Fresh Air System Deep Guide: Total Heat Exchange & Indoor Air Quality
- Published on
What's the difference between opening windows and a fresh air system? How does total heat exchange work? How do you calculate fresh air volume? Which is more important, PM2.5 or CO2? Can fresh air systems and air purifiers replace each other? Do you need a dropped ceiling to install a fresh air system? Behind these questions lies a deep understanding of building ventilation science, thermodynamic exchange, and indoor air quality. Starting from aerodynamics, this article systematically breaks down the scientific principles of fresh air systems.
1. In-Depth Analysis of Indoor Air Pollution Sources
Pollutant Classification and Sources
| Category | Main Pollutants | Sources | Health Effects |
|---|---|---|---|
| Particulate Matter | PM2.5/PM10 | Outdoor infiltration, cooking, pets | Respiratory diseases |
| Gaseous Pollutants | Formaldehyde/TVOC | Building materials, furniture | Carcinogenic, allergenic |
| Biological Pollutants | Bacteria/Mold/Dust Mites | Humid environments, AC systems | Allergies, asthma |
| Combustion Byproducts | CO/NO2 | Gas stoves, water heaters | Poisoning, inflammation |
| Metabolic Byproducts | CO2 | Human respiration | Dizziness, drowsiness |
CO2 Concentration and Human Effects
| Concentration (ppm) | Environment | Effects |
|---|---|---|
| 350-450 | Normal outdoor | Baseline |
| 600-800 | Well-ventilated indoor | No sensation |
| 800-1000 | Typical indoor | Slight discomfort |
| 1000-2000 | Poorly ventilated indoor | Dizziness, reduced concentration |
| 2000-5000 | Enclosed space | Significant discomfort, drowsiness |
| >5000 | Extremely enclosed | Headache, nausea |
Formaldehyde Release Patterns
- Temperature Dependence: Release rate increases 12%-15% for every 1°C rise
- Humidity Dependence: Release rate increases 5%-8% for every 10% increase in humidity
- Release Cycle: Peak period is 3-6 months after renovation, lasting 3-15 years
- Safety Standards: National standard ≤0.08mg/m³, WHO recommends ≤0.1mg/m³
2. Core Principles of Fresh Air Systems
Positive Pressure vs. Negative Pressure vs. Balanced Flow
1. Positive Pressure Fresh Air
- Principle: Forced supply air → indoor positive pressure → stale air exits through door/window gaps
- Pros: Simple structure, low cost
- Cons: Unorganized exhaust, no heat recovery, humidity uncontrollable
- Best For: Warm southern regions, post-renovation occupied homes
2. Negative Pressure Fresh Air
- Principle: Forced exhaust → indoor negative pressure → fresh air naturally enters through intake vents
- Pros: Controllable exhaust path
- Cons: Untreated fresh air enters directly, no heat recovery
- Best For: Transitional solution, not recommended for long-term use
3. Balanced Flow Fresh Air
- Principle: Independent supply + independent exhaust, creating organized ventilation
- Pros: Controllable ventilation path, heat recovery possible, filtration possible
- Cons: Complex ductwork, high cost
- Best For: Preferred solution for new renovations
4. Balanced Flow with Total Heat Exchange (Recommended)
- Adds a total heat exchange core to balanced flow
- Heat recovery rate: 60%-90%
- Humidity recovery: 30%-60%
In-Depth Analysis of Total Heat Exchange Core Technology
Sensible Heat Exchange vs. Total Heat Exchange
| Parameter | Sensible Heat Exchange | Total Heat Exchange |
|---|---|---|
| Heat Recovered | Temperature | Temperature + Humidity |
| Heat Recovery Rate | 60%-80% | 60%-90% |
| Humidity Recovery | None | 30%-60% |
| Suitable Climate | Dry regions | All climates |
| Price | Low | High |
Exchange Core Material Comparison
| Material | Heat Recovery Rate | Humidity Recovery | Resistance | Lifespan | Cost |
|---|---|---|---|---|---|
| Paper | 70%-80% | 40%-60% | Low | 2-3 years | Low |
| Membrane (ERV) | 75%-85% | 30%-50% | Medium | 5-8 years | Medium |
