Percent of Outside Air Calculator

1. What is a Percent of Outside Air Calculator?

Definition: This calculator computes the percentage of outdoor air in the total supply air of an HVAC system, based on the outdoor air flow rate and total supply air flow rate.

Purpose: It is used in HVAC system design to assess the proportion of outdoor air being supplied, helping to evaluate ventilation effectiveness, energy efficiency, and indoor air quality.

2. How Does the Calculator Work?

The calculator uses the following formula for the percent of outside air:

Percent of Outside Air: \[ \text{Percent of outside air} = \frac{Q_{oa}}{Q_s} \times 100 \]

Where:

\( \text{Percent of outside air} \): Percentage of outdoor air (%)
\( Q_{oa} \): Outdoor air flow rate (cfm, m³/s, L/s)
\( Q_s \): Total supply air flow rate (cfm, m³/s, L/s)

Unit Conversions:

Air Flow Rates (\( Q_{oa} \), \( Q_s \)): cfm, m³/s (1 m³/s = 2118.88 cfm), L/s (1 L/s = 2.11888 cfm)

Steps:

Enter the outdoor air flow rate (\( Q_{oa} \)) and total supply air flow rate (\( Q_s \)), and select their units.
Convert both flow rates to cfm.
Calculate the percentage of outside air 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 Percent of Outside Air Calculation

Calculating the percent of outside air is crucial for:

Indoor Air Quality: Determines the proportion of fresh outdoor air in the supply, ensuring sufficient ventilation to dilute indoor pollutants.
Energy Efficiency: Helps balance outdoor air intake with energy consumption, as higher outdoor air percentages may increase heating or cooling loads.
HVAC Design: Assists in designing ventilation systems that meet air quality standards (e.g., ASHRAE guidelines) while optimizing system performance.

4. Using the Calculator

Examples:

Example 1: For \( Q_{oa} = 1080 \, \text{cfm} \), \( Q_s = 3600 \, \text{cfm} \), percent of outside air:
- \( \text{Percent of outside air} = \frac{1080}{3600} \times 100 \)
- \( \text{Percent of outside air} = 0.3 \times 100 = 30.00000 \)
- Since 30.00000 < 10000 and > 0.00001, display with 5 decimal places: \( 30.00000 \)
Example 2: For \( Q_{oa} = 0.5 \, \text{m³/s} \), \( Q_s = 2 \, \text{m³/s} \), percent of outside air:
- Convert: \( Q_{oa} = 0.5 \times 2118.88 = 1059.44 \, \text{cfm} \), \( Q_s = 2 \times 2118.88 = 4237.76 \, \text{cfm} \)
- \( \text{Percent of outside air} = \frac{1059.44}{4237.76} \times 100 \)
- \( \text{Percent of outside air} \approx 0.25 \times 100 = 25.00000 \)
- Since 25.00000 < 10000 and > 0.00001, display with 5 decimal places: \( 25.00000 \)
Example 3: For \( Q_{oa} = 1 \, \text{L/s} \), \( Q_s = 100 \, \text{L/s} \), percent of outside air:
- Convert: \( Q_{oa} = 1 \times 2.11888 = 2.11888 \, \text{cfm} \), \( Q_s = 100 \times 2.11888 = 211.888 \, \text{cfm} \)
- \( \text{Percent of outside air} = \frac{2.11888}{211.888} \times 100 \)
- \( \text{Percent of outside air} \approx 0.01 \times 100 = 1.00000 \)
- Since 1.00000 < 10000 and > 0.00001, display with 5 decimal places: \( 1.00000 \)

5. Frequently Asked Questions (FAQ)

Q: What does the percent of outside air mean in HVAC systems?
A: The percent of outside air indicates the proportion of fresh outdoor air in the total air supplied by an HVAC system, reflecting the balance between ventilation and recirculation.

Q: Why is the percent of outside air typically less than 100%?
A: HVAC systems often recirculate indoor air to reduce energy costs for heating or cooling. The percent of outside air is less than 100% when recirculation is used, with the remainder being recirculated air, often filtered to maintain air quality.

Q: How can I determine the outdoor and supply air flow rates for my system?
A: Outdoor air flow rate (\( Q_{oa} \)) and total supply air flow rate (\( Q_s \)) can be determined from HVAC system specifications, design standards (e.g., ASHRAE), or measured using airflow sensors in the system.