The call usually starts the same way. The air smells musty when the system kicks on, and it fades once it has been running a while. Or somebody looked into a supply register with a flashlight and saw something dark on the inside of the boot.
Both are worth taking seriously. Neither tells you where the growth actually is.
Start at the air handler, not the ducts
An air conditioning system in Central Texas runs most of the year, and while it runs it is deliberately making a cold wet surface inside a box. That is what an evaporator coil is. Warm humid air passes over it, water condenses out, and it drains away.
Everything about that arrangement favours growth, and the components in question are all in one place.
The evaporator coil. Permanently wet during operation, with fins close enough together to trap dust. Dust plus water is a growing medium. A coil that has never been cleaned in fifteen years is the most likely single source of a musty smell in a Brazos County house.
The drain pan. Standing water sits here whenever the drain is slow. Older galvanised pans rust through; the biological film that develops in a slow-draining pan is exactly what you are smelling.
The condensate line. Blocked lines back water up into the pan and, in attic units, into the secondary pan and then the ceiling. This is also the cause of a good share of the ceiling water damage we get called to.
The blower compartment and plenum. Interior fibreglass duct board liner, once it gets damp, cannot be cleaned. It has to come out.
The filter, and the gap around it. An undersized or badly seated filter lets unfiltered air bypass straight onto the coil, which is how coils load up with dust in the first place.
When the ducts genuinely are affected
Flex duct with a fibreglass inner liner can support growth once it has been wetted. In this area that happens for three reasons: duct running through a hot humid attic sweats where insulation has failed, supply air is delivered so cold in a very humid house that condensation forms inside the boot, or the system was run during construction or after a flood with no filter in place.
The visual tell is growth at the register boots and the first few feet of duct, worst on the runs furthest from the air handler.
Why we will sometimes tell you not to clean the ducts
Routine duct cleaning as a maintenance ritual has weak evidence behind it, and we would rather say so than sell it.
It is worth doing when there is visible growth inside the duct or on the boots, when the system was operated during a water event or a fire, when there has been rodent or insect infestation, or when a remediation upstream requires the system to be verified clean.
It is not worth doing because a company knocked on the door with a coupon, or because it is spring. And it is actively pointless when the source is a colonised coil, because you are cleaning downstream of a problem that will re-contaminate the ducts within weeks.
How the work is done properly
System off and contained. The air handler is shut down and locked out. Supply and return openings are sealed during work so debris is not distributed.
Coil cleaning. The coil is accessed, sprayed with an appropriate cleaner, and rinsed through to the pan. Badly fouled coils sometimes have to be pulled to reach the upstream face, which is where the loading actually is. Anyone who cleans only the visible downstream face has cleaned the wrong side.
Pan and drain. The pan is cleaned or replaced, the trap is cleared, the line is flushed and its slope checked. A pan tablet is a maintenance aid afterwards, not a substitute.
Duct work. Source removal with brushing and high-power HEPA vacuum extraction under negative pressure. Flexible duct that is contaminated on the inner liner is replaced rather than cleaned, because you cannot mechanically clean a fibreglass liner without shedding it. Metal duct with fibreglass duct board interior liner: same answer.
Fogging, and what it is worth. Applying an antimicrobial after source removal is reasonable. Applying it instead of source removal is not, and a system that has only been fogged smells better for a few weeks and then does not.
The UV question. A properly sized UV-C lamp aimed at the coil does suppress regrowth on the coil surface. It does nothing for the ducts and nothing for a drain pan full of water. Useful as maintenance after cleaning, useless as a substitute for it.
The humidity problem behind all of it
Here is the part specific to this climate, and it explains why systems here foul faster than the same equipment installed further north.
An air conditioner removes humidity as a side effect of cooling. If the system is oversized for the house — which a great many are, because bigger felt safer to whoever specified it — it satisfies the thermostat quickly and shuts off before it has run long enough to dehumidify. Short cycling produces a cold house with high relative humidity.
Indoor relative humidity above roughly 60% supports growth on almost any surface in the building, not just inside the equipment. If you have persistent musty smells in several rooms rather than one, put a cheap hygrometer in the hallway before you spend money on duct cleaning. The number it gives you will tell you whether you have an equipment problem or a whole-house humidity problem.
Where the reading stays high, the answers are a correctly sized system, a variable-speed blower that runs longer at lower capacity, or a whole-house dehumidifier tied into the return. That is a mechanical contractor's work rather than ours, and we will tell you when that is what you actually need.
Related: attic mold removal, since attic duct problems and attic growth usually arrive together, and mold testing if you want the air sampled before and after.
Common questions
My house smells musty when the AC starts. What is it?
Most often the evaporator coil or the drain pan. The smell being strongest at start-up and fading is characteristic, because the first air across the coil carries the accumulated odour.
Should I get my ducts cleaned every year?
No. Clean them when there is a reason: visible growth, a water event, pests, or a remediation that requires it. Annual cleaning with no findings behind it is not money well spent.
Can flex duct be cleaned?
The interior fibreglass liner cannot be mechanically cleaned without damaging it. Contaminated flex gets replaced. Bare sheet metal duct cleans fine.
Will a UV light fix this?
It helps keep a clean coil clean. It will not clean a dirty one, and it does nothing beyond its line of sight.
Is my system making my house humid?
It can be, if it is oversized and short cycling. Measure indoor relative humidity for a few days. Consistently above 60% points to a system or building problem rather than a duct problem.
Do you replace the air handler?
No, that is a licensed mechanical contractor's work. We clean, remediate and document, and we will tell you plainly when the honest answer is new equipment rather than cleaning old equipment.
Get a free assessment
Tell us what you are seeing and where. We come out, find the moisture source, and put the scope and the price in writing before anything is torn out.