Your AC Is Running Ten Months a Year. Mold Knows That.
In South Louisiana, air conditioning isn't a luxury — it's infrastructure. Your system runs from March through December, cycling thousands of cubic feet of air per hour through ductwork that collects condensation, dust, skin cells, and organic debris. That combination of moisture and nutrient material inside a dark, enclosed space is exactly what mold needs. When it colonizes your HVAC system, every room in the house receives a steady supply of spores every time the blower kicks on.
HVAC mold is one of the most common — and most overlooked — contamination sources we find during home inspections in Terrebonne Parish. Homeowners treat surface mold in bathrooms and bedrooms without realizing the ductwork is recontaminating the air as fast as they clean. We address mold at its source: the air handler, evaporator coil, condensate system, and the full duct network from supply plenums to return grilles.
Why HVAC Mold Is So Common in Terrebonne Parish
Every factor that makes South Louisiana comfortable to live in makes it hostile to HVAC systems. The combination of run time, humidity load, and construction patterns creates mold conditions that don't exist in most of the country.
Condensation Inside the System
When your AC runs, the evaporator coil cools incoming air below its dew point, pulling moisture out of the air as condensation. That's how air conditioning dehumidifies — by design. In a dry climate, this moisture drains quickly and the system stays relatively dry between cycles. In Terrebonne Parish, the humidity load is so high that the evaporator coil and the drain pan beneath it are wet almost continuously during the warm season. The interior surfaces of the air handler cabinet, the supply plenum immediately downstream of the coil, and the first several feet of supply ductwork are also consistently damp from condensation.
This persistent moisture, combined with the dust and organic debris that accumulates on coil fins, in drain pans, and along duct interiors, creates an ideal environment for mold growth. The system essentially builds its own mold habitat: a dark, wet, nutrient-rich surface with a constant supply of fresh spores drawn in from outdoor and indoor air.
Extended Run Time & Aging Systems
AC systems in this climate run nine to ten months a year — far longer than systems in most of the country. This extended run time accelerates wear on components that are critical for moisture management. Condensate drain lines clog more frequently. Drain pans develop biofilm and rust through faster. Evaporator coils accumulate buildup that reduces their efficiency and increases the amount of condensation that stays on the coil surface instead of draining properly.
Many homes in Houma and surrounding communities have HVAC systems that are ten, fifteen, or twenty years old — systems that have been running in this climate for their entire lifespan without the duct cleaning or coil maintenance that would prevent mold buildup. The ductwork in these homes has accumulated years of dust, debris, and microbial growth that a filter change alone can't address. We commonly find systems where the interior of the ductwork has a visible layer of buildup that supports active mold colonies along its entire length.
Attic-Mounted Equipment
In newer construction and many renovated homes across Terrebonne Parish, the air handler is located in the attic. This placement exposes the unit and its associated ductwork to extreme temperature swings — attic temperatures that exceed 140°F during summer days drop rapidly when the sun goes down, causing condensation on the exterior of duct surfaces and inside the air handler cabinet. Attic-mounted ductwork also runs through unconditioned space, which means the temperature differential between the cold supply air inside the duct and the hot attic air outside creates condensation on both sides of the duct wall.
Improperly insulated or poorly sealed duct joints in an attic installation leak conditioned air into the attic space and draw hot, humid attic air into the duct system — both of which increase moisture levels inside the ductwork. We see mold growth concentrated around duct joints, at connections to the air handler, and on flexible duct sections where sagging creates low spots that trap condensation.
The Condensate Drain Problem
The condensate drain line is supposed to carry moisture from the drain pan to the exterior of the home. In practice, these lines clog frequently — algae and biofilm buildup in the warm, wet line is the most common cause, followed by debris, insect nests, and physical damage to the line. When the drain clogs, water backs up into the drain pan, overflows into the air handler cabinet, and can spill into the attic floor, ceiling cavity below, or the interior of the ductwork.
A clogged condensate drain in summer can produce several gallons of water per day that has nowhere to go. We've responded to homes where a clogged drain went unnoticed for weeks — particularly in homes where the owner was offshore — resulting in extensive water damage to the attic floor, ceiling drywall below, and heavy mold growth throughout the air handler and adjacent ductwork. Regular drain line maintenance is one of the simplest and most effective things a homeowner can do to prevent HVAC mold, and we include drain line assessment in every HVAC mold inspection.
