Oxidation on Ductwork

This is not from an inspection but from a friends home. He’s got what looks to be oxidation on his ductwork. Can anyone tell me what might be causing it? His HVAC guy didn’t know either. From looking at it, it almost LOOKS like it is galvanized. Could it be?

Mark am I looking at bare duct in your pic’s If I am it appears to be rusting from the inside out as is very common for duct with high humidity levels during the cooling mode and could also be a humidifier problem in the heat mode if one is installed. One of the reasons that I squeeze a duct in the crawl space when inspecting especially if insulation is installed have had them crush with just a gentle squeeze. Galvanized sheet metal ducts do not have excessive longevity when exposed to high moisture.

Yes they are bare. The gray ashy color you see is what I am referring to. I don’t know if it’s galvinization, or if it’s oxidation. I can’t say that I squeezed them, but I did touch them, and they seemed firm. Also, notice in the pictures the support brackets. They’ve got the same “coating”.

Mark have observed that many many times on galvanized duct with and without insulation. High moisture causes galvanized metal to turn whiteish to gray looking. Check the humidity level in the crawl space probally find it excessively high most of the time what kind of ventilation does it have. I have been in crawls where water was actually dripping from the floor joist and not a water leak just humidity.

Im with Charley, Very high moisture area where ducts are run,Matt

This was in a basement. I wasn’t doing an inspection, but it didn’t appear wet, and I didn’t notice any signs of moisture intrusion. I’ll have him check his humidity. Thanks guys! Learn something every day :o)

As an after thought where was the dryer vent venting

I didn’t notice. I’ll be sure to ask.

There’s a switch going on. Unvented and properly prepared crawl spaces are being seen to be drier than vented crawl spaces!

Don’t believe then I have ever seen a properly prepared crawl then. You have to have something to start with.

I’ve seen basement ducts sweat like those obviously have been doing when a basement window was left open during the cooling season.
(Naturally they wont sweat during the heating season.) :slight_smile: