Mold in an HVAC system usually starts in the air handler, not in the ducts. The evaporator coil is wet every time the system cools, the drain pan holds water, and the blower sends whatever grows there into every room the system serves. That is what turns a room problem into a building problem.
This page stays on the air handler: coil, condensate pan and line, blower, plenum, filter rack and cabinet. Ductwork has its own page.
Nothing else in a building assembles all four growth requirements as reliably. The coil is wet by design, because air conditioning dehumidifies by condensation: cooling works by making a surface cold enough that water comes out of the air onto it.
The cabinet is dark and enclosed, so nothing dries between cycles, and whatever dust gets past the filter lands on wet aluminum as food. Water, food, darkness and a steady temperature is close to ideal.
What makes it a building problem is the blower. A damp closet contaminates a closet. An air handler is connected to every room and runs several times an hour all summer.
What forms between the fins is biofilm, a mat of dust bound together by growth, in a pack with twelve to sixteen fins per inch. Most sits on the upstream face, so a technician who lights the coil and calls it clean has seen one side.
The first symptom is airflow, not smell. A loaded coil restricts air and transfers heat poorly, so you get weak delivery at the far registers, longer run times, higher bills and less dehumidification.
Cleaning means reaching both faces, working a cleaner through the pack, and rinsing until the loosened material leaves rather than settling in the pan. Where fins are bent flat or growth sits deep, replacement is the honest answer.
A clogged condensate line is the most common cause of water inside an air handler, and the most preventable.
The pan drains through a small line, usually three quarter inch PVC with a trap. Warm, wet and dark, it grows slime and algae until a plug forms, and the pan fills with standing water under the coil.
A switch that keeps tripping is an annoyance, and the fast fix is to bypass it, which removes the only thing between a blocked drain and a ruined ceiling. If anyone jumps a float switch without clearing the line, find another contractor.
The blower is a squirrel cage wheel, and each small blade holds a cup of caked dust once it has run in dirty air. That cuts airflow and fouls a large surface downstream of a wet coil.
It cannot be cleaned in place: the cups face inward and a brush does not reach them. Cleaning means pulling the assembly, taking the wheel off the shaft and washing it outside the unit.
The supply plenum takes the coldest, wettest air in the system, and it is often duct board or sheet metal lined on the inside with fiberglass.
Growth on metal cleans off. Growth into a fibrous liner does not, because the material holds it where cleaning cannot reach, and scrubbing shreds the liner into the airstream. Contaminated liner gets replaced, usually with a metal plenum insulated on the outside, which is the physical source removal ANSI/IICRC S520 calls for everywhere else.
The filter is only as good as its seal. Gaps around the frame, a missing door or a filter narrower than the slot let air travel around it.
That is why a MERV 13 filter in a leaky rack can perform worse than a MERV 8 in a sealed one: the better filter is more restrictive, which raises the pressure across it and pushes more air through the gaps. Seal the rack before upgrading the filter.
The cabinet is lined with fiberglass, a porous material inside the wettest box in the building. Once an overflowing pan has soaked it, it grows the way a duct liner does, so it gets cut out and replaced, commonly with closed cell foam board.
Two different devices get sold as one idea, and the evidence behind them is not the same.
Coil-mounted UV-C is a lamp inside the cabinet aimed at the coil and pan. It does what it claims: it lights a fixed surface at close range with unlimited exposure, and keeps a cleaned coil from fouling as quickly.
In-duct air treatment puts lamps or devices in the airstream to treat air as it passes. Results are far more variable, and the reason is exposure time, since air passes a lamp in a fraction of a second. The same caution applies to ionizers and photocatalytic add-ons, where performance varies by product and some designs make ozone.
Neither removes existing growth, and neither fixes a drainage failure. Clean, fix the drainage, then add a lamp if you want one.
A maintenance call is a filter change, a coil rinse from the accessible side, a drain blow out and a performance check. A remediation-grade cleaning means opening the equipment and taking the internals out:
That requires a licensed HVAC technician, so the remediator and the HVAC contractor coordinate. It is a different job from the cheap whole-system special that arrives by phone, which rarely includes pulling a blower wheel. If a quote does not say what comes out of the cabinet, it is not a system cleaning.
Read more: spotting remediation scams · Read more: HEPA vacuums
A running HVAC system during remediation is a distribution machine pointed at the rest of the building.
It gets shut down and sealed off for the duration. Registers in and near the containment are covered with plastic and tape and the return is sealed, so it cannot pull contaminated air out of the work area. Where a building cannot go without cooling, only the equipment serving the affected zone stays off. A new filter goes in at closeout, because the old one puts its captured load straight back into the airstream.
Read more: how containment is built · Read more: clearance testing
None confirms mold alone, but together they justify a look at the system.
Almost every air handler mold problem traces to one of four things.
Read more: dehumidifiers and humidity control
Some of it, and the split is clean.
Reasonable for a property owner. Changing filters. Sealing obvious gaps in a filter rack. Flushing a condensate drain line. Watching indoor humidity and the supply registers.
Not a homeowner job. Opening an air handler and cleaning a coil is HVAC work, involving electrical disconnection, fins that bend and refrigerant lines you do not want to disturb. Pulling a blower wheel is the same, and contaminated plenum liner belongs to a remediation contractor.
A small, visible, isolated area with an obvious moisture source is reasonable for a careful owner with proper PPE. Beyond that, or wherever growth is hidden, the HVAC system is involved, or anyone has health concerns, the job needs a built containment and a contractor. HVAC is on that list because a system reaching every room removes the isolation that makes a small job small.
Read more: the mold remediation process
Yes, and it is one of the most likely places in a building for it. The coil is wet whenever the system cools, the drain pan holds water, the cabinet is dark and enclosed, and dust arrives continuously as food. Those conditions suit growth almost perfectly.
It depends, and an assessment settles it. If there is active growth in or near the system, running it distributes spores to every room it serves. But shutting cooling off in a hot humid climate drives humidity up, which can make things worse. Isolating the affected zone usually beats shutting everything off.
Coil-mounted UV-C genuinely helps keep a cleaned coil and drain pan from fouling again, because it lights a fixed surface continuously at close range. In-duct air treatment devices are far more variable, since air passes the lamp in a fraction of a second. Neither removes growth already there.
Most can be cleaned. A full cleaning means removing the blower assembly, cleaning the coil through the fin pack, clearing the pan and drain line, and replacing porous liner and insulation rather than treating it. Replacement is the answer for a coil too fouled to clean.
Because the smell is coming from the equipment, not the room. Odor that appears at the start of a cycle and fades as the system runs usually means growth on the evaporator coil, water in the drain pan, or a fouled blower wheel. Act on it rather than mask it.
Flush it at the start of cooling season and again partway through, so twice a year in most climates and more often where the system runs nearly year round. Water or vinegar poured through the access tee takes minutes, and a blocked line is the most preventable cause of water inside an air handler.
MI&T is an inspection and testing company. We do not perform mold remediation, we do not clean HVAC systems, and we do not sell equipment or treatment devices, so we have nothing to gain from what a contractor recommends.
What we do is establish whether there is a mold problem, how far it extends, and whether the HVAC system is spreading it, then verify independently that the cleaning worked. That verification matters more here than almost anywhere, because the system delivers its result to every room.
Find your nearest MI&T location to schedule an inspection or clearance test.