How We Remediate HVAC & Duct Mold
HVAC mold remediation is more involved than a standard duct cleaning. Cleaning removes dust and debris; remediation eliminates active mold contamination, treats the surfaces where growth occurred, and addresses the moisture conditions that allowed colonization in the first place.
HVAC Inspection & Sampling
We inspect every accessible component of the system: the return air grille and return duct, the filter housing, the blower compartment, the evaporator coil and drain pan, the supply plenum, and representative sections of supply ductwork including at register boots. We use borescope cameras to inspect inside ductwork without disassembly, and we take air quality samples at supply registers and at the return to compare spore levels entering and leaving the system.
Surface samples are collected from the evaporator coil, drain pan, and any visible growth inside the ductwork for lab identification. This tells us what mold species are present in the system and informs the treatment approach. We also take moisture readings on the air handler cabinet, ductwork, and surrounding materials to identify condensation patterns and any active water intrusion from drain line failures.
System Isolation & Containment
Before beginning remediation, we shut down the HVAC system and seal supply and return registers throughout the home to prevent cross-contamination during the cleaning process. For systems with heavy contamination — particularly where toxigenic species like Aspergillus or Stachybotrys have been identified in the ductwork — we establish negative air pressure containment around the air handler and the primary duct runs we'll be accessing.
This step is important because disturbing mold growth inside ductwork releases spores into the airstream. Without isolation, running the system or even the air movement created by opening duct access panels can push those spores into every room of the house. We keep the system off and the ducts sealed until remediation is complete and clearance testing confirms the air quality meets acceptable standards.
Coil, Drain Pan & Air Handler Treatment
The evaporator coil is cleaned with a coil-specific antimicrobial cleaner that removes biofilm, mold growth, and accumulated debris from the coil fins without damaging the aluminum. The drain pan is cleaned, inspected for corrosion or damage, and treated with a sustained-release biocide tablet to inhibit future microbial growth. The drain line is flushed and cleared, and we verify that it drains freely to the exterior.
The interior of the air handler cabinet — the blower compartment, the supply plenum, and all accessible interior surfaces — is HEPA-vacuumed, wiped down with antimicrobial solution, and inspected to confirm that all visible growth has been eliminated. The blower wheel and housing, which accumulate significant debris over years of operation, are cleaned as part of this process. A clean blower wheel also improves system airflow and efficiency, which helps reduce condensation issues going forward.
Duct Cleaning & Treatment
Supply and return ducts are cleaned using a combination of mechanical agitation (brushes and air whips) and HEPA-filtered negative air vacuum to remove accumulated debris, dust, and mold growth from the interior duct surfaces. For metal ductwork, we follow the mechanical cleaning with an antimicrobial fog that treats the interior surface to eliminate residual mold and inhibit recolonization.
For flexible ductwork that is heavily contaminated — particularly flex duct that has been wet from condensation, attic water intrusion, or drain line overflow — replacement is often the better option. Flex duct's corrugated interior traps debris and moisture in ways that make thorough cleaning difficult, and the insulation layer surrounding the duct can harbor mold that's inaccessible from the interior. We evaluate each duct run individually and recommend replacement only when cleaning won't achieve an acceptable result.
Moisture Source Correction
Cleaning and treating the system addresses the existing contamination, but preventing recurrence requires fixing the moisture conditions that caused it. Depending on what the inspection reveals, this may include clearing and treating the condensate drain line, repairing or replacing a corroded drain pan, adding or improving duct insulation to reduce exterior condensation, sealing duct joints and connections to prevent humid air infiltration, and verifying that the system is properly sized for the home — an undersized system runs longer cycles that increase moisture buildup without adequate dehumidification.
Post-Remediation Testing
After the system is cleaned, treated, and reassembled, we run it through a full cycle and then collect post-remediation air quality samples at supply registers and the return air intake. These samples are sent to the same accredited lab used for the initial testing, and the results confirm that spore levels in the supply air have returned to acceptable ranges. The complete report — initial and post-remediation lab data, photos, treatment documentation, and maintenance recommendations — is provided for your records and is formatted for insurance submission where applicable.
Signs Your HVAC System Has a Mold Problem
Musty Smell When the AC Runs
A musty, earthy, or stale odor that appears when the air conditioning cycles on and fades when it shuts off is the most common indicator of mold inside the ductwork or air handler. The blower pushes air across contaminated surfaces and delivers the odor to every room through the supply registers. If the smell is stronger at certain registers, the contamination may be concentrated in the duct runs feeding those vents.
Visible Growth at Registers
Dark spots, discoloration, or fuzzy growth on or around supply register grilles is mold that has colonized the last few inches of the duct where it meets the register boot. This is visible mold — but it's almost always just the tip of the problem. If mold is growing at the register, conditions inside the duct behind it are favorable for growth along a much larger area. The register growth is what you can see; the duct growth is what's producing the spore load.
Allergy Symptoms Tied to AC Use
If household members experience congestion, sneezing, coughing, headaches, or eye irritation that worsens when the AC is running and improves when windows are open or when you're away from home, airborne mold spores from the HVAC system are a likely cause. This pattern — symptoms that follow the AC cycle — is one of the most reliable indicators we hear from homeowners during initial phone consultations.
Black Dust Around Vents
Dark residue or "black dust" on walls and ceilings around supply registers indicates that contaminated particulate is being blown out of the ductwork. This residue is a combination of dust, microbial growth, and spore deposits that accumulate where the supply air exits the register and meets the room air. If you're cleaning dark marks around your vents regularly and they keep coming back, the source is inside the system.
Water Stains Near the Air Handler
Water stains on the ceiling below an attic-mounted air handler, or water pooling around a closet-mounted unit, indicates a condensate drain failure. The moisture that should be leaving the building through the drain line is instead accumulating inside and around the air handler — creating exactly the conditions mold needs. A current or recent drain overflow also means the interior of the air handler has been wet, which likely means mold is already growing inside the cabinet and on the evaporator coil.
System Running Constantly
An AC system that runs without cycling off is struggling to reach thermostat setpoint — often because the evaporator coil is coated with biofilm and debris that reduces its ability to transfer heat. A dirty coil not only grows mold but also forces the system to run longer, which increases condensation, which feeds more mold growth. It's a cycle that worsens over time and increases both your energy bill and your mold exposure simultaneously.
Maintenance Steps That Actually Matter
You can't eliminate HVAC mold risk in this climate, but you can significantly reduce it with targeted maintenance. These are the interventions that make the biggest difference — ranked by impact, not by how often they're talked about.
Keep the Condensate Drain Clear
A clogged condensate drain is the single most common cause of HVAC mold we encounter. Flushing the drain line with a cup of distilled white vinegar every one to two months during the cooling season prevents algae and biofilm from blocking the line. If your system has an accessible drain pan, check it during filter changes — any standing water means the drain isn't flowing freely. A float switch on the drain pan provides a safety shutoff if the line clogs, preventing overflow damage. This one maintenance step prevents more HVAC mold problems than any other.
Change Filters on Schedule
A dirty filter restricts airflow across the evaporator coil, which lowers the coil temperature and increases condensation. It also allows more dust and organic debris to reach the coil and ductwork, providing nutrients for mold growth. In Terrebonne Parish's high-dust, high-humidity environment, filters need changing more often than the manufacturer's generic recommendation — every 30 to 60 days for pleated filters during the cooling season, not every 90 days. Use the highest MERV rating your system can handle without restricting airflow; your HVAC technician can advise on the right balance for your equipment.
Annual Coil Cleaning
Having the evaporator coil professionally cleaned once a year — ideally at the start of cooling season in spring — removes the biofilm and debris buildup that supports mold growth on the coil surface. A clean coil also dehumidifies more effectively and runs more efficiently, which reduces both moisture levels in the system and your energy costs. This is different from a duct cleaning; it's a targeted treatment of the component that's most vulnerable to mold colonization.
Don't Turn the System Off for Extended Periods
Homeowners who travel or work offshore sometimes turn off the AC to save energy while they're away. In Terrebonne Parish's climate, this can be counterproductive — indoor humidity climbs rapidly in an unoccupied, unconditioned home, and the moisture that accumulates on duct surfaces, furniture, and building materials during a two-week absence can produce mold growth throughout the home, not just in the HVAC system. Setting the thermostat to 78–80°F while away keeps the system cycling enough to manage humidity without running up a large energy bill.
