I've been sealing the ducts in our basement with mastic. I've done a decent job, but I'm trying to find every leak in the basement, where it's accessible.

For instance, I stand on the ladder in one area and I feel cold air with the a/c running. But, I can't figure out where it's coming from, and there are ducts on three sides of me, all within a foot of my head. If I hold up a 1 ply torn corner of a Klennex it flutters a bit but I can't determine which direction, let alone specifically where so I can throw a little more mastic there.

Everything I've read online is about testing leakage to get a number, etc. but I just want to know where exactly in the basement any leaks that are left are from.

I've thought about an incense stick, but am hoping for something with no chance of burning anything, something I can pick up and put down repeatedly with no worries.

3 Answers 3


Google "Smoke Pencil" or "Wizard Stick". Both use fog machine technology to vaporize a small amount of glycerine, producing a cool stream of smoke. The former is the professional product; the latter is a cheaper toy, but probably good enough for your purposes. (And, you definitely don't want to be messing around with flames in a dense tangle of air ducts. Could get exciting fast.)


Depending on how big is the area on which you suspect you can try one simple method: take the soap and mix it with a bit of water (like when you are washing your hands) and then apply it on the surface. You’ll know that you have found a leak because soap will start making bubbles

  • This is a great trick for leak testing gas lines, but I'm not sure how well it would work on HVAC supply lines. It's an interesting idea though, and I might just give it a go to see if it works.
    – Tester101
    Commented Jun 16, 2015 at 11:37
  • I agree that it is not ideal for HVAC but it is simple and cost efficient Commented Jun 17, 2015 at 7:18

You can inspect using a toilet paper for wind blowing and sucking for return ducts. I found a video which showed testing and using a tape for fixing.

Hoping this helps.. Cheers !

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