CFM25 Explained: How Duct Leakage Results Are Compared
Two reports can both contain “CFM” and describe entirely different physical questions. CFM25 is the airflow a calibrated tester needs to maintain a 25-pascal pressure difference across the tested duct boundary; it is not the cooling airflow delivered by the equipment.
The unit becomes useful only when the test type and normalization basis are attached. A bare number cannot show whether a system is tight for its size, whether leakage reaches outdoors, or whether the result meets a particular code or program.
The 25 identifies the pressure used for the leakage test
Airflow through an opening changes with pressure. Reporting the test pressure allows different technicians to compare measurements taken at a common reference condition rather than at arbitrary fan speeds.
Twenty-five pascals is approximately one-tenth inch of water column, but that conversion does not make CFM25 an operating-static measurement. Registers are sealed and a calibrated fan establishes the leakage test.
CFM25 is leakage airflow, not equipment airflow
If a tester reports 120 CFM25, that means 120 cubic feet per minute through the calibrated fan was needed to hold the specified duct pressure under the defined setup. It does not mean the blower is missing 120 CFM during normal operation.
Actual losses while heating or cooling depend on operating pressures at each leak, supply-versus-return location, fan mode, and the space receiving or supplying the leakage. CFM-per-ton guidance addresses delivered system airflow and should not be used to reinterpret a leakage result.
Normalization makes systems of different size comparable
Programs often divide measured CFM25 by conditioned floor area served and express the result per 100 square feet. Other specifications may use nominal system airflow or a different denominator. The report must state both the raw flow and the basis.
A normalized percentage can look similar across two houses while hiding very different leak locations. Floor-area normalization supports compliance comparison; it does not determine energy loss without boundary and climate context.
Test stage changes what is included
A rough-in test occurs before all finish surfaces and registers are complete, while a final test includes finished boots, grilles, and usually the installed air handler according to the chosen protocol. The applicable leakage limit may differ by stage.
Comparing rough-in CFM25 directly with a final value can misstate progress because the tested boundary changed. Before-and-after repair testing should reproduce the same construction stage as closely as practical or explain every difference.
Total and outside values need separate labels
Total CFM25 includes all measured leakage from the connected system. CFM25 to outside isolates leakage across the building air barrier by controlling house pressure with an enclosure fan.
A smaller outside value does not invalidate the larger total result. It shows that some measured openings communicate with conditioned space. Whether those interior leaks matter for comfort may require room airflow or building-cavity investigation.
Pass or fail comes from the governing requirement
Codes, utility programs, certification standards, and contracts can specify different limits and exceptions. Existing-system diagnostics may have no formal pass threshold at all; the goal may be to identify repair opportunity and verify improvement.
The technician should cite the exact requirement, edition, test type, stage, and denominator rather than presenting one number as a universal national rule. Manufacturer airflow limits and duct-design targets answer separate questions.
A useful report allows the math to be checked
Look for raw CFM25, normalized CFM25, served floor area or other denominator, total-versus-outside label, system identifier, test pressure, equipment inclusion, and construction stage. A calculation should be reproducible from those entries.
When comfort remains poor despite a low normalized leakage result, continue with register airflow diagnostics, return-path checks, and equipment airflow verification rather than assuming the leakage test disproved the complaint.
A normalization example needs stated assumptions
Suppose a report lists raw leakage and the floor area served by that specific air handler. Dividing the measured CFM25 by the served square feet and multiplying by 100 produces a floor-area-normalized value. Shared spaces or multiple systems must not be counted twice.
That arithmetic only changes the reporting scale. It does not convert CFM25 into a percentage of cooling airflow unless the chosen program explicitly uses a different denominator and names it. Always retain the raw value so another reviewer can reconstruct the calculation.
Rounding should occur after the calculation at the precision required by the program. Premature rounding, an incorrect served-area figure, or combining two systems can shift a borderline normalized result without any change in the measured leakage.
Frequently Asked Questions
Is CFM25 the amount of conditioned air lost while the AC runs?
No. It is leakage airflow at a controlled test pressure with registers sealed. Operating loss requires more information about actual pressures, leak location, and system mode.
What does CFM25 per 100 square feet mean?
It is the measured leakage divided by the conditioned floor area served, then expressed for each 100 square feet. The denominator and applicable limit must be stated.
Can I compare CFM25 from two different contractors?
Yes only when test type, pressure reference, system boundary, construction stage, normalization basis, and included equipment are equivalent or clearly reconciled. Instrument calibration and fan configuration should also be documented.
Sources and verification
The technical statements in CFM25 Explained: How Duct Leakage Results Are Compared were cross-checked against the primary references below. Final sizing, airflow, electrical, combustion, and refrigerant decisions must also follow the exact equipment instructions and applicable local code.