Infiltration Rate (Air Change Method) Calculator

1. What is an Infiltration Rate Calculator?

Definition: This calculator computes the infiltration air flow rate (\( cfm \)) using the air change method, based on the air changes per hour (\( ACH \)) and the gross space volume (\( V \)).

Purpose: It is used in HVAC design to estimate the rate of outdoor air entering a building due to leaks or ventilation, aiding in the sizing of heating, cooling, and ventilation systems.

2. How Does the Calculator Work?

The calculator uses the following formula for infiltration rate:

Infiltration Rate: \[ cfm = \frac{ACH \times V}{60} \]

Where:

\( cfm \): Infiltration rate (ft³/min, m³/s)
\( ACH \): Air changes per hour (hr⁻¹)
\( V \): Gross space volume (ft³, m³)

Unit Conversions:

Volume (\( V \)): ft³, m³ (1 m³ = 35.3147 ft³)
Infiltration Rate (\( cfm \)): ft³/min, m³/s (1 ft³/min = 0.000471947 m³/s)

Steps:

Enter the air changes per hour (\( ACH \)) and gross space volume (\( V \)), and select the volume unit.
Convert volume to ft³.
Calculate the infiltration rate using the formula.
Convert the result to the selected unit (ft³/min or m³/s).
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 Infiltration Rate Calculation

Calculating the infiltration rate is crucial for:

HVAC Design: Determines the amount of outdoor air entering a building, affecting the sizing of heating and cooling systems.
Energy Efficiency: Helps identify excessive infiltration, guiding improvements in building sealing to reduce energy losses.
Indoor Air Quality: Ensures adequate ventilation rates to maintain healthy indoor environments.

4. Using the Calculator

Examples:

Example 1: For \( ACH = 2 \, \text{hr⁻¹} \), \( V = 3600 \, \text{ft³} \), infiltration rate in ft³/min:
- \( cfm = \frac{2 \times 3600}{60} = 120 \)
- Since 120 < 10000 and > 0.00001, display with 5 decimal places: \( 120.00000 \)
Example 2: For \( ACH = 1.5 \, \text{hr⁻¹} \), \( V = 100 \, \text{m³} \), infiltration rate in m³/s:
- Convert: \( V = 100 \times 35.3147 = 3531.47 \, \text{ft³} \)
- \( cfm = \frac{1.5 \times 3531.47}{60} \approx 88.28675 \, \text{ft³/min} \)
- Convert to m³/s: \( 88.28675 \times 0.000471947 \approx 0.041667 \)
- Since 0.041667 < 10000 and > 0.00001, display with 5 decimal places: \( 0.04167 \)
Example 3: For \( ACH = 0.8 \, \text{hr⁻¹} \), \( V = 5000 \, \text{ft³} \), infiltration rate in ft³/min:
- \( cfm = \frac{0.8 \times 5000}{60} \approx 66.66667 \)
- Since 66.66667 < 10000 and > 0.00001, display with 5 decimal places: \( 66.66667 \)

5. Frequently Asked Questions (FAQ)

Q: What does the infiltration rate represent?
A: The infiltration rate (\( cfm \)) quantifies the volume of outdoor air entering a building per minute due to leaks or ventilation, expressed as a flow rate.

Q: How can I determine the air changes per hour (ACH)?
A: \( ACH \) values are typically obtained from standards like ASHRAE (e.g., Table 5-12), building codes, or experimental measurements, depending on building type and construction.

Q: Why is infiltration rate important in HVAC design?
A: It affects the heating and cooling load, as infiltrated air must be conditioned, impacting system sizing and energy consumption.