Mould In The Ducts
Mould in a duct system is almost never in the middle of a duct run. It is at the coil, in the drain pan, or on a wet duct liner.
Short answer
Growth needs moisture, and the moisture in a forced-air system is at the evaporator coil and its drain pan during cooling, on a duct running through a cold unheated space, and on a humidifier left connected over summer. Cleaning the ducts without addressing those is treating the wrong thing.
Where the moisture is
The evaporator coil, which is wet by design whenever the air conditioning runs, and the drain pan beneath it.
A blocked condensate drain, which leaves standing water in the pan — the single most common source of a musty smell when the cooling starts.
A duct running through a cold crawlspace, an unheated garage or against an exterior wall, where warm moist air condenses on the inside.
A furnace humidifier left connected over the summer, which is a wet component in the cooling supply air.
The air handler cabinet and the filter slot, which draw unfiltered air from whatever space they sit in.
And flexible duct with a wet interior liner, which is porous and is not cleanable to a neutral state.
Check the drain pan
Look in the drain pan under the evaporator coil. Standing water, or water in the secondary pan, is the answer in a large share of cases, and it is a drain clearance rather than a cleaning contract. If the system has a float switch that keeps tripping, that is the same story from the other end.
The remediation order
Clear the condensate line, correct its fall, and wash out the pan.
Clean the coil properly, which means opening the cabinet rather than spraying through a register.
Clean the blower wheel and the cabinet interior, which hold more than the ducts do.
Seal the filter slot and the cabinet panels so unfiltered air is not bypassing.
Replace flexible duct with an affected liner rather than cleaning it.
Insulate or reroute duct running through a cold space, which is what stops the condensation recurring.
And then clean the ducts, which at that point is finishing the job rather than the job itself.
What not to buy
A fogging or sanitizing pass with the drain pan still holding water, which treats the symptom for a few weeks.
A duct cleaning that does not include the coil, the blower and the cabinet.
Cleaning scheduled before a renovation finishes, which just reloads.
An ultraviolet lamp presented as a substitute for cleaning, rather than as a supplement after it.
A biocide applied inside the ductwork without a product label that permits that use, which is worth asking about.
And a quote that does not name where the moisture is coming from, because without that it is not a remediation.
Common questions
Is mould usually in the duct runs?
No. It is at the coil, in the drain pan, or on duct in a cold space.
What should I check first?
The condensate drain pan under the evaporator coil.
Can flexible duct be cleaned?
Not once the liner is affected. That run gets replaced.