Biofilm is a layer of microorganisms bound together in a self-produced slime matrix, stuck to a surface. In HVAC systems it forms on evaporator coils, in drain pans and inside ductwork wherever condensation provides steady moisture. It matters because the matrix physically shields the organisms inside it — which is why a system can be sprayed with disinfectant and still smell a week later.
Why HVAC systems grow it
An air conditioning system produces condensation by design. Warm humid air passes over a cold evaporator coil, water condenses, and it drains away — or does not, if the pan is sloped wrong or the line is partially blocked.
That gives you constant moisture, a steady supply of organic material in the form of dust and skin cells pulled through the return, a stable temperature and no light. It is close to ideal conditions, and it is why HVAC is so often the part of a building that stays contaminated after everything else is cleaned.
What makes biofilm different from surface growth
The matrix — extracellular polymeric substance — is the whole problem. It is sticky, water-resistant and it physically limits what reaches the organisms underneath.
Consequences that matter practically:
- Disinfectants underperform. A product that kills on contact still has to make contact. Much of it is neutralised in the outer layers.
- It regrows fast. Surviving organisms rebuild from an established base.
- It holds odour. The matrix traps the compounds producing the smell, so the smell returns as soon as the system runs.
- It reduces efficiency. A fouled coil transfers heat poorly, so the system works harder for the same output.
Why "spray and walk away" fails
The standard approach is spraying a coil cleaner or disinfectant and letting it drain. On a lightly soiled coil that is fine. On an established biofilm it treats the surface of the matrix and leaves the structure intact, which is the pattern behind smells that come back every cooling season.
What actually works is breaking down the matrix itself rather than trying to kill through it. Enzymes degrade the organic polymers holding it together, so the structure comes apart and what is inside is exposed and removable. Duct-Coil Klear is formulated for coils and duct interiors specifically, and cuts biofilm as part of that.
Signs you have it
- A musty smell that is strongest in the first minutes after the system starts
- A smell that returns within weeks of a duct cleaning
- Visible slime or dark film on the evaporator coil or in the drain pan
- Standing water in the pan, or a slow drain line
- Cooling performance that has drifted without an obvious mechanical cause
Doing it properly
- Replace the filter first, so you are not pulling debris through a cleaned system.
- Clear the drain pan and drain line. Standing water regrows everything.
- Treat the coil with a product that breaks down the matrix, and give it dwell time.
- Fog the ductwork so you are not cleaning the coil and leaving the ducts.
- Fix the underlying moisture problem — pan slope, drain blockage, oversized system short-cycling.
One gallon covers coils and duct fogging for a home up to 2,000 square feet.
Common questions
What is biofilm?
A community of microorganisms held to a surface in a self-produced protective matrix.
Why does my AC smell even after cleaning?
Usually because the biofilm matrix was treated on its surface rather than broken down, so the organisms inside survived and regrew.
Does disinfectant remove biofilm?
It kills what it reaches. The matrix limits what it reaches, which is why disinfection alone often underperforms on established biofilm.
Can biofilm affect cooling performance?
Yes. A fouled coil transfers heat less efficiently, so the system runs longer for the same result.
