Duct Leakage Test on a Zoned System With a Sealed Register
You are performing a total duct leakage test to a target pressure of 25 Pa on a two-story house with a single central air handler and two motorized zone dampers. Before the test you tape off all supply and return registers and seal the air handler cabinet opening. With the duct pressurization fan running, the gauge holds 25 Pa and reads 145 CFM of leakage. While walking the house you notice one second-floor supply register boot was missed during masking; its damper is closed but the boot is open to the room. The zone damper serving that branch is also closed. You also observe that the return plenum has a factory-installed barometric bypass damper that is propped open by a piece of tape. The house volume is 18,000 cubic feet and the duct system is entirely inside conditioned space. The client wants a number they can compare to a program threshold of 6 CFM per 100 square feet of floor area, and the floor area is 2,400 square feet.
1. How should you treat the missed register boot and the propped bypass damper before accepting the 145 CFM result?
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Neither condition can be left as-is, but they fail in opposite directions and must be handled differently.
Missed boot (open to conditioned space): A supply boot open to the room is an intentional leakage path in the middle of a test whose purpose is to quantify leakage. Because the branch zone damper is closed, that path is isolated from the rest of the duct system, so it probably contributes little to the measured 145 CFM - but you cannot assume that. The correct action is to stop, seal the boot, and re-run. If the number does not change, you have documented that the closed damper isolated it; if it drops, the original reading was inflated.
Propped bypass damper: A barometric bypass is a designed pressure-relief path, not accidental leakage. Propping it open with tape creates a permanent open path that will read as leakage and will also depressurize the return relative to the supply, distorting the supply/return split. Remove the tape and let the damper seat normally, or if it must be defeated for the test, seal it closed and note that in the report.
Decision: Do not report 145 CFM. Seal the boot, restore the bypass to its normal operating state, re-establish 25 Pa, and record the new value with a note describing both corrections. Any result reported without those corrections is not defensible against the program threshold.
2. The corrected test reads 132 CFM at 25 Pa. The program threshold is 6 CFM per 100 square feet of floor area. Evaluate compliance and explain what the number does and does not tell the client.
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Threshold math: 6 CFM per 100 ft² × (2,400 ft² / 100) = 6 × 24 = 144 CFM allowable.
Result: 132 CFM is below 144 CFM, so the system passes by 12 CFM - a margin of about 8 percent. That is thin enough that the measurement uncertainty matters. Field duct tests commonly carry uncertainty of several percent from gauge calibration, temperature and barometric variation, and imperfect masking, so a 12 CFM margin should be reported as a pass with a caveat rather than a comfortable pass. Note the test pressure (25 Pa), the corrections made, and the calibration date of the gauge.
What the number tells the client: At a 25 Pa reference pressure, the duct system leaks 132 CFM, which is within the program allowance. It gives a comparable, repeatable figure for program compliance and for before/after comparison if sealing work is done.
What it does not tell the client: It does not say how much leakage occurs at normal operating pressures, which are far below 25 Pa, so the real operating loss is smaller and is not linearly scalable. It does not locate the leaks. It does not separate supply from return leakage, and return leakage in a house with ducts in conditioned space has different consequences than supply leakage. Finally, because the entire duct system is inside conditioned space, much of this leakage is energy-neutral - it moves air between the duct and the conditioned zone rather than to unconditioned space - so the client should not expect the full 132 CFM to translate into energy savings.
3. The client asks whether the same 132 CFM figure can be used to claim envelope leakage compliance too, since the ducts are inside the conditioned space. How do you respond?
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No. Duct leakage and envelope leakage are separate metrics measured with different setups and are not interchangeable.
Different test configurations: A duct leakage test pressurizes the duct system with the house at a defined reference condition and all registers masked; it measures air escaping the ducts. An envelope test (blower door) depressurizes the whole house to a reference pressure such as 50 Pa with registers and interior doors in their normal state; it measures air crossing the building shell. The 132 CFM was obtained with the ducts as the pressure boundary, not the shell.
Why inside-duct location matters: Because the ducts are entirely within conditioned space, duct leakage largely recirculates within the envelope and does not show up as envelope leakage. Conversely, envelope leakage paths - penetrations, attic hatches, rim joists - are invisible to the duct test. One number cannot stand in for the other.
What to tell the client: The 132 CFM is a duct leakage result only. If envelope compliance is required, a separate blower door test must be run and reported in its own units (for example CFM50 or ACH50), with the house prepared properly. Offer to schedule it, and make clear in the written report that the duct result does not satisfy or substitute for an envelope requirement.