Air Changes per Hour (ACH) Calculator

Supply Air Flow Rate (\( cfm \)):

Room Length:

Room Width:

Room Height:

1. What is an Air Changes per Hour (ACH) Calculator?

Definition: This calculator computes the air changes per hour (\( ach \)) in a space, indicating how often the air is replaced, based on the supply air flow rate and room volume, which is calculated from the room's length, width, and height.

Purpose: It is used in HVAC systems to assess ventilation effectiveness, ensuring adequate air turnover to maintain indoor air quality, thermal comfort, and health standards in a space.

2. How Does the Calculator Work?

The calculator uses the following formulas:

Room Volume: \[ \text{volume} = \text{length} \times \text{width} \times \text{height} \]

Air Changes per Hour: \[ ach = \frac{cfm \times 60}{\text{volume}} \]

Where:

\( ach \): Air changes per hour (hr⁻¹)
\( cfm \): Supply air flow rate (ft³/min, m³/s, L/s)
\( \text{volume} \): Room volume (ft³, calculated from dimensions)
\( \text{length} \), \( \text{width} \), \( \text{height} \): Room dimensions (ft, m, cm)
Constant 60: Converts minutes to hours (since \( cfm \) is in ft³/min and \( ach \) is in hr⁻¹)

Unit Conversions:

Supply Air Flow Rate (\( cfm \)): ft³/min, m³/s (1 m³/s = 2118.88 ft³/min), L/s (1 L/s = 2.11888 ft³/min)
Room Dimensions (\( \text{length} \), \( \text{width} \), \( \text{height} \)): ft, m (1 m = 3.28084 ft), cm (1 cm = 0.0328084 ft)

Steps:

Enter the supply air flow rate (\( cfm \)), room length, width, and height, and select their respective units.
Convert supply air flow rate to ft³/min and room dimensions to ft.
Calculate the room volume using the dimensions.
Calculate the air changes per hour using the formula.
Display the result with 5 decimal places, or in scientific notation if the value is greater than 10,000 or less than 0.00001.

3. Importance of Air Changes per Hour (ACH) Calculation

Calculating the air changes per hour is crucial for:

Indoor Air Quality: Ensures sufficient air turnover to dilute pollutants, odors, and contaminants, maintaining a healthy indoor environment.
HVAC Design: Helps design ventilation systems that meet air change requirements for specific spaces (e.g., hospitals, offices), ensuring compliance with standards like ASHRAE.
Energy Efficiency: Balances ventilation rates to achieve desired air quality without over-ventilating, reducing energy consumption for heating or cooling.

4. Using the Calculator

Examples:

Example 1: For \( cfm = 1080 \, \text{ft³/min} \), room dimensions \( \text{length} = 41.9 \, \text{ft} \), \( \text{width} = 29.8 \, \text{ft} \), \( \text{height} = 10 \, \text{ft} \), ACH in hr⁻¹:
- Volume: \( 41.9 \times 29.8 \times 10 = 12490 \, \text{ft³} \)
- \( ach = \frac{1080 \times 60}{12490} \)
- \( ach = \frac{64800}{12490} \approx 5.18815 \)
- Since 5.18815 < 10000 and > 0.00001, display with 5 decimal places: \( 5.18815 \)
Example 2: For \( cfm = 0.5 \, \text{m³/s} \), room dimensions \( \text{length} = 5 \, \text{m} \), \( \text{width} = 500 \, \text{cm} \), \( \text{height} = 200 \, \text{cm} \), ACH in hr⁻¹:
- Convert: \( cfm = 0.5 \times 2118.88 = 1059.44 \, \text{ft³/min} \)
- Convert dimensions: \( \text{length} = 5 \times 3.28084 = 16.4042 \, \text{ft} \), \( \text{width} = 500 \times 0.0328084 = 16.4042 \, \text{ft} \), \( \text{height} = 200 \times 0.0328084 = 6.56168 \, \text{ft} \)
- Volume: \( 16.4042 \times 16.4042 \times 6.56168 \approx 1765.735 \, \text{ft³} \)
- \( ach = \frac{1059.44 \times 60}{1765.735} \)
- \( ach = \frac{63566.4}{1765.735} \approx 36.00015 \)
- Since 36.00015 < 10000 and > 0.00001, display with 5 decimal places: \( 36.00015 \)
Example 3: For \( cfm = 0.01 \, \text{L/s} \), room dimensions \( \text{length} = 10 \, \text{m} \), \( \text{width} = 1000 \, \text{cm} \), \( \text{height} = 100 \, \text{cm} \), ACH in hr⁻¹:
- Convert: \( cfm = 0.01 \times 2.11888 = 0.0211888 \, \text{ft³/min} \)
- Convert dimensions: \( \text{length} = 10 \times 3.28084 = 32.8084 \, \text{ft} \), \( \text{width} = 1000 \times 0.0328084 = 32.8084 \, \text{ft} \), \( \text{height} = 100 \times 0.0328084 = 3.28084 \, \text{ft} \)
- Volume: \( 32.8084 \times 32.8084 \times 3.28084 \approx 3531.47 \, \text{ft³} \)
- \( ach = \frac{0.0211888 \times 60}{3531.47} \)
- \( ach = \frac{1.271328}{3531.47} \approx 0.00036 \)
- Since 0.00036 < 10000 and > 0.00001, display with 5 decimal places: \( 0.00036 \)

5. Frequently Asked Questions (FAQ)

Q: What does air changes per hour (ACH) mean in HVAC systems?
A: Air changes per hour (\( ach \)) measures how many times the entire volume of air in a space is replaced with new air in one hour, indicating the ventilation rate and its effectiveness in refreshing indoor air.

Q: Why is ACH important for indoor air quality?
A: ACH is important because it quantifies how quickly indoor air is replaced, which helps dilute pollutants, control humidity, and maintain a healthy environment, especially in spaces with high occupancy or specific air quality requirements.

Q: How can I measure the dimensions of my room to calculate the volume?
A: Measure the length, width, and height of the room using a tape measure or laser distance measurer in your preferred units (e.g., feet, meters, or centimeters). The calculator will convert these dimensions to feet to compute the volume and ACH.