Understanding Filter Media in Fresh-Air Air Conditioners

A Three-Stage Filtration System: A Coarse-to-Fine Line of Defense

Fresh-air air conditioners typically use a multi-layer filter system in which each layer has a specific function:

Pre-filter: As the first line of defense, the pre-filter mainly captures large contaminants such as hair and dust. It is usually made of nonwoven fabric or nylon mesh. Some washable models, such as metal mesh filters, can be rinsed every 1–2 months and reused for 3–5 years. Nonwashable nonwoven filters should be replaced every 3–6 months, or every 2–3 months during periods of heavy smog or in locations beside busy roads.

HEPA filter: This is the main purification layer. It captures fine particles such as PM2.5 and bacteria; H13 filters can reach 99.97% efficiency. HEPA media are made of ultrafine glass fiber or polypropylene. In ordinary urban environments, replacement is recommended every 8–12 months. In heavily polluted areas, such as northern China during winter, the interval should be shortened to 3–6 months. HEPA filters must not be washed because water damages the fiber structure.

Activated-carbon filter: This layer is designed to adsorb formaldehyde and odors, but it may release pollutants again after becoming saturated. In newly renovated homes, replacement is recommended every 3–6 months. Under routine conditions, such as homes with pets or kitchen odors, the interval can be 6–12 months. In humid regions, replacement every six months helps prevent mold.

Replacement Intervals: Do Not Let an Overused Filter Become a Pollution Source

Filter life is not fixed and should be adjusted to the environment:

Basic guidance: Manufacturers generally recommend replacing pre-filters every 3–6 months, HEPA filters every 6–12 months, and activated-carbon filters every 6–12 months. The actual schedule should reflect operating conditions.

Polluted environments: Where PM2.5 levels are high or the property is beside a busy road or construction site, shorten the replacement intervals for HEPA and activated-carbon filters by 50%, and inspect the pre-filter every two months.

Usage patterns: Continuous 24-hour operation or high-airflow mode can reduce filter life by 20%–30%. In homes with pets, the activated-carbon filter may need replacement after 4–8 months.

Warning signs: Replace the filter if its surface has turned black, less light passes through it, outlet airflow has weakened enough that it cannot lift a sheet of A4 paper, or fan noise has increased. Some smart models also send replacement reminders through an app.

Maintenance and Replacement: Simple, Low-Cost Steps That Preserve Performance

Cleaning restrictions: Never wash HEPA or activated-carbon filters. Water can damage their filtration structure or encourage bacterial growth. Nonwoven pre-filters must be replaced, while metal or nylon mesh pre-filters may be washed, thoroughly dried, and reused.

DIY replacement: Switch off the power, open the filter compartment secured by clips or screws, remove the old filter while noting its orientation, install the new filter, and fasten the cover. The process takes about five minutes, and a replacement generally costs from a few dozen to around one hundred yuan. If you are unsure how to proceed, have a professional replace the filter to prevent unfiltered air from bypassing an incorrectly installed unit.

Tips for extending service life: Fit a stainless-steel dust screen over the outdoor air inlet to reduce the load on the pre-filter. Regularly vacuuming a washable pre-filter can extend HEPA filter life by 20%–30%.

Common Mistakes: Attempts to Save Money That Can Harm Your Health

Using filters beyond their service life: A clogged filter reduces incoming airflow and ventilation efficiency and may even support mold growth. Saturated activated carbon may release formaldehyde and other pollutants back into the air.

Washing filters indiscriminately: Washing a HEPA filter can reduce filtration efficiency by more than 50%, allowing fine particles to pass through.

Neglecting pre-filter maintenance: Excessive dust on the pre-filter increases the load on the HEPA filter and can shorten its service life by 30%–50%.

Although the filter media in a fresh-air air conditioner may be easy to overlook, they are the gatekeepers of indoor air. Regular inspection and appropriate replacement help keep every breath clean and healthy.