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From time to time, there's this god awful stench coming from my floor vents. It's not potent enough to where you can smell it directly at the vent, but strong enough to linger in the house and stink it up. Some days, it smells like death. Other days, it just smells like a dirty sink or bathroom.

I've ran an endoscope down my vents only to find your common dust and other misc. items (like ear plugs - why?). I also went under the house (crawl-space) and took a look around and found nothing. Insulation seems good, no leaks that I can see, and my A/C condenser appears to be running water to my sump pump correctly.

What are some common causes of smelly HVAC? I'm really wanting to figure this out because the stench is not very pleasing to guests.

Additional Info: It only seems to occur on hotter days (>90deg F). Pressumably when the A/C has to work extra hard.

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    You have something growing in your evaporator or the drip pan. Use caution some chemicals can damage the aluminum fins/tubes of the evaporator. Never mix chemicals I do t think you were but wanted to state that since I wasn't sure. Your condensate drain if dripping needs to be cleaned you should not have standing water in the pan as many bad things can grow including your fowl order. There are tablets/ pads that can be added to the pan to prevent the "green gunkies" from growing and they do not smell bad. I use one from nu-calgon or foaming coil cleaner like simple green, no acid based cleaner
    – Ed Beal
    Sep 25, 2017 at 19:34

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The stench is certainly—due to your hot-weather-only mention—due to condensate dripping where it shouldn't. While the majority seems to make it into the drip pan where it is safely pumped away, some condensate may splatter into the filter, plenum, and/or ducts where it activates dormant bacteria. Also the "return" refrigerant line may well be cold enough to create condensate on its surface and drip to bad places.

I would start by taking a good look around the evaporator and air filter. Temporarily remove the filter and see (smell) if the stench ceases. Wipe down all surfaces around there (while the air handler is "off"). Maybe even spray everything with a 100% white vinegar or 5% bleach solution.

Also look at the evaporator itself: is it partially blocked? Does it look clean? What about the edges which don't "work" as hard in not-so-hot weather? Cleaning it requires great care not to block it further (by fin folding), but a hand-held spray bottle of water can gently work wonders in many cases.

As for earplugs in the ducts: My wife sometimes uses them to get to sleep; they easily fall out during the night and travel surprisingly far by morning—either onto the floor or between the sheets toward the feet. If there is a floor register within a few feet of the bed, I would not be surprised if they rolled right on down there.

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