Ventilation Rate Calculations for HVAC Engineers

Ventilation Rate Calculations for HVAC Engineers

ASHRAE Standard 62.1 defines minimum ventilation rates to maintain acceptable indoor air quality. Proper calculations ensure code compliance while avoiding over-ventilation energy penalties.

Zone Outdoor Air Flow

Ventilation rate procedure:

$$V_{oz} = R_p \times P_z + R_a \times A_z$$

Where:

• $V_{oz}$ = zone outdoor airflow (CFM)
• $R_p$ = people outdoor air rate (CFM/person)
• $P_z$ = zone population
• $R_a$ = area outdoor air rate (CFM/ft²)
• $A_z$ = zone floor area (ft²)

Typical values (per Standard 62.1 Table 6-1):

System Outdoor Air Flow

Multiple-zone recirculating systems:

$$V_{ot} = \frac{\sum{(R_p \times P_z \times D)} + \sum{(R_a \times A_z)}}{E_z}$$

Where:

• $D$ = occupant diversity (not all spaces fully occupied simultaneously)
• $E_z$ = zone air distribution effectiveness

Zone air distribution effectiveness:

• Ceiling supply, floor return: $E_z = 1.0$
• Floor supply, ceiling return: $E_z = 1.2$ (displacement ventilation)
• Poor mixing: $E_z = 0.8$

Ventilation Efficiency

System ventilation efficiency accounts for uneven outdoor air distribution to zones:

$$E_v = \frac{V_{ot}}{V_{oa}}$$

Where $V_{oa}$ = actual outdoor air intake

Worst-case zone method:

$$E_v = \frac{V_{oz,min}/V_{dz,min}}{\max(V_{oz}/V_{dz})}$$

Typically $E_v = 0.6-0.8$ for VAV systems

CO₂-Based Demand Control Ventilation

Steady-state CO₂ concentration:

$$C_s = C_o + \frac{N \times G}{V_{oz} \times \rho_{air}}$$

Where:

• $C_s$ = steady-state CO₂ (ppm)
• $C_o$ = outdoor CO₂ (typically 400-450 ppm)
• $N$ = number of occupants
• $G$ = CO₂ generation rate per person (~0.0052 CFM at 0.3 L/min metabolic rate)

Target: Maintain < 1,000-1,200 ppm in occupied spaces

Energy Impact

Annual ventilation heating energy:

$$Q_{heat} = 1.08 \times CFM_{OA} \times HDD_{65} \times 24$$

Annual ventilation cooling energy:

$$Q_{cool} = 1.08 \times CFM_{OA} \times CDD_{65} \times 24$$

Typical: Ventilation represents 20-40% of total HVAC energy in commercial buildings

Practical Applications

1. Office building: 5 CFM/person + 0.06 CFM/ft² minimum
2. Schools: 10 CFM/person + 0.12 CFM/ft² (children metabolize more)
3. DCV: Reduce outdoor air when spaces unoccupied (saves 20-40% ventilation energy)

Related Technical Guides:

• Psychrometric Processes
• Energy Recovery Systems
• Load Calculation Methodology

References:

• ASHRAE Standard 62.1: Ventilation for Acceptable Indoor Air Quality
• ASHRAE Standard 62.1 User’s Manual