| Aluminum (Sensible) | 80%-90% | None | Medium | 10+ years | Medium |
| Ceramic | 70%-80% | 20%-40% | High | 10+ years | High |
Exchange Efficiency Calculation
- Temperature Exchange Efficiency = (T_supply - T_outdoor) / (T_exhaust - T_outdoor) × 100%
- Enthalpy Exchange Efficiency = (h_supply - h_outdoor) / (h_exhaust - h_outdoor) × 100%
- Note: Rated efficiency is lab data; actual performance is affected by airflow speed and temperature difference
3. Fresh Air Volume Calculation and Design
Fresh Air Volume Standards
| Standard | Per-Person Fresh Air Volume | Applicable Scenario |
|---|---|---|
| National Standard GB/T 18883 | 30 m³/h | Residential |
| National Standard GB 50365 | 30-50 m³/h | Office |
| ASHRAE 62.1 | 36 m³/h | US standard residential |
| DIN 1946-6 | 40-60 m³/h | German standard residential |
Calculation by Room Area
-
Air Change Rate Method:
- Residential: 0.5-1.0 air changes/hour
- Bedroom: 1.0-1.5 air changes/hour
- Formula: Fresh air volume = Room volume × Air change rate
-
Example:
- Bedroom 15m² × 2.5m ceiling height = 37.5m³
- 1.0 air change/hour → Fresh air volume ≥ 37.5m³/h
- Accounting for duct losses → Select 50m³/h unit
Whole-House Fresh Air Volume Calculation
Example: 3-bedroom, 2-living-room (100m²):
├── Master bedroom: 15m² → 50m³/h
├── Bedroom 2: 12m² → 40m³/h
├── Bedroom 3: 10m² → 35m³/h
├── Living room: 30m² → 75m³/h
├── Total demand: 200m³/h
├── Duct loss factor: 1.2-1.3
└── Actual unit selection: ≥250m³/h
Static Pressure and Duct Design
- Duct Resistance: ~2-5Pa per meter of straight duct, ~10-20Pa per elbow
- Static Pressure Requirement: Total duct resistance + terminal residual pressure (≥30Pa)
- Duct Diameter Selection:
- Φ75mm: Airflow ≤30m³/h
- Φ90mm: Airflow 30-60m³/h
- Φ110mm: Airflow 60-100m³/h
- Φ160mm: Airflow 100-250m³/h
- Duct Principles: Short, straight, minimal bends
4. In-Depth Analysis of Filtration Systems
Filtration Grade System
| Grade | Filtration Efficiency | Particle Size | Function |
|---|---|---|---|
| G4 | 90% (≥5μm) | Large particles | Pre-filter |
| F7 | 80% (0.4μm) | Medium particles | Main medium filter |
| F9 | 95% (0.4μm) | Medium particles | High-efficiency medium filter |
| H11 | 95% (0.3μm) | PM2.5 | Sub-HEPA |
| H13 | 99.95% (0.3μm) | PM2.5 | HEPA |
| H14 | 99.995% (0.3μm) | PM2.5 | Ultra-HEPA |
Fresh Air Filtration Configurations
Option 1: Basic (G4+F7)
- Best For: Areas with good air quality
- Filtration Performance: PM2.5 removal ~60%-70%
- Maintenance Cost: Low
Option 2: Standard (G4+F7+H11)
- Best For: Moderately polluted areas
- Filtration Performance: PM2.5 removal ~90%-95%
- Maintenance Cost: Medium
Option 3: High-Efficiency (G4+F7+H13)
- Best For: Heavily polluted areas
- Filtration Performance: PM2.5 removal ~99%
- Maintenance Cost: High
- Note: H13 has high resistance, requires a high-static-pressure motor
Gaseous Pollutant Filtration
-
Activated Carbon Filter:
- Formaldehyde CADR: Increases with thickness and iodine value
- Iodine value ≥800mg/g is high quality
- Saturation cycle: 3-6 months (depends on pollution level)
- Cannot be washed or regenerated
-
Photocatalyst (TiO₂):
- Catalytically breaks down formaldehyde into CO₂ + H₂O
- Requires UV light source for activation
- Effectiveness: Supplementary formaldehyde removal
-
Cold Catalyst:
- Catalytic breakdown at room temperature
- No UV light source required
- Effectiveness: Moderate, long lifespan
5. Fresh Air System Installation Methods
Dropped Ceiling (Central Fresh Air)
- Installation Timing: During renovation, before ceiling installation
- Unit Location: Kitchen/bathroom ceiling, balcony, utility room
- Ductwork: PVC/PE pipes + distribution box
- Supply Vents: Bedroom/living room ceiling or wall
- Return Vents: Hallway/living room
- Pros: High airflow, low noise, whole-house coverage
