Mold in air conditioner return

4 years ago

We got two new air conditioners about 1 year ago, and we've been noticing a funky smell when the AC kicks on. We had the company that installed the new systems do an inspection, and they found mold in the return. When talking about mitigation, they mentioned they could either install a UV system or they can replace the return box with a new one. Both options cost $$ and I'm not sure I understand why there's mold in there in the first place?

I'm wondering if anyone had some insight or thoughts into why there might be mold showing up. We live in Texas so it's humid, but no roof leaks or detectable moisture in the attic where these are located. The upstairs unit was replaced from a 2.5t unit to a 3t unit, perhaps it's oversized? But we got multiple quotes and many of them suggested a 3t unit for upstairs.

Here's some pictures of my return (moldy) and evaporator coils

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PRO

Austin Air Companie

4 years ago

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.

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Comments (8)

dadoes
4 years ago
It's very common. My 17+yo system (also in TX) replaced two months ago had some smutz. A pic of the old blower and couple exterior of the evaporator .... I didn't get any of the inside the A-coil.
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PRO
Charles Ross Homes
4 years ago
Hi, Chase,

Mold and mildew are common; spores are in the air we breathe. Keeping them from colonizing inside your home is a function of controlling conditions including moisture, and temperature, as well as limiting potential food sources.

Your picture shows a plenum that's either constructed of duct board or insulated with rigid fiberglass on the interior. Both are great at trapping potential food sources for mold/mildew and they're impossible to clean. I think you'll have a healthier indoor environment if you change it out for a plenum constructed from sheet metal and insulate it on the exterior--not the interior.

Each time you change out a filter is an opportunity to inspect a portion of your HVAC system. Take along a flashlight, have a look, and have the ductwork cleaned as needed.
Stax
4 years ago
OP: Is your plenum drain clear?
Chase Wilson
Original Author
4 years ago
@Stax I cleaned it out yesterday with vinegar. A bunch of red gunk came out but I don't think it was clogged.
Stax
4 years ago
Is it possible to run a water hose down the plenum drain?
PRO
Gainesville Mold Remediation
3 months ago
The oversized unit explanation is worth taking seriously. A 3-ton unit that short-cycles because it's cooling the space faster than it can dehumidify will leave more moisture in the air than a correctly sized unit running longer cycles. Texas humidity combined with a unit that hits the set temp before it can pull moisture is a pretty common mold setup. The mold in the return box is a moisture problem, not a filter problem. UV lights and air scrubbers treat symptoms. The question to answer is whether the unit is running long enough cycles to actually dehumidify the space.I was suprised when our HVAC tech explained that a bigger unit isn't always better in humid climates. Run time matters as much as capacity. If your unit hits the set temp in 8 minutes and shuts off, it never gets to the dehumidification phase of the cycle.Before spending money on UV systems or a new return box, ask the installing company for the load calculation showing the 3-ton was the right size. If they sized it right the problem is elsewhere. If they oversized it, that's the conversation to have.
dan1888
2 months ago
last modified: 2 months ago
Separately controlled dehumidifier should be considered as part of your system. The YouTube channel The Build Show has done episodes including "The Most Overlooked Part of HVAC". Whole house dehumidification like the Sante Fe products are shown. You need a product that turns on your fan at a set point and reduces humidity down 10 points or whatever you set it at.