HVAC decontamination — evaporator biofilm and where odour really comes from

HVAC decontamination — evaporator biofilm and where odour really comes from

When you smell “stale socks” or mild duct odour after turning on A/C, you are not starting the day with a clean cabin — you are circulating bacterial biofilm on the evaporator. The problem grows slowly, invisibly, and spray “A/C cleaners” almost never fix it.

Biofilm growth over time

Move the month slider and watch how much biofilm accumulates on the fins. The reset button simulates studio sanitisation — see the difference between untouched A/C and 18 months of use.

HVAC anatomy and where the problem is

Components

1. Outside air intake — entry under the bonnet, usually with cabin filter (pollen filter).
2. Blower motor — drives air through the evaporator.
3. Evaporator — aluminium fins with refrigerant inside. Air cools here from 25 °C to 7 °C → moisture condenses.
4. Heater core — warms cooled air when heat is needed.
5. Distribution flaps and ducts — route air to vents.

Condensate = permanent environment for biological growth

During normal driving with A/C, 150–300 ml of water per hour condenses on evaporator fins — most drains away, but moisture always remains on fin surfaces and in small creases between them.

What you find there:

• Temperature 5–10 °C → optimum for psychrophilic bacteria (Micrococcus, Pseudomonas, Sphingomonas).
• Humidity 95–100 % → moulds (Aspergillus, Cladosporium, Penicillium).
• Dark, no UV → bacteria and fungi lack inactivation.
• Constant nutrient supply → organic particles from cabin filter (skin flakes, pollen, dust).

A biofilm forms — a slimy polysaccharide matrix where bacterial cells cooperate and resist conventional disinfectants (the matrix protects them).

Odour source — VOCs and sulfides

Biofilm releases:

• Volatile sulfur compounds (DMS, MM, H₂S) — typical “stale socks” smell.
• Volatile amines — fishy odour in advanced stages.
• Aldehydes (from partial oxidation of organic matter) — sharp unpleasant tone.
• MVOCs — microbial volatile organic compounds, typical “musty” feel.

Odour does not spread from ducts alone but from the evaporator fins themselves — air drawn by the blower passes directly through biofilm that actively releases these compounds.

That is why classic “cabin ozonation” does not fix the smell — ozone usually does not reach behind the evaporator cover plastic; it only oxidises dashboard surfaces.

Why enzymatic cabin sprays are not enough

Enzyme sprays (typically proteases + lipases) sold as “A/C cleaner” look logical on paper — enzymes break organic biofilm components. In practice:

After an enzyme spray you mostly smell briefly better from fragrance additives, not removed biofilm. In 2–4 weeks odour returns.

What actually works

DIY level (preventive maintenance)

DIY HVAC maintenance — monthly

1. Cabin filter every 15,000 km or once a year

   Filter is first line of defence. Clogged filter passes more dust to evaporator → more nutrients for biofilm.

2. Dry A/C before shutdown

   Last 5–10 minutes of drive: A/C OFF, fan on max, warm air. Dries condensate on fins.

3. Regular A/C use in winter

   Once a month in winter run A/C 10–15 minutes. Refrigerant circulation, seal lubrication, stagnant moisture purge.

4. Cabin clean Interior Cleaner via towel

   Vents, trims, ducts — wherever touched, dust accumulates (biofilm nutrient).

Studio level (curative intervention)

For a vehicle with existing odour, prevention is not enough. Studio protocol:

1. Endoscopic inspection — camera through evaporator drain channel shows true fin condition.
2. Pre-flush — pressurised water through fins (controlled, to avoid flooding electronics).
3. ClO₂ application — chlorine dioxide generator in closed cabin with A/C on recirculation. 10–20 min exposure.
4. Combined oxidative protocol — if biofilm is mature, ClO₂ + short O₃ post-flush.
5. Full ventilation 15–30 min with fan on max.
6. FEYNLAB® Interior Cleaner on cabin surfaces — removes VOC from plastics that may have absorbed odour.

Studio intervention cost: CZK 1,500 – 3,500 depending on problem age.

Key takeaways

• A/C odour is not “dirty ducts” — it is bacterial biofilm on the evaporator.
• DIY enzyme sprays do not fix the problem — they mask with fragrance.
• Monthly prevention (filter, A/C drying, winter use) strongly slows biofilm growth.
• For odour older than ~2 months, studio ClO₂ intervention is the functional approach.

Further reading

• Ozone vs chlorine dioxide — risk and use comparison
• FEYNLAB® interior portfolio
• Decontamination and prep — why it matters before coating