[Daily Mildew Prevention for Air Conditioners] Can "running the air conditioner for 15 minutes before shutting down the air conditioner really prevent mildew?" An in-depth analysis of the causes of biofilm and sour taste in water collection trays

Moldy air conditioner smell: an invisible killer of indoor air health

In Taiwan’s humid and sultry rainy season and midsummer, air conditioning is an indispensable equipment for every household. But many people often encounter an embarrassing situation: within the first few minutes of turning on the air conditioner, a moist gas with a strong "musty rag smell" or "sour vinegar smell" will be emitted from the air outlet. The smell will gradually fade away after the air conditioner has been turned on for more than ten minutes. This musty smell is not only unpleasant, but also means that the inside of the air conditioner has become a breeding ground for mold and bacteria. Whenever the wind drum rotates, thousands of mold spores and allergens will be spread indoors by the wind, causing frequent sneezing, skin allergies, or asthma in your family. To eradicate this musty smell, in addition to regular cleaning of the filter, the most core daily prevention method is to eliminate the inside of the air conditioner.

The physical causes of mold: the formation of condensation water and biofilm (Biofilm)

To understand why air conditioners become moldy, you must first understand the cooling principle of air conditioners:

1. Condensation phenomenon: When the air conditioner is cooling (Cooling), the low-temperature evaporator aluminum fins will come into contact with the hot air in the room. When the water vapor in the air is cooled, it will condense into small water droplets on the surface of the fins.
2. Drainage and drainage: The condensed water will slide down along the fins, collect into the plastic water collecting tray (water tray) below, and then be discharged to the outside through the drain pipe.
3. Moist confined spaces: When we press the off button, the air-conditioning valve closes and the compressor and fan stop operating. At this time, the trace amounts of moisture remaining on the fins and the stagnant water that cannot be completely drained in the drain pan will be trapped in the dark, airtight and warm casing of the air conditioner. With Taiwan’s high humidity averaging over 70%, this is a perfect breeding ground for mold to grow.
4. The birth of biofilm: Dust, dander and bacteria in the air are mixed in stagnant water and ferment on the inner wall of the water collection pan to form a sticky, smelly "biofilm". This is the main chemical and biological source of the sour and musty smell of air conditioners.

No-cost daily anti-mildew trick: The scientific basis for "air supply operation" before shutting down

The most effective and cost-free way to prevent biofilm and mold from settling inside your air conditioner is. Develop the following habits every time before turning off the air conditioner:

• Switch to "Fan Only" mode and run for 15 to 30 minutes: This is the core physical method of preventing mildew. After switching to air supply, the compressor stops cooling, but the fan continues to rotate. The large air flow will quickly evaporate the residual condensed water on the evaporator fins and dry the moisture around the air outlet and air drum. Let the inside of the machine return to a dry state before shutting it down officially. Mold will not be able to breed on dry plastic and metal surfaces.
• Make good use of the "anti-mildew operation (self-cleaning)" function of modern air conditioners:Many new types of separate inverter air conditioners (such as Hitachi, Daikin, Mitsubishi Electric and other brands) have built-in "automatic mildew prevention" or "body mildew prevention" functions. After enabling this function, when you press the off button, the air conditioner will not turn off the panel immediately. Instead, it will automatically operate at a weak air volume and low speed to supply air (even drying with slight heating first) for about 20 to 40 minutes before actually shutting down. Homeowners should take the initiative to read the manual and set this function to "always on."

Practical maintenance strategies for commercial properties and tenants: reducing tenant cancellation rates

For lessors or property managers who operate sublet apartments in Taipei and New Taipei, the musty smell of air conditioners is a hidden landmine that affects tenants' willingness to renew their leases (Churn Rate). When a tenant complains that the air-conditioner has a sour smell, if a plumber is asked to come and clean it every time, the single-time salary will be as high as NT$ 2,500 to NT$ 3,500. To reduce operating expenses (OpEx):

[Management Strategy] When the tenant moves in, the management staff should proactively hand over an "Air Conditioning Compliance Usage Card", guide the tenant step by step in setting the automatic mildew-proof operation of the air conditioner, and include this as part of the handover instructions. This can not only significantly slow down the loss of mold and scale on the air-conditioning drain pan, but also extend the professional cleaning cycle from 1 year to 3 years. It can also ensure the air quality in the suite and improve the living experience and satisfaction of high-asset tenants.

Conclusion: Use good habits to create a healthy breathing space

The core of daily mildew prevention for air conditioners lies in the word "drying". The small habit of blowing air for 15 minutes before shutting down the machine can cut off the chain of condensation water turning into moldy biofilm at the physical source. Whether you are an owner-occupier who values ​​your living health, or a real estate dealer who values ​​the efficiency of asset maintenance, implementing this simple electrical and mechanical usage guide will not only save you expensive cleaning and repair costs, but also make every breath in your home full of freshness and peace of mind.

New knowledge on air-conditioning power consumption and Taiwan climate maintenance regulations in recent years (2025-2026 current situation)

With the high temperatures in summer breaking historical records and the adjustment of electricity price structure in recent years, the latest statistics from the Energy Administration of the Ministry of Economic Affairs show that air-conditioning equipment accounts for more than 47% of residential electricity consumption in summer. Many people mistakenly believe that if the air conditioner is not cold, they only need to "replenish the refrigerant." However, in fact, refrigerant is not naturally consumed during normal operation in a closed circuit. If the refrigerant is insufficient, it means that there are micro leaks in the copper pipes or joints. Simply replenishing the air conditioner will only treat the symptoms but not the root cause. Refrigerant leakage will aggravate the high temperature overload of the compressor and shorten its service life.

According to Taiwan's standard operating specifications for air-conditioning projects, it is recommended that professional technicians with a refrigeration and air-conditioning decoration technician license perform "full machine high-pressure water washing and evaporator deep sterilization" every 1 to 2 years for detached air-conditioning indoor units. Especially in the humid summer in the Shuangbei Basin, the air drum and water collection tray are easy to breed black mold and jelly-like biofilm. This is also the main reason for the sour smell in the air-conditioning outlet and the backflow of the drainage pipe. Regular maintenance can maintain CSPF (Air Conditioning Seasonal Performance Factor) level one energy efficiency level, which can save an average household 15% to 20% of air conditioning electricity bills every year.

Autonomous simple maintenance vs professional maintenance safety red line

• Homeowners can do this themselves: Regularly remove and clean the nylon filter every 2 to 3 weeks (rinse with clean water and dry in a cool place, do not expose it to the sun to avoid deformation and embrittlement); clean the air outlet blades of the indoor unit with a dry cloth; check whether there is any accumulation of debris within 50 cm around the outdoor unit that hinders heat dissipation.
• Professional technicians must be entrusted according to regulations: Refrigerant pressure testing and leakage repair welding, high-pressure cleaning of indoor unit air drums and aluminum fins with liquid chemicals, reset of the drainage slope of hidden cold water pipes in the ceiling, and 220V dedicated power circuit configuration and leakage circuit breaker replacement.