HVAC & Duct Mold Questions
Is HVAC mold removal the same as duct cleaning?
No. Standard duct cleaning removes dust and debris from the interior of ductwork using brushes, air whips, and vacuum. It does not specifically address mold contamination, treat surfaces with antimicrobial agents, or verify post-cleaning air quality with lab testing. Mold remediation includes the mechanical cleaning but adds species identification through lab sampling, antimicrobial treatment of contaminated surfaces, assessment and correction of moisture sources (drain issues, condensation, duct insulation), and post-remediation air quality verification. If your system has active mold growth, duct cleaning alone won't resolve the problem — and if the moisture source isn't addressed, the mold will recolonize the cleaned surfaces within weeks.
How often should I have my ducts cleaned in this climate?
For homes in Terrebonne Parish, we recommend a professional duct inspection every two to three years and cleaning when the inspection shows it's warranted — which, given the humidity and run time in this climate, is more often than national averages suggest. If you have known moisture issues (a history of drain line problems, attic-mounted equipment, or a home that floods), annual inspection is appropriate. Between professional cleanings, changing filters on schedule and maintaining the condensate drain are the most effective things you can do to keep the system clean.
Can HVAC mold make you sick?
Yes. A contaminated HVAC system delivers mold spores to every room in the house through the supply air. Occupants inhale these spores continuously while the system is running. Common symptoms include chronic sinus congestion, recurring headaches, coughing, sneezing, eye and throat irritation, and worsening of asthma symptoms. The severity depends on the mold species present, the spore concentration, the duration of exposure, and individual sensitivity. Children, elderly residents, and people with respiratory conditions or immune compromise are at higher risk. In homes where the HVAC system is the primary mold source, occupants often report that symptoms improve dramatically within days of remediation.
Should I replace my ductwork or can it be cleaned?
Metal ductwork (galvanized steel or aluminum) can almost always be effectively cleaned and treated. The smooth interior surface allows thorough mechanical cleaning and antimicrobial treatment. Flexible ductwork is more complicated — the corrugated interior traps debris and moisture, and the fiberglass insulation layer surrounding the inner liner can harbor mold that's inaccessible from inside the duct. For flex duct with light surface contamination, cleaning may be sufficient. For flex duct that has been significantly wet (from condensation accumulation, attic leaks, or drain overflow) or has heavy mold growth, replacement is usually more cost-effective and produces a better long-term result. We evaluate each duct run individually and recommend the most practical approach.
Will UV lights in my HVAC system prevent mold?
UV-C germicidal lights installed in the air handler — specifically positioned to irradiate the evaporator coil surface — can help suppress mold growth on the coil by disrupting microbial reproduction. They work best as a preventive measure after the system has been professionally cleaned, not as a substitute for remediation of existing contamination. UV lights have limited effectiveness against mold that's already established on surfaces, and they do nothing to address contamination inside ductwork beyond the immediate area of the lamp. Think of them as one layer of prevention, not a standalone solution. A UV light on a dirty, mold-contaminated coil is like putting an air freshener in a room that hasn't been cleaned — it addresses the symptom without touching the problem.
Does insurance cover HVAC mold removal?
Coverage depends on the cause. If HVAC mold resulted from a covered event — water damage from a storm, a burst pipe that flooded the air handler, or a mechanical failure — most policies will cover the remediation. Mold that developed gradually from deferred maintenance (years of skipped filter changes and no coil cleaning) is typically classified as a maintenance issue and not covered. Our inspection and lab documentation establishes the cause and timeline of the contamination, which helps your adjuster determine coverage. In cases where the HVAC mold is part of a larger remediation — for example, the system was contaminated during a flood that also affected the rest of the house — it's usually covered as part of the overall water damage claim.
Addressing Mold Beyond the HVAC System
Mold Inspection & Testing
Air quality sampling at supply registers and return intakes tells you exactly what your HVAC system is putting into your air — and whether it's the source of your symptoms.
Learn more →Attic Mold Removal
Attic-mounted air handlers and ductwork often develop mold alongside the attic structure itself. We remediate both the system and the space around it.
Learn more →Black Mold Removal
When lab testing identifies toxigenic species like Aspergillus or Stachybotrys in the HVAC system, full containment remediation protocols apply.
Learn more →