dark stains + dry = dirt or dust ---- dark stains + damp (or wet) = mold potential.

Filtration: the better filter you use, the less dirt goes thru -- however a too restrictive filter and your system won't work or you cut the capacity of the system and your light bill goes up.

funky smells and Texas go hand in hand. When you switch from heat to cool and then from cool to heat like we've been doing here lately --- it's not uncommon for funky smells to rear up. Especially when humidity levels start going back up.

Bacteria -- is what causes the smells most often, not mold. Mold often will make you sick first no smells and discovered after you get sick and start investigating.

Bacteria usually gives off smells when heated, but if you have a heat pump not uncommon to get funky smells in heating mode or AC mode after recently interchanging between the two. Why does that happen?

Humidity plays a role in that. Especially when you run the AC in more mild like humid type conditions the AC doesn't run long enough to pull it out.

The pictures you show are about as common as the day is long.

What can you do? Set your temperature set points higher or lower forcing the equipment to run longer. If the smells don't go away after several hours of run time (heat or AC) then you may need to have the E-coil professionally cleaned as this is where the bacteria grows.

Shut the system off, avoid using it during times like this in our climate. Bacteria needs the moisture of the AC cycle to grow. Heat cycle kills it, so when it dies it gives off smells.

You can put in UV bulbs and a whole conglomeration of other gadgets, but if you're not prepared to replace these parts on an annual maintenance plan then keep your money in your wallet. It is argumentative that these sorts of products do much of anything.

If you are an alergy sufferer there are products for that -- but there are additional costs to maintain those on an annual basis. The expense never ends.

I service the Katy, Texas area.