A musty smell from AC unit is one of the most common comfort complaints we hear from homeowners in Hanover and across the South Shore. That damp, stale odor usually shows up when the system kicks on and can quickly make your home feel uncomfortable or even unhealthy. In many cases, the smell points to moisture problems inside the system rather than a failure of the AC itself.
At Efficiency Plumbing, Heating & Air, we help homeowners identify why that musty smell from AC unit keeps coming back and what it takes to fix it for good—not just mask it with air fresheners.
Why a Musty Smell From AC Unit Is So Common
Air conditioners naturally remove humidity from the air. When everything works properly, that moisture drains away. When it doesn’t, water lingers inside the system and creates the perfect environment for mold, mildew, and bacteria.
In Massachusetts homes, the problem often worsens during humid summer days or shoulder seasons when the AC runs inconsistently. Short cycles, poor drainage, and restricted airflow all contribute to moisture buildup that leads to odors.
If your home smells fine until the AC starts, that’s a strong sign the issue lives somewhere inside the cooling system.
Mold and Mildew Growth Inside the System
One of the most frequent causes of a musty smell from AC unit is mold or mildew growth on internal components. The evaporator coil, drain pan, and nearby ductwork stay cool and damp, which makes them prime targets for microbial growth.
Over time, mold spores spread through the system and circulate throughout your home every time the AC runs. This not only causes odors but can also aggravate allergies and respiratory issues.
The smell often resembles wet laundry, old basements, or mildew. If the odor grows stronger the longer the system runs, microbial growth is a likely culprit.
Clogged or Dirty Condensate Drain Line
Your AC system relies on a condensate drain line to remove moisture collected during cooling. When that line clogs with dirt, algae, or debris, water backs up into the drain pan or air handler.
Standing water quickly becomes stagnant and produces a musty or sour smell that enters the airflow. In some cases, homeowners notice water damage or leaks alongside the odor.
Drain line issues are especially common in older systems or units that haven’t received regular maintenance.
Dirty Air Filters and Restricted Airflow: Musty Smell in Your AC Unit
Airflow problems don’t just affect comfort—they also contribute to odors. When air filters become clogged, airflow slows down and allows moisture to linger on coils and internal surfaces.
Poor airflow can cause:
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Excess condensation on evaporator coils
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Longer drying times inside the system
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Increased mold and mildew growth
Replacing filters regularly helps keep air moving and reduces the conditions that cause a musty smell from AC unit problems.
Ductwork Issues and Odors
Sometimes the smell doesn’t start inside the AC unit itself. Dirty or contaminated ductwork can trap moisture, dust, and organic debris that produce odors when air passes through.
Leaks in ductwork can also pull humid air from crawl spaces, basements, or attics into the system. That air often carries mold spores and musty smells that spread through your home.
In older Hanover homes, duct systems may contain years of buildup that only becomes noticeable when cooling season begins.
Oversized AC Systems and Short Cycling
An oversized air conditioner cools the home too quickly and shuts off before removing enough humidity. While the temperature may feel comfortable, moisture remains in the air and inside the system.
This short cycling prevents proper drying and allows odors to develop. Over time, homeowners notice persistent musty smells even though the AC appears to work “fine.”
Proper system sizing plays a major role in humidity control, especially in coastal Massachusetts climates.
Why DIY Fixes Rarely Solve the Problem
Many homeowners try to solve a musty smell from AC unit issues with scented filters, vent deodorizers, or room sprays. While these may mask the odor temporarily, they don’t address the root cause.
In fact, covering up smells without removing moisture or microbial growth often allows the problem to worsen. Mold and bacteria continue to grow quietly until odors return stronger than before.
Lasting solutions require identifying where moisture builds up and correcting airflow, drainage, or cleanliness issues within the system.
How Professional AC Service Fixes a Musty Smell in Your AC Unit
Professional service targets the source of the smell instead of the symptoms. At Efficiency Plumbing, Heating & Air, we inspect the entire cooling system to find where moisture or contamination develops.
Depending on the cause, solutions may include:
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Cleaning evaporator coils and drain pans
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Clearing and treating condensate drain lines
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Improving airflow and filtration
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Sealing duct leaks or cleaning ductwork
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Adjusting system performance for better humidity control
In some cases, we may recommend additional solutions such as UV lights or dehumidification to prevent future odor problems.
Health and Comfort Concerns You Shouldn’t Ignore
A musty smell from AC unit issues often signals poor indoor air quality. Mold spores and bacteria circulating through the home can trigger headaches, congestion, coughing, and allergy symptoms.
Children, older adults, and those with asthma or respiratory conditions tend to feel the effects first. Even if health symptoms don’t appear right away, prolonged exposure isn’t something to ignore.
Fixing the odor improves not just comfort, but also the air your family breathes every day.
Preventing a Musty Smell From AC Unit Problems
Prevention starts with regular maintenance. Annual AC tune-ups help keep drain lines clear, coils clean, and airflow balanced. Filter changes every one to three months also reduce moisture-related issues.
Keeping humidity under control inside the home plays a major role as well. Using bathroom exhaust fans, addressing leaks, and maintaining proper insulation all help limit excess moisture that feeds odor problems.
Routine care often prevents musty smells before they start.
When to Call Efficiency Plumbing, Heating & Air
If you notice a persistent musty smell from AC unit operation, don’t wait for it to get worse. Odors rarely disappear on their own and usually point to underlying issues that need professional attention.
Efficiency Plumbing, Heating & Air helps homeowners throughout Hanover, MA diagnose and eliminate AC odor problems at the source. Our technicians focus on long-term fixes that restore clean airflow and dependable comfort.
When your AC smells fresh again, your home feels healthier, cooler, and far more comfortable—exactly how it should. Call Efficiency Plumbing, Heating & Air today to schedule AC service and get rid of that musty smell for good.