- Cons: Requires renovation coordination, complex installation
Wall-Mounted (Surface-Mounted Fresh Air)
- Installation Timing: Can be installed anytime
- Installation Method: Wall core drilling (Φ100-160mm)
- Airflow: 30-150m³/h
- Pros: No dropped ceiling needed, single-room use
- Cons: Limited airflow, some noise, wall penetration required
Cabinet Fresh Air
- Installation Method: Floor-standing + exterior wall opening
- Airflow: 200-500m³/h
- Pros: High airflow, flexible placement
- Cons: Takes up floor space, visually obtrusive
PE Pipe vs. PVC Pipe
| Parameter | PE Pipe | PVC Pipe |
|---|---|---|
| Connection Method | Distribution box + quick-connect | Glue bonding |
| Resistance | Low | High |
| Air Leakage Rate | Very low | Relatively high |
| Cleanability | Washable | Difficult to clean |
| Secondary Pollution | Low | High (glue off-gassing) |
| Cost | High | Low |
| Recommendation | ★★★★★ | ★★★ |
6. Noise Control
Noise Sources
- Unit Noise: Motor + fan (primary source)
- Duct Noise: Airflow + duct wall vibration
- Vent Noise: Turbulence at air outlet
- Sound Transmission: Solid-borne noise through ducts
Noise Standards
| Location | Nighttime Noise Limit | Daytime Noise Limit |
|---|---|---|
| Master Bedroom | ≤25dB(A) | ≤35dB(A) |
| Secondary Bedroom | ≤30dB(A) | ≤35dB(A) |
| Living Room | ≤35dB(A) | ≤40dB(A) |
Noise Reduction Solutions
- Unit Selection: Choose low-noise models (lowest setting ≤25dB)
- Unit Location: Keep away from bedrooms, install in kitchen/bathroom
- Silencer Duct: Connect 1-1.5m of silencer flex duct to unit outlet
- Distribution Box: Standard for PE pipe systems, reduces noise + ensures even airflow
- Supply Vents: Choose adjustable airflow type, reduce opening size appropriately
- Vibration Dampening: Use vibration-dampening hangers for unit mounting
- Sound Insulation: Wrap unit with soundproofing material
7. Fresh Air vs. Air Purifier
| Parameter | Fresh Air System | Air Purifier |
|---|---|---|
| Core Function | Introduce outdoor fresh air + filtration | Recirculate and filter indoor air |
| CO2 Control | Effective | Ineffective |
| PM2.5 Control | Effective | Highly effective |
| Formaldehyde Control | Effective (continuous dilution) | Effective (recirculation adsorption) |
| Oxygen Replenishment | Yes | No |
| Installation | Complex / requires renovation | Plug-and-play |
| Cost | High (unit + installation) | Low |
| Best Combination | — | Fresh air + purifier combo |
Optimal Combination Strategy
- Fresh Air System: Solves CO2 + oxygen levels + whole-house air exchange
- Air Purifier: Localized high-efficiency PM2.5/formaldehyde removal
- Combined Effect:
- Fresh air maintains CO2 <800ppm
- Purifier rapidly reduces localized PM2.5
- Complementary, not interchangeable
8. Fresh Air System Buying Checklist
Central Fresh Air Buying Points
- Airflow ≥ whole-house demand × 1.2 (duct losses)
- Total heat exchange efficiency ≥70%
- Filtration grade ≥ F7+H11 (H13 for polluted areas)
- Lowest setting noise ≤25dB
- Motor: EC DC inverter (50%+ energy savings)
- PE pipes + distribution box (not PVC)
- Control: App + indoor panel
- Built-in CO2/PM2.5 sensor
- Bypass function (bypasses heat exchange core in spring/autumn)
- Filter replacement reminder
- Integrated brand installation and after-sales service
Wall-Mounted Fresh Air Buying Points
- Airflow ≥ room volume (air change rate ≥1 time/hour)
- Filtration grade ≥ H11
- Lowest setting noise ≤30dB
- Electric auxiliary heating (essential for northern winters)
- Multiple airflow speed settings
- App remote control
- Installation service included
- Acceptable filter replacement cost (≤500 RMB/year)
9. Pitfall Avoidance Guide
- "Fresh air is just a big air purifier": The core of fresh air is air exchange (introducing outdoor air); purifiers recirculate and filter. They are fundamentally different.
