Pressure Pan Readings Disagree With the Whole-House Test
A 2,200 square foot ranch has a centrally located air handler with supply ducts in the vented attic and return ducts in a chase that runs through the attic. You run a blower door to -50 Pa and take pressure pan readings at each register. Most first-floor registers read 1–2 Pa. Two registers at the far end of the supply trunk read 6 and 7 Pa. The whole-house blower door test gives 3,100 CFM50. The client has complained of hot rooms at the end of the house in summer and cold rooms there in winter, and a contractor has proposed sealing the two high-reading boots as the fix. The homeowner wants to know if that will solve the comfort complaint.
1. Interpret the pressure pan pattern and explain what the 6–7 Pa readings most likely indicate.
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What pressure pans measure: With the house at -50 Pa, a pressure pan over a register reads the pressure difference between the room and the duct behind that register. A reading near zero means the register boot and duct are well connected to the house pressure - little leakage from that terminal. A higher reading means the duct at that terminal is holding a different pressure, i.e., it is leaking to a space that is not at house pressure (here, the vented attic).
Pattern: Most registers at 1–2 Pa indicate the near-trunk boots are reasonably tight. The 6 and 7 Pa readings at the far end indicate those two boots and their adjacent duct runs leak meaningfully to the attic. That is consistent with long duct runs, more joints and elbows, and less attention during installation at the far end.
Caveat: Pressure pan is a screening tool, not a quantification. A 6–7 Pa reading flags a location for inspection; it does not tell you the CFM. Confirm by visually inspecting those boots and the duct joints above them before committing to a repair scope.
2. The contractor proposes sealing only the two high-reading boots. Evaluate that scope against the comfort complaint and the whole-house result.
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The proposed scope is too narrow for the stated complaint.
Why sealing the boots alone is unlikely to fix the comfort problem: The complaint is hot in summer and cold in winter at the same end of the house. That pattern points to insufficient delivered airflow and/or duct heat gain and loss along the run, not just boot leakage. Sealing two boots reduces attic leakage at those terminals but does nothing about:
- Duct surface area and insulation condition along the long run through a hot attic (heat gain in summer, loss in winter).
- Return path restrictions - the return chase through the attic is also a leakage and thermal liability.
- Balancing: if the far rooms are starved of airflow, sealing boots can slightly increase delivered air but will not rebalance the system.
What the whole-house number adds: 3,100 CFM50 is the envelope-plus-duct leakage at 50 Pa. It confirms the house is leaky overall but does not allocate leakage to the two boots. Using it to justify a boot-only scope overstates what the data supports.
Better scope: Inspect and seal all accessible attic duct joints, the air handler connections, and the return chase; verify insulation and that the ducts are not crushed or disconnected; then check room-by-room airflow and balance. Recommend a targeted duct leakage test before and after so the improvement is measurable. Tell the homeowner plainly that boot sealing alone is unlikely to resolve the comfort complaint.
3. The homeowner asks how much of the 3,100 CFM50 is duct leakage. What can you honestly say?
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You cannot apportion it from the data you have. The blower door measures the combined leakage of the envelope and any duct paths that connect to outside or to unconditioned space. It does not separate them.
What would separate them: A dedicated duct leakage test - for example a total duct leakage test at 25 Pa with registers masked, or a leakage-to-outside test with the house depressurized - run as its own procedure. Pressure pan readings screen for locations but do not produce a CFM total.
What you can say now: The pressure pan pattern shows that at least some leakage exists at the two far-end boots and, by inference, along that attic run. The magnitude is unknown. You can also note qualitatively that ducts in a vented attic plus a return chase through the attic create real opportunities for both leakage and thermal loss, which is consistent with the comfort complaint.
Honest framing for the client: "The blower door tells us the house as a whole is leaky; the pressure pans tell us where to look. Neither one gives us a duct leakage CFM. If you want that number, we run a separate duct test." Avoid assigning a percentage split - it would be fabricated.