How to Fix a Leaky Air Duct?

ByMarc Silberberg


It’s already early June and our HVAC systems could be heard humming as they cool our homes and commercial facilities. However, not everyone is having the same feeling as they walk into the air conditioned space. Some will feel that cool, calming sensation when they walk into a home and the central cooling is working efficiently, while others are just not getting that experience and are waiting for someone to service their system. Unfortunately, there are many waiting for  an HVAC technician to come check out the cooling system but have still yet to be seen due to the recent labor shortage. 

While you’re waiting for the technician to show up, you may want to do some trouble shooting yourself to give some more insight to the serviceman before he or she arrives. One of the many reasons as to why HVAC systems under perform is due to leaks in the air ducts. When there is an air duct leak, conditioned air will escape and can negatively impact how your home gets cooled down on a warm summer day. If homeowners can identify if air is escaping from the ductwork, the technician can run a duct leakage test to see how much CFM (cubic feet per minute) is escaping the system. According to code, there is an allowable duct leakage of 2%, which is multiplied by the amount of conditioned space in square feet. For example, an area of 24,000 square feet has duct leakage leeway of 480 square feet (24,000 x .02 = 480 CFM). Anything more than that should be addressed in order for your HVAC system to run efficiently. 

How To Do a Visual Duct Leakage Test?

Depending  if your ductwork is visible will determine if you can do a visual duct leakage test. Many homes in the United States are conditioned with push cooling and heat. The supply and return vents pull and push the air in and out of the system, ultimately warming or cooling the home. In most homes that use push heating and air conditioning the ductwork will be beneath the living area. These vents are sometimes viable from the basement and crawl space ceilings. It’s generally easier to diagnose a leaky vent in a basement where there is ample room to move around and treat it on the spot with high grade foil tape or mastic. While foil tape is easier to handle cement like mastic will seep into the air leaks providing a stronger air seal.   

When in your basement or crawl space, guide your hand carefully to feel for any air escaping the vents. Sometimes you might hear a hissing sound if it’s not too noisy. Mark the spot and keep moving all around the metal ductwork. Part of the ductwork consists of metal trunks connecting one another. Often they are supported by metal cleats which if not air tight can possibly be the source of a duct leak. As you come to joints where the venting will take a different direction, pay close attention. The bulk of air leaks in HVAC vents tend to be where the joints connect. It might not have been firmly tightened or the connection got jammed and the HVAC installer just left it that way. For such an air leak, mastic would be the preferred method of sealing as the substance will fill that space and the surrounding areas. When the cement-like substance hardens, it will expand and fill the gaps that are causing the air leak. As per a visual duct leakage inspection in a crawl space, the reason why it’s more difficult is due to the short ceiling that makes it difficult to maneuver when inspecting the venting. Regardless if your system is not performing well and you suspect leaky ductwork it’s definitely worth giving it a shot to try and diagnose a leaky vent. You might get a little dirty down there but if you do find an air leak there you can save yourself from a service call and solve the problem on your own. Just keep in mind that not always is it one leaky vent that’s the problem.

What To Do When My Vents Are Covered With Insulation 

The challenge when your duct work is covered by insulation, is that the actual metal vents are hard to access even when visible in a basement. Sometimes you can very well have an air leak in your ducts but would never be able to find it because it’s hidden under the insulation. Other times you may notice the insulation getting blown down from an air leak and identify the problem that way. What you will have to do is remove the insulation to be able to access the metal vents and check for air leaks. Once your inspection is complete then you can reinstall the insulation. Before you put the insulation back on the vents make sure any treatment that was done to the vents is completely dry.

Visual Ductwork Can Be Inspected for Leaks But What About Everywhere Else?

This is quite difficult and you may need to hire a professional duct leakage expert to figure this out. Before the actual test is done there is a pre-test that gives an accurate reading of how the current system is performing measured against the duct leakage test that will be done next. Once all the equipment is in place the test will be run by blowing air from a fan into the duct work to see how much air is escaping the ducts. Since the equipment is hooked up to a computer it will give a reading that determines the system performance to see if air is escaping the ducts. 

If the results are good, then the job is complete. If the results show air escaping the ductwork the technician will move on to a sealant treatment. The substance that’s got to be used is a milky and rubbery liquid that gets blown into the duct work in an aerosol to travel throughout the supply and return ducts. The sealant which is used is a vinyl acetate polymer liquid that gets sprayed into the duct work and will fill any hole or leak it can find. When a hole or crack is found it will naturally build up in that specific spot and create an airtight plug that will not allow any air to escape.

During the air duct sealing process, the testing equipment is hooked up to a computer that measures how much air is escaping the ducts in CFM (cubic feet per minute). As the aerosol treatment is taking place the technician performing the duct leakage test will be monitoring the program on a tablet that will inform him of the improvement of overall system performance. Results can be noticed when you place your hand over the vent and feel a stronger push of cool air. 

Plate Covering Hole From Vent Cut Out After Air Duct Cleaning

In order for an air duct cleaning to be done right the equipment needs to be inserted at different points in the vents. The way that happens is by making a circular cut out in the metal trunks and inserting the air duct cleaning equipment there. After the work is complete the air duct cleaning company will patch up the hole with a metal plate and screws. The problem is the circular cut out can leak significant amounts of air that can hamper your HVAC’s system’s performance. In addition, there are holes made from metal screws that also need to be accounted for potential loss of air. The correct way to seal the hole is to make sure that absolutely no air is escaping by dressing the metal plate with a layer of mastic for an airtight seal. Often this can be overlooked by the air duct cleaning company and over the course of time will allow more and more air to escape. Always make sure to hire a licensed air duct cleaning professional who will take the time to make sure that there are no air leaks.

Final Words

Having an air cooling system which underperforms can be upsetting to any homeowner. You can have a brand new HVAC system installed and for some unknown reason your house is still not cooling down. Other times you may have parts of your home which are not getting cool at all. If this is the case, you may want to inspect for possible air leaks in your ductwork. The cool air that has been conditioned may not be getting to the living areas due to air escaping the vents. The key is to find where the air leaks are and to seal them up properly with material that provides a nice air tight seal. If you have done a thorough inspection to the best of your ability and still have not resolved the problem you may need to call a duct leakage expert that can identify leaks beyond a visual inspection. You will be surprised by what a positive impact treating leaky ductwork can have on your HVAC system’s performance. 

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