“Why Does My AC Smell Like Mildew?” Matt in Sacramento Asks an HVAC Expert

Matt in Sacramento has been turning his AC on a lot more lately, like all of us. Unfortunately, something besides cool air is coming out of his vents—something that stinks. He says:

“As I’m sure you’ve noticed, it’s gotten pretty hot in Sacramento lately, like it always does at this time of year. I’ve lived here all my life, and I know some days you just have to give in and run your air conditioner. The problem is that this year, something is up with mine. It’s cooling my house, but while it’s doing that a terrible smell is also coming out of the vents. Now, I obviously don’t want to have to plug my nose every time the temperature outside climbs above 90. So I want to know, why does my AC smell like mildew? And what can I do to fix this?”

First of all, Matt, I agree that plugging your nose every time the temperature gets above 90 is not an ideal situation, especially not here in Sacramento where the average high is above 90 throughout both July and August. Your arm would get tired and your nose would get sore. Let’s focus then on what’s making your AC smell like mildew and why. That way, you can start to work toward a solution.

So, Exactly Why Does My AC Smell Like Mildew?

Matt should be thankful that he’s identified mildew as the smell in his vents. Far too often, I have homeowners who can’t pinpoint exactly what bad odor their AC is generating, which makes it harder to know where to start fixing it. In Matt’s case, I already have a few ideas about what’s causing the problem, and how it can be cleaned up.

Mildew is actually far and away the most common nasty smell that comes out of AC units, and it happens because mold, mildew, or fungus has started to grow inside…

Mildew is actually far and away the most common nasty smell that comes out of AC units, and it happens because mold, mildew, or fungus has started to grow inside, or nearby, the system. Now, I expect this to be an even more prevalent problem than usual in Sacramento this summer because of how much moisture was in the air all winter along. Chances are, Matt will have lots of company.

In addition to our above average amount of winter rain, part of any air conditioner’s job is to remove moisture from the air, and if that moisture isn’t subsequently drained properly, mildew can grow. Your AC unit is probably set up to automatically release moisture, but a clog or a malfunction can interfere.  In fact, even having a dirty filter can lead to moisture getting trapped, causing mildew to grow on it. As air then passes through the moldy filter, it brings the stench with it. The solution for this is to have a professional take a good long look at your AC unit—and the space around it—so that they can pinpoint the source of the smell.

Why Else Might My Air Conditioner Stink When I Turn It On?

Mildew is not the only bad smell that comes out of AC units. In my years as an HVAC specialist, I’ve come across many air conditioners with odorous issues, and I can tell you that there’s a great deal of variety among the types of bed smells they can produce. Let’s take a look at some of the other offensive scents, as well as at what exactly might be causing them.

• Rotten eggs: A rotten egg smell is far less common than mildew, but still a concern, especially if the smell was suddenly present the first time an air conditioner is turned on for the year. This odor often means that a bird or other small animal crawled into the vents over the winter and died there. It’s going to take professional intervention to safely clear this problem from your home.
• Sewage:  A sewage smell can indicate a backed up sewer line or a ruptured vent pipe somewhere near the AC’s duct system. Now, we talked at length this winter about the plumbing problems that resulted throughout Sacramento during the winter storms, like basement toilets backing up and what to do about burst pipes. These sewer backups are often caused by debris in the pipes causing a clog, which can be really noticeable around fixtures you use every day, like a toilet or bathtub, but much less so when it comes to those that are deep inside your home. Then, you have to wait until the problem manifests itself in the form of a smell, which in this case is near your AC ducts.
• Feet: A sweaty foot smell from your AC usually means that stagnant water is collecting inside and not being drained properly, creating a musty smell coming from your vents that might seem like sweaty socks but definitely isn’t. It’s a close cousin to the mildew smell, and the main difference is that this one is in a far earlier stage in which mildew hasn’t grown yet. This means it can also be easily rectified by having a professional make sure the system is draining properly.
• Gunpowder or exhaust: A gunpowder or exhaust fume smell is another common AC ailment, though it’s likely not all that applicable to Matt’s situation. See, Matt said his AC was otherwise working just fine. That’s generally not the case with this odor, which is usually caused by a circuit board or fan motor shorting out and sending this burning smell and smoke up through the vents. Basically, if you’re smelling this, your AC is probably not working at all.

What To Do If Your Home Air Conditioner Smells Bad

When someone asks me why their home air conditioner smells so bad, I usually give them the old good news, bad news answer. The bad news is that there’s a definite, and often serious, cause for their odor ailment. The good news, however, is that the vast majority of these problems are easily fixable, and the others can all be fixed with a bit of professional work. In other words, a smell coming from your AC is not a guarantee that the whole system needs to be replaced.

In my opinion, however, it does mean that a professional needs to be called. All of these are internal situations, some of which involve dangerous situations like mold, dead animals, and burning circuit boards. Don’t tackle that kind of work yourself. Call an experienced and trained professional who can identify where the smell is coming from and make it go away safely and effectively.

When you turn on your AC, you want one thing coming out of your vents: cool air. And, you want it to help you relax on hot summer days. Matt, and anyone else with this type of problem, certainly doesn’t want that relaxation to come at the cost of smelling mildew. In fact, I can’t think of a single situation where a bad smell is a good thing, except maybe with the sauerkraut my grandma used to make when I was a kid. So, don’t put up with an onerous odor. Call a trained HVAC specialist and get the scent cleared up today.

Take action against the bad smell coming from your AC. Contact the trained professionals at Bell Brothers today to make sure only cool air is coming from your vents.

Image courtesy Unsplash user Jimmy Bay