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Duct leakage as a percentage of airflow is a critical metric for evaluating the efficiency and performance of an HVAC system. It quantifies the amount of conditioned air that escapes from the ductwork relative to the total airflow that the HVAC system is designed to move. Understanding and minimizing duct leakage is essential for improving energy efficiency, ensuring consistent indoor comfort, and reducing energy bills.

Explanation and Importance

Duct Leakage: This refers to the unintended loss of air from the duct system through holes, gaps, or disconnections. Leakage can occur in both the supply and return sides of the duct system, leading to wasted energy and uneven heating or cooling.

Percentage of Airflow: The duct leakage as a percentage of airflow is calculated by comparing the amount of air lost (duct leakage) to the total airflow the HVAC system is designed to manage. This percentage provides a clear indication of the system’s efficiency. For example, if an HVAC system is designed to move 1000 cubic feet per minute (CFM) of air, and 100 CFM is lost due to leakage, the duct leakage percentage is 10%.

Measured Airflow vs. Nominal Airflow

Measured Airflow: This is the actual airflow moving through the duct system as determined by testing with specialized equipment such as a duct blower. It accounts for real-world conditions, including any losses due to leakage or other inefficiencies.

Nominal Airflow: This represents the theoretical or design airflow specified by the HVAC system manufacturer. It assumes ideal conditions without accounting for any potential leaks or inefficiencies in the ductwork.

Step-by-Step Directions to Conduct the Test

Prepare the HVAC System:
Turn off the HVAC system.
Seal all supply and return registers with tape or temporary covers to prevent air from escaping through them.
Close all exterior doors, windows, and openings to isolate the duct system.
Install the Duct Blower:
Attach the duct blower (also known as a duct tester) to the air handler, either at the return grille or directly at the air handler unit. Ensure the connection is airtight.
Set Up Pressure Measurement:
Connect a pressure gauge to measure the pressure difference between the inside of the duct system and the outside.
Set up the blower door if necessary to measure the house pressure, but typically only the duct system pressure is measured.
Conduct the Duct Leakage Test:
Turn on the duct blower and pressurize the duct system to a standard test pressure, typically 25 Pascals (Pa).
Measure the airflow (in cubic feet per minute or CFM) required to maintain this pressure. This airflow is the Total Duct Leakage at the test pressure (CFM25).
Determine the Total Airflow of the HVAC System:
Measure the total airflow of the HVAC system using a calibrated device (anemometer [flow hood], digital TrueFlow Kit, etc.).
OR
Obtain the total design airflow of the HVAC system from the system specifications or manual (often provided in CFM).
Calculate the Duct Leakage as a Percentage of Total Airflow

By following these steps, contractors can provide homeowners with a clear, quantifiable assessment of their duct system’s efficiency. This information helps in making informed decisions about necessary repairs or upgrades to improve overall system performance and reduce energy costs. Highlighting the difference between measured and nominal airflow also underscores the importance of addressing duct leakage to achieve the intended performance of the HVAC system.

Duct Leakage to the Outside measures the amount of conditioned air that escapes from the HVAC duct system to the outside environment. This leakage is significant as it directly impacts the energy efficiency and performance of a home’s heating and cooling systems. By understanding and minimizing this leakage, homeowners can reduce energy consumption, lower utility bills, and enhance indoor comfort.

Why Is It Important?

Energy Efficiency: Leaky ducts mean that conditioned air intended for the living spaces is lost to the outside. This forces the HVAC system to work harder, increasing energy usage and costs.
Indoor Comfort: Duct leakage can lead to uneven heating or cooling, resulting in hot or cold spots within the home. Ensuring the duct system is sealed properly helps maintain a consistent and comfortable indoor temperature.
Air Quality: Leaks in the duct system can allow contaminants from unconditioned spaces (like attics or crawl spaces) to enter the ductwork and circulate throughout the home, potentially affecting indoor air quality and health.
System Performance: A well-sealed duct system ensures that the HVAC system operates efficiently and as designed, prolonging the lifespan of the equipment and reducing the need for frequent repairs.

Step-by-Step Directions for Conducting a Duct Leakage to the Outside Test

Preparation:
Ensure the HVAC system is turned off.
Seal all supply and return registers with tape or temporary covers to prevent air leakage through them.
Close all exterior doors, windows, and openings to isolate the duct system.
Install the Duct Blower:
Attach the duct blower (duct tester) to the air handler, either at the return grille or directly at the air handler unit. Ensure the connection is airtight.
Set Up Pressure Measurement:
Connect a pressure gauge to measure the pressure difference between the inside of the duct system and the outside.
Set up a blower door at the main entrance of the house to either depressurize or pressurize the house to the same pressure as the ducts. This step ensures that the measurement reflects only the leakage to the outside.
Conduct the Test:
With the blower door running, depressurize (or pressurize) the house to -25 Pascals (or +25 Pascals).
Simultaneously, the duct blower will pressurize (or depressurize) the ducts to the same pressure.
Measure the airflow (in cubic feet per minute or CFM) required to maintain this pressure in the ducts. This airflow measurement is the Duct Leakage to the Outside at the test pressure (CFM25).
Record Measurements:
Note the airflow (CFM) reading from the duct blower. This measurement is the Duct Leakage to the Outside at the test pressure (CFM25).

Using This Information with Homeowners

Contractors can use the results of the Duct Leakage to the Outside test to explain to homeowners the importance of duct sealing. By providing a quantifiable measurement of the air leakage and illustrating how it impacts energy bills and indoor comfort, contractors can effectively communicate the benefits of sealing ducts. This data helps homeowners understand the tangible benefits of investing in duct sealing, leading to improved HVAC efficiency, reduced energy costs, and enhanced indoor comfort.

By following these steps and using the measurements obtained from the Duct Leakage to the Outside test, contractors can provide valuable insights and recommendations to homeowners, fostering better energy management and improved living conditions.

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September 2025