Will the real leaker please stand up. His name is Duct Work.

Using artificial smoke will help you find leaks in duct work. It’s a simple DIY project and the artificial smoke is not expensive. Simply turn your heating systems fan to the “fan on” position and while holding the smoke generator introduce the smoke along all the duct seams and connections.

On the supply or leaving air side of the furnace the air leakage will divert the smoke making leaks obvious. On the return side (filter side) the smoke will be sucked into the system.

Watch this really short video that shows a Wizard Stick locating some leaks. I made these leaks to demonstrate how to use artificial smoke. My duct system is pretty tight.

The smoke generator I use in this short video is called a Wizard Stick. Amazon says they are currently not available but that changes. I found a similar unit called a Smoke Pencil ONE Air Leak Detection Hazer pictured below. You can click on the image if you want to check it out.

Duct work leaks are one of the most energy consuming problems in most heating and air conditioning systems. Adding high efficiency equipment to a duct system full of leaks is not the answer.

You are way better off fixing the duct leakage and keeping the low efficiency furnace unless it’s just time to replace the furnace anyway. Sealing and insulating your duct work will give you a positive return on the cost of closing up all the cracks.

One Major Leak Area Is Duct Flex Connectors

These flex connector facilitate the connection between two sections of duct or duct and a furnace as you can see above. They are assembled by folding at the corners and then stapled at the canvas like material that provides the flexing.

You could have more than one flex connector. They also help eliminate vibration originating from the equipment.

The stapled part is sometimes a pretty good leak. Look for where these ends meet and seal them with a good pliable duct sealer or caulk.

You should seal up all the leaks as you find them. You will notice the system to run quieter as you begin to get all the leaks stopped. An added benefit.

Older Duct Work In Attics And Crawl Spaces Prone To Leak

You may have to get dirty and get down in that crawl space to examine your ducts. Look for sagging duct and flex duct that has separated from the metal connections.

If you physically are unable to inspect your duct work find a youngster who can and send them down there with a camera. Or, hire someone. It’s that important.

Animals like rats, mice, and cats love to snuggle into the warm spots especially on flex duct and scratch through the liners into the duct itself creating huge leaks and very unsanitary conditions.

The only way to repair such an invasion into flex duct is to replace the entire section of duct.

Mastic Duct Sealant Works To Seal Most Metal Duct Connections

Mastic duct sealing products are a great way to quickly close up those air leaks. It paints with a brush like paint and it’s water soluble. It will dry in no time and lasts for a very long time. I love using this stuff.

The image below shows me testing an elbow on a section of duct that I haven’t put mastic on completely.

It Leaks. It’s A Small Leak. But It Still Leaks

It’s a little hard to see the leak moving the smoke, but it does. Each one of these connections is a small leak. The gores (the joint in an elbow) that allow the elbow to turn or adjust are also each, a small leak.

Add up all the connections or joints in a system. Even though they are only hair line cracks they add up.

The average thickness of a hair is about 1/1000 of an inch. Considering that heating system might have about ten 6″ elbows and 6″ elbows are almost 19″ around.

If my math is right that adds up to an accumulated 1″ hole in your duct work. And those are the leaks in just the elbows. Would you go and drill a 1″ hole in your ducting? Or spend $25 and buy a bucket of mastic? There’s a ton of products made to seal duct work. Check them out on Amazon, any of them is better than leaky duct work.

Sealing Ductwork From The Inside

Years ago, I’m guessing about 20 years ago, we were investigating a new product on the market called Aeroseal. It was a pretty exciting way to seal ductwork from the inside.

We use my house as a test house and I could not believe how much difference this process made on the efficiency and noise level of my heating system.

Aeroseal is an application that blows a polymer through the inside of your duct system. Nothing is introduced into the house as all registers are closed and sealed. This polymer finds all the cracks and crevices and seals them.

It’s an amazing process. Search for Aeroseal near you and get some prices.

Caution, 100% Duct Sealing Can Reduce Efficiency

Some duct systems are undersized from the get go. Sealing a duct work system that doesn’t properly distribute the air could cause overheating in winter and frozen AC coils in the summer.

Best test to have done is a static pressure test. I have an article that addresses static pressure here. Check it out. I discuss adding more efficient filters and how they can cause problems to systems that aren’t capable of moving enough air to compensate for such filters.


Everyday you wait is money down the duct and out the crack. You are either heating the attic or the crawl space depending on where your duct is located in your home.

If your return is in the attic then you are sucking in hot air in the summer and paying to cool it again or you are sucking in cold air in the winter and paying to heat it again.

If your supply ducts or the duct work delivering air into your house is leaking then your house will be in a negative pressure sucking in outside air that is unconditioned.

Their are more scenarios in regard to pressures in a house that are created by leaking duct work but I think you should get the picture by now and hopefully be inspired to do something about sealing up the leaks in your duct work.

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