Why Your HVAC System Smells Musty, and What It Means

A musty HVAC smell usually means moisture or microbial buildup inside the system or ducts.

As cooler weather arrives on Delmarva, many homeowners notice a familiar but unwelcome scent when they turn on their heat for the first time: a musty, damp smell coming from the vents. It’s a common sign that your home’s indoor air quality needs attention.

A musty smell doesn’t always mean something serious, but if you don’t address the root cause, you may be facing a serious issue later. Poor indoor air quality and high humidity in your home can lead to mold growth, damage to your home, and even health issues over time.

Let’s cover all the possible causes of that foul-smelling air, and what you can do about it.

Indoor HVAC condenser unit in the utility room of a Delmarva home.

What Causes a Musty Smell in an HVAC System?

Possibility 1: Moisture Buildup in the Ducts

Condensation naturally forms inside your air ducts, especially when warm, humid air meets cool surfaces. If that moisture lingers, it can create the perfect environment for mildew and bacteria to grow. That growth can then cause a stale or musty odor every time your system runs.

Possibility 2: Dirty or Clogged Air Filters

When air filters become saturated with dust and humidity, they trap odor-causing particles. The result: a musty, “wet dust” smell that circulates through your home. Replacing filters regularly helps prevent these odors and keeps your system efficient.

Here’s a link to our fall HVAC maintenance checklist, with tips on how regularly to change air filters, depending on your home’s unique needs.

A Comfort Plus Services HVAC technician installs a new air filter during a maintenance visit to a Sussex County home.

Possibility 3: Mold or Debris on the Evaporator Coil

Your HVAC coil removes humidity from the air. When it gets coated in dust or debris, the moisture it collects can stagnate, leading to microbial growth. That damp smell is often the first sign your system needs cleaning.

Possibility 4: High Indoor Humidity

During the mild weather we experience in fall and spring, a typical HVAC system in our area runs less frequently, which means humidity levels can rise indoors. Elevated humidity (above 50 percent) contributes to that persistent, musty odor. Keep reading to see how a dehumidifier is an essential component of your home comfort system!

How Do I Fix a Smelly HVAC System?

Schedule a Professional HVAC Inspection

A certified HVAC repair technician can locate the source of odors in your ductwork, drain pan, or coils, and clean the affected areas safely. At Comfort Plus Services, our NATE-certified technicians inspect, clean, and sanitize system components to restore fresh airflow. Early inspection prevents small issues from turning into expensive repairs.

Replace or Upgrade Your Filters

Change filters every 1-3 months, or more often if you have pets or allergies. Consider high-efficiency filters that capture smaller particles and improve airflow. When humidity outside is at its highest, consider changing your filter more frequently. It may tend to get dirty faster when the air moving through it contains a lot of moisture.

An HVAC technician prepares to inspect a whole-home air purifier filter.

Book Seasonal Maintenance

Just before the weather changes is the perfect time to schedule HVAC maintenance, and that’s why we usually schedule tune-ups for our HVAC Service Plan customers starting at the end of summer. A professional tune-up ensures your system is clean, well-lubricated, and ready for the winter heating season, while also preventing odors when you turn up the heat.

Comfort Plus HVAC technicians conduct a full HVAC system inspection during a seasonal maintenance service call.

Improve Indoor Air Quality

Musty smells can be eliminated for good with better ventilation and air purification. Whole-home dehumidifiers, UV lights, and air purifiers work together to clean the air and reduce moisture. Explore Comfort Plus Services’ indoor air quality solutions to protect your home year-round.

Watch the video to learn more about how our most popular whole-home air purifier works.

Video: How the REME HALO® Whole Home In-Duct Air Purifier cleans the air in your home

Enroll in a Service Plan

Preventive care pays off. CPS offers Service Plans for all heating system types: electric HVAC, propane or natural gas heating systems, and heating oil systems. Service Plans include seasonal tune-ups, priority scheduling, and repair discounts, making it easy and convenient for you to keep your home comfortable and odor-free all year long.

When to Call the Experts

A musty smell may start small, but it’s a warning sign your HVAC system needs professional attention. The NATE-certified technicians at Comfort Plus Services can inspect, clean, and repair your system to restore fresh airflow and comfort.

Serving homeowners across Maryland’s Eastern Shore and Delaware, Comfort Plus Services is the most trusted (and highest rated) local team for year-round indoor comfort and 24/7 emergency heating and HVAC repairs.

Frequently Asked Questions (FAQs)

A musty smell often appears when cooler weather causes your HVAC system to cycle less frequently, allowing humidity to build up in the ducts or on the evaporator coil. Dust, condensation, or a clogged air filter can trap moisture and create that damp odor. Homeowners across the Eastern Shore and Delaware notice this most during fall and spring transitions. A professional HVAC maintenance visit can clear out buildup, check drainage, and restore fresh, clean airflow.

Yes, poor indoor air quality (IAQ) is often to blame for persistent bad smells, even if the original cause of the smell was an HVAC system in need of servicing. Excess humidity, inadequate ventilation, and hidden microbial growth can all contribute to musty odors that linger even after cleaning.

Installing a whole-home dehumidifier, air purifier, or UV light can help remove these contaminants. If you live on Maryland’s Eastern Shore or in Delaware, Comfort Plus Services offers indoor air quality solutions designed to eliminate odors at the source and keep your air healthy year-round.

A Comfort Plus Services technician prepares to enter a Rehoboth Beach home, carefully covering his shoes to protect the homeowner's floor.

Call on Comfort Plus Services whenever you need up-front pricing, quality work with a satisfaction guarantee, and quick attention to your business’ HVAC and plumbing needs. Since 2013, Comfort Plus has been providing professional and dependable service, and hundreds of five-star Google Reviews are just a snapshot into how our customers feel about our commitment to going above and beyond, in all that we do.

Still Have Questions?

Contact Comfort Plus Services today! Our friendly, knowledgeable staff is always happy to help you find the right HVAC or plumbing solution for your home.