- "Higher airflow is always better": Excessive airflow → high noise, high energy consumption, reduced heat recovery efficiency.
- "Total heat exchange means you don't need AC": Total heat exchange only recovers heat; it cannot heat or cool.
- "Wall-mounted units don't need drilling": They absolutely require a hole to the outside; the hole is just smaller than for central systems.
- "Filters don't need replacing": Saturated filters become ineffective and can become a pollution source themselves.
- "Installation doesn't need design": Poor duct design → 50%+ airflow loss, double the noise.
- "Opening windows removes formaldehyde faster than fresh air": Fresh air provides continuous exchange, which is more stable and efficient than intermittent window opening.
- "Fresh air is too cold in winter": Total heat exchange recovers 70%+ of heat; auxiliary heating covers the remaining temperature difference.
- "PVC pipes are good enough": PVC glue off-gasses secondary pollution + duct walls accumulate dust that cannot be cleaned.
- "Once installed, you can forget about it": Requires regular filter replacement, exchange core cleaning, and duct inspection.
10. Maintenance Schedule
| Item | Interval | Notes |
|---|---|---|
| Pre-filter | 1-2 months | Washable / vacuum |
| Medium filter | 3-6 months | Replace |
| HEPA filter | 6-12 months | Replace |
| Activated carbon filter | 3-6 months | Replace (not washable) |
| Total heat exchange core | 1-2 years | Clean / replace |
| Duct inspection | 1-2 years | Professional cleaning |
| Unit inspection | 1 year | Motor + seal integrity |
| Distribution box | 1 year | Check seals + clean |
Annual Maintenance Cost Reference
- Central Fresh Air: 800-2000 RMB/year (filters + exchange core)
- Wall-Mounted Fresh Air: 300-800 RMB/year (filters)
Key Takeaway: The essence of a fresh air system is "organized ventilation." Its core value is solving CO2 buildup and oxygen levels—something air purifiers cannot do. When choosing a fresh air system, focus on three things: total heat exchange efficiency, filtration grade, and installation quality. A poorly installed system is worse than no system at all. Design matters more than the equipment itself.
Related Guides
Home Appliance Energy Saving Myths: What Actually Works
Learn the truth about energy-saving practices for home appliances and discover simple usage changes that can significantly reduce your electricity bill without sacrificing comfort.
Is a Home Espresso Machine Worth It? A Year of Pulling Shots at Home
A home espresso machine can save money or become an expensive shelf ornament. After a year, the deciding factors are the grinder, the learning curve, and daily cleaning — not the machine's pressure number.
Do Air Purifiers Actually Work? What a Year of Ownership Reveals
Air purifiers do something you can't see, which makes "is it worth it?" hard to judge. After living with one, the real answers are about sizing, filter cost, and noise — here's the honest experience.
6 Vacuum Cleaner Buying Mistakes: Why Suction Watts Aren't the Whole Story
Wattage on the box, a battery that dies mid-clean, and a filter that leaks dust back into the air are common vacuum mistakes. Here's what actually decides how well it cleans.