 1200 cfm = .36). or, That is 36% return duct leakage. That’s through the roof! A maximum amount of return duct leakage is typically 5%. Once again, if your customer is willing to pay for the repair, lower is better. If the system is sized really close to the building load, paying you to reduce leakage further may be worth it to your customer.

Explain Return Duct Leaks to Your Customer
When you begin to explain these issues to your customer, keep in mind, most of them have no idea what a return duct is. No really, they don’t. So, keep it very simple.

Here’s an example of what you might say, adjusting your explanation to their situation.

“I’ve found the reason your air conditioning system isn’t keeping up. The air conditioning equipment is working fine. The problem is with the ducts that move the cold air around your house.”

“I’ve found several large leaks in the duct (show a picture) that carries air from inside the house back to the air conditioner. On a hot day, the air in your attic climbs up to 120. This hot attic air is pulled into your air conditioning system; heating the air in your house. All that added heat makes it so your air conditioner cannot keep up.”

“We can fix this problem and really improve your comfort and reduce your utility bills by renovating your duct system. After we complete this work, your home will be much more comfortable. Your air conditioner will run a lot less and your utility bills will go down.”

Of course, this is just an example explanation, but notice how simple the description is. Once customers understand their problem, then you can mention the percent of reduction in system capacity and percent of duct leakage to help them understand the magnitude of the problem. Just keep it very simple.

As soon as they understand and express a desire to have it fixed, stop telling them about their problems and focus on the benefits and price of repairs. Then have them approve the work.

Typical Repairs
Typical repairs include repairing return duct leaks or installing additional duct insulation. You should also offer to renovate and upgrade the supply ducts at the same time. Be sure to test again once your work is completed. Share the improvement with your customer to assure them the repairs hit the target you were aiming for.

Rob “Doc” Falke serves the industry as president of National Comfort Institute, Inc., an HVAC-based training company and membership organization. If you're an HVAC contractor or technician interested in a free test procedure to measure and plot fan airflow, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at for free information, articles, and downloads.

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