Using a Thermal Imaging Camera to Locate Hidden Supply Air Vents

It is possible, right now; that you have an abandoned or hidden supply air vent in the attic or walls of your home. In this post, I want to help you locate hidden supply air vents by using a proper thermal imaging camera.

With the huge decrease in prices for today’s thermal cameras, contractors and homeowners now have access to one of the most beneficial home maintenance tools on the market. Like most tools, a certain level of training and experience is necessary before the owner begins to reap the benefits and thermal cameras are no different.

There are specific building and atmospheric conditions that must be established for a thermal camera to reveal potential defects that exist in your home. Trained and experienced professionals can use a thermal camera without fear of misidentifying a thermal pattern as a defect. Cutting into a wall and finding out that your interpretation of a thermal pattern was wrong is embarrassing and costly.

There is a large margin for error and false detection that can happen if you are untrained, so it is critical that contractors and homeowners understand the basics of how a thermal camera works before they get too adventurous!

Locating Hidden Supply Air Vents

There are multiple applications in which a thermal camera can be used to help you locate costly defects in your home’s ventilation system (heating and cooling air ducts). But this post is only about finding hidden, blocked, or abandoned supply air registers. Finding these defects tends to be more comfortable for the novice because there will be a significant temperature difference at the location of the hidden vent.

To detect hidden vents in the walls, ceilings, or floors of your home, you’ll need to place your heating or cooling system into operation. It doesn’t matter which mode you choose for the scan, but of course, I wouldn’t turn my furnace on in the middle of the summer and never place a cooling system into operation when the outdoor temperature is below 60 degrees.

For simplicity, let’s say that you want to perform the scan in the summer and you place the cooling system into operation. For best results, make the system run for at least a half-hour and then get your camera ready for the scan.

I like to perform my scans on the building envelope in the rainbow palette because it gives more color variations, thus, more sensitivity. Iron is also a good choice for this type of scan.

Now that we’ve had the cooling system in operation for a while and we have the camera ready, the next step is to scan the walls, ceilings, and floors throughout the home.

Flir C3 Thermal Image of an exterior wall
Thermal Image Taken With a Flir C3

I’ve attached a photo from a Flir C3 that shows an exterior wall with no defects. The blue vertical lines are the wall studs and ceiling joists. If you want to achieve an image that is similar to mine, you’ll have to prepare the home in a little more depth, which I will cover in a different article.

We are looking for large blotches, typically a square foot in size. The blotch, in this case, should show up as a blue color due to it lowering the surface temperature of the material around it.

If you find a potential anomaly that resembles our hidden vent, try and gain access to that location within your attic to verify your finding.

If there is no attic access, switch the thermostat from the cooling mode to the heating mode. Wait about 15 minutes and then scan again.

If your blue blotch is now red, you may have found a hidden or blocked vent.

It is also possible that you’ve found a significant tear or leak in your ducts; either way, you’ll want to consider having some assistance with the problem before damaging your home. Remember, massive air leaks and blocked vents in your walls can, and likely will, lead to mold growth during the summer months.

Real World Example #1

Thermal Pattern Showing a Covered Supply Air Vent Behind a Wall
Thermal Pattern Showing a Covered Supply Air Vent Behind a Wall

I was conducting a thermal scan around the interior of a brand new home when I  found a surface temperature anomaly with my thermal camera.

On this inspection, the outside ambient air temperature was 65 degrees F. Because I wanted to increase the temperature difference between the indoor and outdoor temperature; I operated the furnaces in the home.

I ran both of the home’s furnaces in the heating mode for a few hours while I inspected other systems and components for my client. After a few hours of run time, I had created a target rich environment, and I was confident that I’d maximized my potential to reveal hidden defects. 

Sheetrock Wall
There was no visible indication of a vent behind the wall

Not long into the thermal scan, I’d found a high-temperature anomaly on the upper entry wall. Because the furnace was in operation, the heat from the duct created a sizeable red smudge on the wall. In the provided thermal image, you can barely see the louvers inside the vent. 

Instantly I knew what I had detected; the sheetrock crew had neglected to cut out the hole for the supply air vent in the wall. Even though I knew what I’d found, I wanted to verify my findings, so I traversed the attic space and peered over the wall from above. As expected, there was an insulated flex duct attached to the framing in the precise location of the thermal anomaly.

Supply air vent installed in a sheetrock wall
The previously covered vent was repaired

I documented the defect in my report, and then on a warranty inspection, I got to see the resulting repair.

In this case, without the thermal camera, I would have never known that there was an additional vent in the front entry of this brand new home.

If you look at the second image, there was no indication that there was a vent behind the wall. The wall was as smooth an unassuming as all the other walls in the home.

If the HVAC contractor that installed the home’s system had a thermal camera, they would have been able to detect this issue, adding additional value to their services.

Real World Example #2

Okay, example #1 was a low-hanging fruit because the vent was directly affecting the surface temperature on the wall, making for an easy find.

Let’s look at a defect that can be a little more tricky to deduce without proper attic access or additional equipment.

Thermal Image of a Covered Supply Air Duct Blowing Cold AirI inspected an 11-year-old home, and after setting up the thermal conditions necessary to perform my inspection; I found a thermal anomaly above the 2nd-floor catwalk.

My instinct was that it was an abandoned supply air vent, but I wasn’t 100 percent sure that it wasn’t a roof leak (even after thousands of inspections). Either way, I knew that it was a problem.

I made my way to the attic and looking in the direction of the presumed abandoned supply vent; I found my reward. About 30 feet from the edge of the attic decking was the abandoned supply air duct. I carefully made my way (I was suspended above the entry floor)Abandoned Supply Air Duct From a Distance

to the duct to confirm my thermal defect. The duct had never been completely installed. It had been like that for 11 years! Because the duct wasn’t pressed directly against the sheetrock, like in example #1, the temperature difference was not as extreme. A less sensitive camera may not have recognized this defect, and if the home wasn’t adequately prepared, I might have missed it as well.

When supply air is discharged into the attic, it has the potential to create a large volume of condensation, which can lead to mold growth!

Eleven years of providing conditioned air to the attic and what’s worse, the seller was likely obligated to negotiate on the repair before closing on the home. Eleven years of energy loss and now we can add the cost of the repair.

Abandoned Supply Air Duct in Attic

If that’s not reason enough to purchase a thermal camera for your home, then we don’t understand building science in the same way!

(For homeowners, I strongly recommend the Flir One or the Flir C-Series)

Those were just two examples of abandoned vents and ducts, of which I have located and reported on many more times.

However; locating hidden supply vents is just one example of many. Thermal cameras are great at finding loose and over-heating electrical connection, water infiltration into the home, missing insulation, and more.

If you have a question about this article, let me know in the comments below. Thanks for reading!

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This Post Has 2 Comments

  1. Flir Boy

    This website is… how do I say it…relevant!! Finally I’ve found something
    which helped me. Thanks!

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