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Calculate fresh air ventilation requirements for homes, commercial buildings, and individual rooms based on ASHRAE standards.
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Ventilation is the process of exchanging indoor air with outdoor air to maintain healthy indoor air quality. Proper ventilation removes pollutants, moisture, odors, and stale air while providing fresh oxygen.
ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) publishes the primary ventilation standards in North America.
Residential buildings
CFM = 0.03 × sq ft + 7.5 × (Nbr + 1)
Where Nbr = number of bedrooms
Commercial buildings
Vbz = Rp × Pz + Ra × Az
Breathing zone outdoor airflow
Fans remove air; fresh air enters through leaks or passive vents. Simple and affordable but doesn't filter incoming air. Best for mild climates.
Fan brings in outdoor air; stale air exits through leaks. Can filter and condition incoming air. Creates positive pressure.
Heat Recovery Ventilator: equal supply and exhaust with heat exchange. Recovers 70-80% of heat energy. Best for cold climates.
Energy Recovery Ventilator: transfers heat AND moisture. Best for humid climates where you want to keep humidity out in summer.
ASHRAE 62.2 provides minimum ventilation rates for homes:
| Home Size (sq ft) | 1 BR | 2 BR | 3 BR | 4 BR |
|---|---|---|---|---|
| 1,000 | 45 CFM | 53 CFM | 60 CFM | 68 CFM |
| 1,500 | 60 CFM | 68 CFM | 75 CFM | 83 CFM |
| 2,000 | 75 CFM | 83 CFM | 90 CFM | 98 CFM |
| 2,500 | 90 CFM | 98 CFM | 105 CFM | 113 CFM |
| 3,000 | 105 CFM | 113 CFM | 120 CFM | 128 CFM |
ASHRAE 62.1 ventilation rates by occupancy category:
| Space Type | CFM/person | CFM/sq ft | Default Density |
|---|---|---|---|
| Office Space | 5 | 0.06 | 1 per 200 sq ft |
| Conference Room | 5 | 0.06 | 1 per 20 sq ft |
| Retail | 7.5 | 0.12 | 1 per 67 sq ft |
| Restaurant | 7.5 | 0.18 | 1 per 15 sq ft |
| Classroom | 10 | 0.12 | 1 per 29 sq ft |
| Gym/Fitness | 20 | 0.18 | 1 per 25 sq ft |
| Room | Minimum CFM | Intermittent | Continuous |
|---|---|---|---|
| Bathroom | 50 CFM or 1 CFM/sq ft | 50 CFM | 20 CFM |
| Kitchen | 100 CFM min | 100 CFM | 5 ACH |
| Laundry | 50 CFM | 50 CFM | — |
| Garage | 100 CFM | 70 CFM/car | — |
Per ASHRAE 62.2, a typical 2,000 sq ft home with 3 bedrooms needs about 90 CFM of continuous ventilation. This ensures adequate fresh air for 4 occupants.
HRV (Heat Recovery Ventilator) transfers heat between incoming and outgoing air. ERV (Energy Recovery Ventilator) transfers both heat AND moisture. ERVs are better for humid climates.
Modern tight homes (built to current energy codes) typically require mechanical ventilation. Older, leaky homes may get adequate ventilation through natural air infiltration, but this is uncontrolled.
Run bathroom exhaust fans during use and for 20-30 minutes after to remove moisture. For continuous exhaust, run at 20 CFM. Some codes require automatic humidity or motion control.
Makeup air is outdoor air that replaces exhausted air. Large exhaust fans (like range hoods over 400 CFM) may require dedicated makeup air systems to prevent negative pressure issues.