How to Choose a Fresh Air System? Total Heat Exchange, Heat Recovery Efficiency, and Filter Grade — Three Core Parameters Explained at Once
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How to Choose a Fresh Air System? Total Heat Exchange, Heat Recovery Efficiency, and Filter Grade — Three Core Parameters Explained at Once
Fresh air systems are becoming increasingly popular, but few people seriously study the parameters during renovation — resulting in either choosing a one-way flow system (wasting money) or getting extremely low heat recovery efficiency (using fresh air in winter is like opening a window to the cold). This article helps you build a selection framework.
Ventilation Method: The Fundamental Logic of a Fresh Air System
One-Way Flow Fresh Air (Exhaust-Only)
Only has exhaust ducts; indoor polluted air is expelled, and fresh air naturally infiltrates through door gaps and wall seams.
Drawbacks:
- Fresh air is not filtered; PM2.5 enters directly
- No heat recovery; in winter it's equivalent to having a window open
- Only suitable for old buildings with very poor airtightness, or as an extremely low-budget temporary solution
Conclusion: Not recommended — don't choose one-way flow just because it's cheap.
Two-Way Flow Fresh Air (ERV — Energy Recovery Ventilator)
Has both supply and exhaust ducts. Fresh air passes through a total heat exchange core where it exchanges heat and humidity with the exhaust air before entering the room.
Working principle:
- Winter: Cold outdoor air passes through the heat exchange core, recovering heat from the warm indoor exhaust air, and is preheated before being supplied to the room
- Summer: Hot, humid outdoor air passes through the exchange core, recovering coolness from the indoor exhaust air, and is cooled and dehumidified before being supplied to the room
- Fresh air and exhaust air never mix directly; they are separated by a heat-conductive membrane, exchanging only heat and humidity
Advantages:
- Filtered fresh air is supplied to the room
- Heat recovery significantly reduces energy consumption in winter and summer
- The room maintains slight positive pressure, reducing infiltration of outdoor pollutants
Conclusion: For home use, always choose two-way flow with total heat exchange — one-way flow is not worth installing.
Heat Recovery Efficiency: The Most Critical Performance Parameter
Sensible Heat Recovery Efficiency: The ability to recover temperature (heat), typically ≥ 70% is acceptable, ≥ 75% is excellent.
Total Heat Recovery Efficiency: Recovers both temperature and humidity simultaneously, more reflective of actual living comfort (especially in humid southern regions and dry northern regions).
Selection standards:
- Total heat recovery efficiency ≥ 70% (passing grade)
- Recommended ≥ 75% (mid-tier level)
- Premium products achieve ≥ 80%
Verification method: Request the product's test report according to the GB/T 21087-2020 standard, and check the "total heat exchange efficiency" value.
PM2.5 Filtration Efficiency: Intercepting Outdoor Pollution
Fresh air must be filtered before entering the room. The filter grade determines how much PM2.5 is intercepted.
| Filter Grade | Filtration Efficiency | Description |
|---|---|---|
| G4 coarse filter | Approx. 50–70% | Only filters large particles, insufficient for PM2.5 |
| F7 medium-efficiency filter | Approx. 80–85% | Basic filtration, barely adequate |
| H11 HEPA | ≥ 95% | Good |
| H13 HEPA | ≥ 99.97% | Recommended grade, close to medical standard |
| H14 HEPA | ≥ 99.995% | Top tier, high price |
Recommendation: Home fresh air systems should use at least H13 HEPA filtration with PM2.5 filtration efficiency ≥ 99.97%. Don't compromise in heavily polluted cities (areas with high year-round AQI).
Note on filter replacement cycle: H13 filters are typically replaced every 6–12 months (depending on local air quality). Replacement cost is part of the long-term ownership cost — confirm consumable prices before purchasing.
Rated Airflow: How to Calculate If It's Sufficient
Rated airflow (m³/h): The volume of fresh air the system can supply per hour.
Recommended air change rates:
- Residential standard: 0.5–1 air changes per hour (i.e., replacing 50%–100% of indoor air volume per hour)
- Heavily polluted areas or homes with infants: ≥ 1 air change per hour
Calculation formula:
Required airflow (m³/h) = Indoor space volume (m³) × Air change rate
Example: 120 m², ceiling height 2.8m → volume 336 m³, target air change rate 0.8 → required airflow approx. 270 m³/h
Selection: Choose a model with rated airflow ≥ the calculated value, with 20% margin (duct resistance reduces actual airflow).
Noise: The Hidden Killer of Sleep Quality
Fresh air systems run 24 hours a day, and noise significantly affects sleep.
| Level | Noise dB(A) | Description |
|---|---|---|
| Excellent | ≤ 26 dB | Virtually imperceptible in the bedroom |
| Acceptable | 27–35 dB | Slight operating sound can be noticed |
| Too loud | > 35 dB | Affects sleep, not recommended |
Check the "sleep mode / low-speed noise" value in the spec sheet, not the high-speed noise.
Installation Considerations
Ceiling installation: The main unit is installed in the ceiling, ducts run through the ceiling space, supply vents are flush with the ceiling surface — most aesthetically pleasing, but requires pre-renovation planning; difficult to retrofit.
Wall-mounted / through-wall installation (single-room type): No ceiling work needed; a hole is drilled through the exterior wall. Suitable for retrofitting a single room in an older building, but not as effective as central fresh air for whole-house coverage.
Recommendation: New renovations must prioritize planning the fresh air system. Trying to fix it after renovation is complete will result in significantly more work and worse results.
Formaldehyde Removal: An Add-On Feature of Some Models
The core function of a fresh air system is ventilation and filtration, not formaldehyde removal. Some models include built-in formaldehyde filtration modules (activated carbon, photocatalytic), which have some auxiliary effect. However, the fundamental solution for formaldehyde during the post-renovation period is still extensive ventilation (open windows directly, not using the fresh air system). Once formaldehyde has been released, the fresh air system serves as a daily maintenance tool.
Parameter standards in this article are sourced from GB/T 21087-2020 "Air-to-Air Energy Recovery Equipment" and GB/T 34012-2017 "Air Cleaning Devices for Ventilation Systems."