If you turn on your air conditioner and suddenly notice a fishy smell coming from the vents, don't ignore it.

A fishy smell is different from the musty or damp odor you might sometimes notice from an HVAC system. In many cases, that smell can point to moisture getting into ductwork or insulation where it shouldn't be. But there is another possibility homeowners need to be aware of: certain overheating electrical components can also produce an odor that people describe as fishy.

Either way, your air conditioner shouldn't smell like that. The important thing is figuring out where the odor is coming from instead of trying to cover it up.

A Fishy Smell Can Be a Moisture Problem

Here in southeastern North Carolina, we deal with a lot of humidity. If your ductwork runs underneath your house through a crawl space, that's one of the first places I would look.

Standing water, high crawl-space humidity, plumbing leaks, poor drainage around the foundation, or ground moisture can create a damp environment underneath the home. If that moisture reaches the ductwork and gets into the insulation surrounding it, the insulation can stay wet.

And wet insulation can develop a pretty distinct odor.

Once your HVAC system starts moving air, that odor can make its way into the house through the duct system. That's why you may notice the fishy smell primarily when your air conditioner comes on.

Check Under the House First

If you have a crawl space, take a look underneath the house if it is safe and accessible to do so.

You're looking for things such as:

-Standing water
-Wet or sagging duct insulation
-Plumbing leaks
-Condensation around the ductwork
-Damp areas around the foundation
-Missing or damaged vapor barrier
-Signs that water has been entering the crawl space

The question isn't just, "Why does my AC smell?"

The bigger question is, "Where is the moisture coming from?"

Until you answer that question, you're probably not going to permanently solve the odor problem.

The Problem Could Also Be in the Attic

Not every home has ductwork underneath it.

If your ducts run through the attic, moisture can still be the culprit. A roof leak, condensation problem, or another source of water can soak the insulation around a duct run.

Once that insulation gets wet and stays wet, you can end up with the same type of odor.

We've also seen situations where liquids have gotten into floor registers or duct runs around kitchens, bathrooms, and other areas of the home. Sometimes the source is more localized than you can expect.

The key is tracing the smell back to where the moisture is actually entering the system.

Drying It Out May Not Be Enough

Once you find the source of the water, that problem needs to be corrected first.

There's not much point in drying wet insulation if it's just going to get wet again the next time it rains or whenever the crawl space humidity climbs.

After the moisture source has been corrected, the affected ductwork and insulation should be inspected. Depending on how wet it became and how long it stayed that way, some insulation may need to be replaced rather than simply dried.

You also want to make sure the ductwork itself is properly sealed. Leaky ductwork in a damp crawl space can pull unwanted air, moisture, and odors into the system and distribute them throughout your home.

There Is Another Cause You Shouldn't Ignore: Electrical Problems

This is an important one.

A smell that homeowners describe as "fishy" can sometimes come from overheating electrical components, wiring, or plastic materials.

That means you shouldn't automatically assume every fishy smell is coming from wet insulation.

Pay attention to where the odor seems strongest. If it smells more like hot plastic or burning electrical material, or the smell is particularly strong around your indoor HVAC equipment, thermostat, electrical panel, outlet, or another electrical component, that's something to take seriously.

If you suspect an electrical or burning smell, turn the HVAC system off and have it professionally inspected.

That's not something I recommend waiting around to see if it goes away.

Don't Try to Cover Up the Smell

One of the biggest mistakes you can make is spraying air freshener into the vents or using another product to mask the odor.

That might make the house smell better temporarily, but it hasn't fixed anything.

Think of the smell as your HVAC system giving you a warning.

If you've got wet insulation, you need to find out why it's wet. If you've got a crawl-space moisture problem, that needs to be corrected. If you've got leaking ductwork, it needs to be sealed. And if you've got an overheating electrical component, it needs immediate attention.

Treat the cause, not the smell.

Here's the Takeaway

A fishy smell coming from your AC isn't normal.

In our humid southeastern North Carolina climate, I would definitely investigate moisture around the ductwork, particularly if your ducts are located in a crawl space. Look for standing water, damp insulation, condensation, plumbing leaks, and other sources of moisture.

If your ductwork is in the attic, look for roof leaks or other places water could be getting to the insulation.

And don't overlook the possibility of an electrical problem. If the odor resembles hot plastic or burning electrical material, shut the system down and have it inspected.

Your air conditioner shouldn't make your house smell fishy. Don't just cover up the odor and move on.

Find the source. Stop the moisture or correct the electrical problem. Then deal with whatever was damaged.

​That's how you fix the problem instead of putting a Band-Aid on it.