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Duct DaddyAir Duct Sealing

Testing & Diagnosis

How Is Duct Leakage Tested?

Duct leakage is tested with a calibrated fan (a duct blaster) connected to the duct system, usually at the air handler. Registers are sealed, the fan pressurises the ducts to a standard reference pressure of 25 Pascals, and the fan reports the airflow — in CFM25 — required to hold that pressure. That airflow is a direct measure of how much air the duct system is losing through leaks.

Key takeaways

  • A calibrated fan pressurises the sealed duct system to a standard 25 Pa reference point.
  • The fan's own airflow readout, CFM25, is the leakage measurement.
  • Total leakage is tested with the house at ambient pressure; leakage to outside adds a house pressurisation step.
  • The test is repeatable, which is what makes before-and-after comparisons meaningful.
  • Results are reported relative to floor area or system airflow, not as a raw number alone.

Most likely causes

What you can safely check yourself

  • Note whether registers and returns are already sealed or covered before a scheduled test — the technician will do this, but it helps to know why.
  • Locate the air handler and main trunk lines so you can observe where the test equipment connects.
  • Have any recent duct modifications or additions on hand to mention, since new branches change the test result.
  • Check that attic or crawlspace access is clear, since a technician may need to inspect ducts during the same visit.

Stick to what you can see, feel and hear. Leave electrical panels, refrigerant, combustion appliances and equipment internals to a licensed professional.

What a professional should measure

  • Total duct leakage test: registers sealed, ducts pressurised to 25 Pa, CFM25 recorded for the whole system.
  • Leakage-to-outside test: adds a blower-door depressurisation of the house to isolate leaks that exit conditioned space from leaks that stay inside it.
  • Visual and manometer inspection of connections, boots and plenum seams to correlate the number with physical leak locations.
  • Airflow verification at registers, to see whether measured leakage lines up with weak or strong delivery in specific rooms.
  • Static pressure readings across the air handler, which help distinguish a leakage problem from a restriction problem.

When duct leakage could be involved

A duct system is a sealed pathway that only works as designed if the air entering it at the air handler is the air arriving at the registers. Pressurisation testing turns that assumption into a number by forcing air into the system and measuring how much escapes before it reaches the intended exit points.

Because the test uses a fixed reference pressure, results from different homes, or the same home at different times, can be compared on equal terms. That is the entire value of the method: it replaces a guess about leakage with a measurement taken the same way every time.

When this probably isn't a duct-sealing problem

  • A test that returns a low CFM25 relative to system airflow indicates the ducts are not the source of an airflow or comfort complaint, and attention should shift to equipment, filters, coils or duct design.
  • Rooms with uneven comfort despite tight ducts point toward sizing, balancing or return-air issues rather than leakage.
  • A recent equipment change with no duct modification does not usually shift a leakage number meaningfully on its own.
  • Humidity or condensation complaints with tight ducts are more likely tied to runtime, sizing or envelope moisture than to leakage.

When to call an HVAC contractor

Duct Daddy specialises in duct leakage testing and duct sealing. We do not perform mechanical HVAC repair, refrigerant work or equipment replacement. Contact a qualified HVAC contractor if any of the following apply:

  • The test points to equipment-side symptoms such as short cycling, refrigerant issues or unusual sounds from the unit itself.
  • Static pressure readings suggest a restriction rather than a leak, such as a clogged filter or undersized return.
  • Thermostat, control board or electrical behaviour needs diagnosis alongside the duct assessment.
  • The system is old enough that equipment performance, not the ducts, is the limiting factor.

Why this matters in Southeast Louisiana

Many Southeast Louisiana homes run ducts through hot, humid attics, where a leaking supply run loses conditioned air into space that is anything but conditioned — one reason a pressurisation test is worth doing before assuming equipment is underperforming.

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Frequently asked questions

Does the house need to be empty or shut up for a duct leakage test?
The house does not need to be empty, but exterior doors and windows are typically closed, and the technician will seal registers and grilles temporarily. Normal household activity can usually continue during the test.
How long does a duct leakage test take?
A total leakage test typically takes well under an hour once access to the air handler and registers is available. Adding a leakage-to-outside measurement extends the visit slightly, since it requires setting up a blower door as well.
Can duct leakage be tested without special equipment?
Not reliably. Visual inspection can spot obvious disconnections or gaps, but quantifying leakage requires a calibrated fan and pressure gauge. Without that equipment, any number quoted is an estimate rather than a measurement.
Is one test enough, or should leakage be tested more than once?
A single test establishes a baseline. A second test after sealing work verifies how much leakage was actually eliminated, which is the only way to confirm that sealing achieved its intended result rather than assuming it.

Get a measured answer, not a guess

A duct leakage test replaces speculation with a number specific to your system. If leakage turns out not to be the issue, you will know that too, and can direct attention where it belongs.

Schedule a Duct Leakage Test

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