Thanks for the quick response!

No - Not yet, at work currently.

If it's just rust can I just switch to plastic or fully painted vents etc or should I worry about more sinister issues potentially?

If rust is forming on the vents then that means a source of humidity. What causes this? For example, maybe condensation occurring within the duct work or is there another usual source - eg the actual hvac system or maybe just a normal issue with these circular metal vents?

It looks like condensation forming on the metal. If you look closely you can see how the condensation is dripping down from the inner ring, which is the coldest, out towards the perimeter.

Have you measured the humidity in the house? I would not be surprised if it is above 60%. I don't think it is rust. You can probably clean it off with Fantastik spray cleaner.

The real solution is to lower the humidity level. Buy an inexpensive temperature and humidity gauge and begin monitoring.

Thanks Mike,

I will watch to see if the staining returns after I clean them and will begin monitoring humidity in the house.

I will also look at the one in the kitchen and the bedrooms once I get home. If the staining looks different I'll probably post a followup set of images but if its consistent with this one probably not.

Thanks for the information, analysis, and responses.

It's condensation at the register. The coloration could be dirt or (most likely) rust. I'm guessing they are metal vents. Switching to plastic (if you can find them) should help. Check the air flow and temperature gradient also. Too low an airflow/too high a temperature drop can cause condensation at the registers, especially metal ones.

There is good advice to check your overall humidity. Get a couple of inexpensive hygrometers so you can check them against each other or one and a sling psychrometer to check it. In addition to that, check the seal around the where the duct connects to the register and where the register meets the ceiling or wall. Often pressure differentials induced by the air handler (and caused by leaky ducts) suck in dirty, humid air from whatever is behind the registers. It mixes with the cold air you get condensation on the cold registers. Changing to plastic might fix the symptom, but don't forget the problem behind it. That should not be happening.

". In addition to that, check the seal around the where the duct connects to the register and where the register meets the ceiling or wall. "

spot on.

you are seeing the symptom of another issue.

duct leakage.

here is how to remedy the leakage at these areas:

take the supply grill down (and clean it) seal the sheetrock ceiling

penetration into the supply boot.

the only long lasting product I've found in 20+ years is

Hardcast brand #1402 Rolled Mastic Tape.

seal tape to ceiling, fold into supply boot. press to seal

well.

Keep tape within area covered by supply grill, you can

trace lightly around the grill before removing it.

once the mastic tape seals...it isn't coming off.

(as long as surfaces are clean and dry)

Do this at all your supply grills. Then go do your

Mom's grills.

best of luck.

A big reason condensation tends to form on vents/air diffusers (especially in humid climates) is because so many of them are typically made from metal. Choosing a product that isn't made from metal prevents the issue from happening in the first place. Feel free to check out our blog post that explains in more detail: https://blog.inviair.com/blog/diffuser-condensation-is-a-serious-problem.-heres-what-you-can-do-to-avoid-it