ACH (Air Changes Per Hour)

What is ACH?

ACH stands for Air Changes Per Hour and is a standard used in HVAC design to specify how many times the entire volume of air in a room should be completely replaced with fresh air per hour.

ACH is a fundamental metric in ventilation design that ensures adequate air quality, moisture control, and contaminant removal in indoor spaces.

How ACH Works

An ACH of 4 means that all the air currently in the room is completely replaced 4 times per hour. This is calculated as:

ACH = (CFM × 60) ÷ Room Volume (cubic feet)

Conversely, to find the CFM needed for a specific ACH requirement:

CFM = (Room Volume × ACH) ÷ 60

ACH Requirements by Space Type

Residential Homes

• Standard: 3-4 ACH
• With mechanical ventilation: 2-3 ACH minimum
• Energy efficient: 2-2.5 ACH with fresh air inlet
• Purpose: Remove odors, moisture, and indoor pollutants

Office Buildings

• Standard: 4-6 ACH
• Outdoor air: 15-20% of total air supply
• Purpose: Maintain comfort and productivity while managing energy costs

Retail Spaces

• Standard: 4-8 ACH depending on occupancy
• High traffic areas: 6-8 ACH
• Purpose: Manage odors and maintain customer comfort

Medical Facilities

• Examination rooms: 6-12 ACH
• Operating rooms: 15-25 ACH (high filtration)
• Isolation rooms: 12-15 ACH with negative pressure
• Purpose: Infection control and sterile environments

Laboratories

• Standard labs: 8-12 ACH
• Specialized labs: 12-20+ ACH
• Fume hood areas: 12-15 ACH (additional capacity)
• Purpose: Contaminant removal and worker safety

Restaurants and Kitchens

• Dining areas: 6-8 ACH
• Kitchen spaces: 15-20+ ACH
• Exhaust hoods: Additional dedicated capacity
• Purpose: Odor and smoke removal

Data Centers

• Standard: 4-6 ACH minimum
• High-density: 6-8+ ACH
• Purpose: Temperature and humidity control for equipment

Factors That Affect ACH Requirements

Occupancy Level

Higher numbers of people increase CO₂ and moisture, requiring higher ACH to maintain air quality.

Activity Type

Cooking, smoking, exercise, and industrial processes increase contaminant generation and ACH needs.

Building Envelope

Air-tight, well-insulated buildings can use lower ACH with proper mechanical ventilation. Leaky buildings require higher ACH to achieve adequate air quality.

Outdoor Air Requirements

Building codes often mandate minimum outdoor air percentages (typically 15-20%), which influences ACH design.

Humidity Control

Moisture-generating activities (cooking, showers, laundry) may require higher ACH or dedicated dehumidification.

Contamination Sources

Presence of chemical, biological, or particulate contaminants increases ACH requirements.

Climate

Heating/cooling loads affect the feasibility of very high ACH and may influence practical design targets.

ACH vs. CFM

ACH is a standard that specifies how many times air should be changed.

CFM is the rate at which that standard is achieved.

They are related by the room's volume:

• Same ACH in a larger room requires more CFM
• Same CFM in rooms of different sizes produces different ACH

Calculating Room Volume for ACH

Formula

Room Volume = Length × Width × Height (all in feet)

Example

• Room: 20 feet long × 15 feet wide × 8 feet high
• Volume = 20 × 15 × 8 = 2,400 cubic feet
• For 4 ACH: CFM = (2,400 × 4) ÷ 60 = 160 CFM

ACH and Energy Efficiency

Tight Building Envelope + Controlled Ventilation

Modern energy codes emphasize:

• Tight, sealed building envelopes with low natural infiltration
• Lower mechanical ACH requirements (2-3 instead of 3-4)
• Dedicated fresh air intakes with heat recovery
• More cost-effective long-term operation

Traditional Buildings

Older, leakier buildings achieve adequate ACH through:

• Natural infiltration and exfiltration
• Less need for mechanical ventilation
• Higher energy costs
• Less consistent air quality control

Meeting ACH Requirements

Natural Ventilation

• Open windows and doors
• Relies on wind and temperature differences
• Variable and uncontrollable
• Increased energy costs

Mechanical Ventilation

• HVAC systems with dedicated fresh air intake
• Consistent and controllable
• Allows heat/energy recovery
• Best practice for code compliance

Balanced System

• Equal supply and exhaust airflow
• Prevents pressure imbalances
• Maintains consistent ACH
• Optimal for comfort and efficiency

Ventilation Best Practices

• Calculate ACH requirements based on space type and occupancy
• Use mechanical ventilation for consistent performance
• Include outdoor air intake with proper filtration
• Consider heat/energy recovery ventilation
• Commission systems to verify ACH is being achieved
• Maintain filters and clean ductwork regularly
• Balance air supplies between zones
• Monitor CO₂ or humidity as indicators of adequacy