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Duct Leakage Testing Explained: What New York Homeowners Should Know
A forced-air system can heat or cool the equipment room correctly while delivering too little conditioned air to the rooms that need it. In Westchester houses with ducts in attics or basements—and in Bronx homes where chases, soffits, or additions complicate the distribution system—loose joints and disconnected runs can turn a comfort complaint into a hidden airflow problem.
Duct leakage testing replaces guesswork with a measured result. ENERGY STAR reports that a typical house can lose roughly 20% to 30% of the air moving through its duct system because of leaks, holes, and poor connections. That national figure is context, not a prediction for any individual home; the useful question is what your own system is doing.
What duct leakage testing actually measures
For the test, the HVAC equipment is turned off and the supply and return registers are temporarily sealed. A calibrated fan is connected to the duct system, usually at a large return or the air handler. The fan creates a standard pressure difference, and a digital gauge measures how much airflow is required to hold that pressure. More airflow needed to maintain the test pressure generally means more leakage.
Results are commonly reported as CFM25: cubic feet of air per minute at 25 pascals of pressure. CFM25 is not a utility-bill estimate. It is a repeatable measurement that lets a trained professional compare the system with an applicable standard, the home's size, the location of the ducts, and the purpose of the test. A number without that context can be misleading.
Total leakage versus leakage to outside
A total-duct-leakage test counts air escaping anywhere in the duct system, including leakage into conditioned space. A leakage-to-outside test isolates the portion communicating with attics, garages, vented crawlspaces, or outdoors. The distinction matters: leakage inside the home's thermal boundary may still affect airflow and comfort, while leakage outside can send paid heating or cooling directly out of the conditioned space.
Why it is not the same as a blower-door test
A blower-door test measures leakage through the building enclosure—the walls, ceilings, floors, windows, doors, and penetrations that separate indoors from outdoors. A duct test measures the air-distribution system. The tools can be used together, but they answer different questions.
That is why a complete diagnostic plan may combine duct testing with pressure measurements, airflow checks, visual inspection, thermal imaging under suitable conditions, or a blower-door test. For a broader view of how those pieces fit together, see NYEA's guide to the home energy audit process.
What the result can—and cannot—tell you
The test produces a system-level leakage number; it does not, by itself, point to every failed joint. The next step is interpretation. A professional may inspect accessible trunks and branches, compare supply and return behavior, or use controlled pressure techniques to narrow the search. The Building Performance Institute maintains training and certification for technicians who perform infiltration and duct-leakage testing to recognized procedures.
Total leakage helps quantify the overall condition of the distribution system.
Leakage to outside helps identify losses connected to unconditioned areas.
Room-by-room comfort complaints help connect the measured result with actual performance.
A post-repair test can document the change using the same method, without promising a particular bill reduction.
Clues that make testing worth discussing include rooms that stay too hot or cold, unusually dusty areas, weak airflow at some registers, ducts routed through an attic or crawlspace, or an HVAC replacement planned for the near future. These signs are not proof of leakage; they are reasons to investigate.
What happens during a professional test
Review comfort complaints, recent renovations, HVAC zones, and where ducts are located.
Turn off the air handler and identify the registers that belong to the system being tested.
Temporarily seal the supply and return openings and connect the calibrated test fan.
Run the test at the specified pressure, record the measurement, and check the setup for accuracy.
Explain whether additional diagnostics, targeted sealing, duct repair, or no immediate action makes sense.
A simple homeowner preparation checklist
Clear furniture, rugs, and belongings away from supply and return registers.
Write down which rooms are uncomfortable and when the problem is most noticeable.
Share information about additions, finished basements, or past HVAC work.
Make attic, basement, mechanical-room, or crawlspace access available only when it is safe.
Secure pets before temporary register covers and test equipment are installed.
Do not remove equipment panels, enter unsafe spaces, or attempt combustion, electrical, or refrigerant work yourself.
Test before you size or replace equipment
Testing is especially useful before replacing a furnace, central air conditioner, or heat pump. NYSERDA advises homeowners to consider major air leaks and duct design before investing in new equipment. A new unit cannot correct a disconnected branch or a return path that pulls air from an unconditioned space.
Duct sealing is not automatically the first or only recommendation. Some systems have crushed flex duct, inadequate return paths, inaccessible sections, or design problems that need repair rather than sealant. Accessible joints may be sealed with materials intended for ductwork, but concealed systems and homes with combustion appliances deserve professional evaluation so pressure changes do not create safety or ventilation problems.
Choose the next step from the evidence
If uneven temperatures, weak airflow, or a planned HVAC upgrade has you questioning the duct system, start with the specific problem—not a predetermined repair. Learn more about NYEA's duct leakage testing service, or contact New York Energy Assessments to discuss an appropriate diagnostic scope for a Westchester or Bronx home.
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