Duct Leakage Testing
Duct leakage testing procedures using a duct blaster, including total duct leakage, leakage to outside, sealing priorities, testing protocols, and the impact of duct leakage on system performance.
- Perform duct leakage testing using a duct blaster
- Distinguish between total duct leakage and leakage to outside
- Interpret duct leakage test results against code and program requirements
- Prioritize duct sealing strategies for maximum performance improvement
Lección 1
Duct Leakage Fundamentals
Why Duct Leakage Is a Major Problem
Duct leakage is one of the three biggest energy wasters in residential HVAC systems (along with incorrect charge and low airflow). Studies by the Department of Energy show that the average residential duct system leaks 20-30% of the air it is supposed to deliver. In a 3-ton system delivering 1,200 CFM, a 25% leak means 300 CFM of conditioned air is lost to unconditioned spaces (attics, crawlspaces, wall cavities).
The energy impact is enormous. A 25% duct leak in a system with ducts in a 130 F attic can increase cooling energy consumption by 30-40%. It also causes comfort problems - rooms at the end of long duct runs are starved of airflow while conditioned air pours into the attic.
Two Types of Duct Leakage
Not all duct leakage has the same impact. BPI and building codes distinguish between:
Total duct leakage - All air leaking from the duct system, regardless of where it goes. This includes leaks into the attic, crawlspace, wall cavities, AND into the conditioned space. Total leakage is measured with the building envelope open (windows and doors open).
Leakage to outside - Only the air leaking from ducts to unconditioned spaces (attic, crawlspace, garage, outside). This is the leakage that wastes energy because the conditioned air is lost from the building entirely. Leakage to outside is the more meaningful metric and is measured with the building pressurized to the same pressure as the duct system.
Leakage to Conditioned Space
Where: Into finished walls, between floors
Energy impact: Minimal - air stays in building
Comfort impact: Minor - air reaches rooms indirectly
Example: Leaky boot connection inside wall cavity
Leakage to Outside
Where: Attic, crawlspace, garage, outdoors
Energy impact: Severe - conditioned air completely lost
Comfort impact: Severe - rooms starved of airflow
Example: Disconnected flex duct in attic
How Duct Leakage Affects System Performance
Supply leaks - Conditioned air escapes before reaching rooms. Rooms at the end of the duct system receive less airflow. The building may depressurize, drawing in hot humid outdoor air through envelope leaks.
Return leaks - Unconditioned air (hot attic air, humid crawlspace air, dusty wall cavity air) is drawn into the return duct and mixed with the return air. This adds load to the system (the coil must cool attic air at 130 F instead of room air at 75 F) and can introduce pollutants, dust, and moisture.
Combined effect - Supply leaks depressurize the building while return leaks pressurize it differently. The net effect depends on the balance of supply vs. return leakage, but in most cases, the system works harder, delivers less capacity, and consumes more energy.
The average residential duct system leaks 20-30% of its airflow. Leakage to outside (attic, crawlspace) is far more damaging than leakage to conditioned space. Supply leaks waste conditioned air and depressurize the building; return leaks draw in hot, humid, or contaminated air. Both reduce capacity, increase energy consumption, and create comfort problems.