A duct leakage tester works by pressurizing the duct system and measuring the airflow required to maintain that pressure, which indicates the amount of leakage. Here’s how the process typically works:
- Preparation: All supply and return registers are sealed off to isolate the duct system12.
- Setup: A calibrated fan (duct tester) is connected to the duct system, usually at the return side3.
- Pressurization: The fan pressurizes the duct system to a standard pressure of 25 Pascals, which approximates typical operating conditions1.
- Measurement: A manometer measures the duct pressure (typically on Channel A) and the fan pressure (on Channel B)3.
- Calculation: The airflow through the fan at the test pressure is measured, quantifying the duct leakage. This is typically expressed in cubic feet per minute (CFM)1.
- Analysis: The leakage rate is often calculated as CFM per 100 square feet of conditioned floor area. For example, a leakage of 4 CFM per 100 square feet is commonly referred to as a 4% leakage rate5.
The test can be performed as a “total duct leakage test” or a “leakage to outside test,” depending on whether the goal is to measure all leaks or only those leaking to unconditioned spaces1. This method allows HVAC professionals to accurately assess duct system performance and identify areas needing improvement.
Interpreting the results of a duct leakage test involves understanding the measurements and comparing them to established standards. Here’s how to interpret the results:
- Measurement Units: Results are typically expressed in CFM25 (Cubic Feet per Minute at 25 Pascals of pressure)35.
- Leakage Rate: The leakage rate is often calculated as CFM per 100 square feet of conditioned floor area. For example, 4 CFM per 100 square feet is referred to as a 4% leakage rate1.
- Acceptable Thresholds:
- For rough-in tests (before drywall installation), total duct leakage should not exceed 3% or 3 CFM per 100 square feet of conditioned floor area1.
- If the air handler is included in the rough-in test, the acceptable threshold increases to 4% or 4 CFM per 100 square feet1.
- For post-construction tests, a leakage rate of 4% or less is generally considered acceptable8.
- Interpreting Results:
- Lower CFM25 values indicate less leakage and better duct system performance.
- Higher values suggest more significant leaks that may require attention.
- Compliance with Building Codes:
- Check local building codes for specific requirements. Many jurisdictions have adopted the 2018 or 2021 International Energy Conservation Code (IECC) standards4.
- Energy Efficiency Programs:
- Programs like ENERGY STAR may have their own criteria for acceptable duct leakage rates3.
If the test results exceed the acceptable thresholds, it indicates that the duct system needs sealing or repairs to improve its efficiency and performance.
Did somebody say building codes?
Yes, there are specific building codes related to duct leakage. The International Energy Conservation Code (IECC) and various state-specific codes provide requirements for duct leakage testing and acceptable leakage rates. Here are some key points:
2021 IECC Requirements
- All ductwork must be tested for leakage, even if it remains within the building’s thermal envelope12.
- For ducts outside the thermal envelope: Leakage must not exceed 4.0 CFM per 100 square feet of conditioned floor area at 25 Pascals2.
- For ducts inside the thermal envelope: Leakage must not exceed 8.0 CFM per 100 square feet of conditioned floor area at 25 Pascals2.
Earlier IECC Versions (2012-2018)
- Total leakage limit: 4 CFM per 100 square feet of conditioned floor area at 25 Pascals4.
- Rough-in test limits: 4 CFM per 100 square feet with air handler installed, 3 CFM without air handler4.
State-Specific Codes
- North Carolina Energy Conservation Code (2018):
- Total duct leakage limit: 5 CFM per 100 square feet of conditioned floor area6.
- High-efficiency option: 4 CFM per 100 square feet of conditioned floor area8.
- ENERGY STAR Version 3 Rev 11 criteria:
- Rough-in: ≤ 4 CFM25 per 100 square feet or ≤ 40 CFM25, whichever is greater3.
- Final: ≤ 8 CFM25 per 100 square feet or ≤ 80 CFM25, whichever is greater3.
It’s important to note that building codes can vary by jurisdiction and are subject to updates. Always check with local authorities for the most current requirements in your area.