Ventilation Rates — Required Air Changes

Overview

Ventilation is the process of supplying fresh outdoor air to an enclosed space and removing stale indoor air. Adequate ventilation is essential for:

• Indoor air quality (IAQ)
• Occupant health and comfort
• Moisture and odor control
• Dilution of indoor pollutants

Air Changes Per Hour (ACH)

The number of times the entire air volume in a room is replaced per hour:

$ACH = \frac{Q}{V}$

where:

• $Q$ = ventilation air flow rate (m³/h)
• $V$ = room volume (m³)

Recommended Air Changes

ASHRAE 62.1 Ventilation Rates

ASHRAE Standard 62.1 specifies minimum ventilation rates based on per person and per floor area:

$V_{\text{bz}} = R_p \cdot P_z + R_a \cdot A_z$

where:

• $R_p$ = outdoor air per person (L/s per person)
• $P_z$ = zone population
• $R_a$ = outdoor air per floor area (L/s per m²)
• $A_z$ = zone floor area (m²)

Occupancy Category	$R_p$ (L/s per person)	$R_a$ (L/s per m²)
Office space	2.5	0.3
Conference/meeting	2.5	0.3
Classroom (5+)	5.0	0.6
Retail sales	3.8	0.6

Key Points

• Ventilation rates are typically higher for spaces with more occupants or more pollutant sources
• Hospitals and laboratories require the highest ventilation rates for contamination control
• Energy-efficient ventilation uses heat recovery (HRV/ERV) to reclaim energy from exhaust air
• CO₂ monitoring can be used for demand-controlled ventilation (DCV)