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Testing & Diagnosis

What Is a Duct Leakage Test?

A duct leakage test is a diagnostic that quantifies how much air escapes a duct system before reaching its registers. A calibrated fan pressurises the sealed ducts to 25 Pascals and reports the airflow, CFM25, needed to sustain that pressure. It can be run as a total leakage test or a leakage-to-outside test, and it measures how much air is lost, not why comfort feels off in a given room.

Key takeaways

  • The test measures airflow lost from the duct system at a standard reference pressure, expressed as CFM25.
  • Total leakage counts all air lost anywhere along the ducts; leakage to outside isolates air lost outside conditioned space.
  • The test does not diagnose comfort complaints on its own — it establishes whether duct leakage is even part of the picture.
  • Results are meaningful mainly in context: relative to floor area, system airflow, and any applicable code or program threshold.
  • It is a repeatable measurement, which makes it useful for verifying sealing work rather than just diagnosing it.

Most likely causes

What you can safely check yourself

  • Have a rough idea of your home's conditioned floor area — it puts any leakage number in context.
  • Note whether ducts run through an attic, crawlspace or other unconditioned area, since that affects whether leakage matters as energy loss.
  • Check for any obvious disconnected duct sections, torn flex duct or fallen insulation that a technician should be told about beforehand.
  • Keep a record of when ducts were last worked on, since recent modifications can change how leakage is distributed.

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

  • Duct pressurisation to 25 Pa with all registers sealed, reading total system CFM25.
  • Leakage-to-outside testing, combining the duct fan with a house blower door to separate interior leaks from leaks to unconditioned space.
  • Visual tracing of return and supply runs to locate leak sources implied by the reading.
  • Comparison of the leakage number against system airflow (CFM) or conditioned floor area, since a raw number alone is not informative.

When duct leakage could be involved

The test exists because duct leakage is otherwise invisible. Ducts are usually hidden in attics, crawlspaces, chases and soffits, so a leak rarely announces itself the way a dripping pipe does. Pressurising the system and measuring the airflow needed to hold a set pressure converts that hidden loss into a specific, comparable figure.

Total leakage and leakage-to-outside answer different questions. Total leakage tells you how much air the system loses overall, including small losses inside conditioned space that barely matter for energy use. Leakage to outside tells you how much conditioned air is actually leaving the house — into an attic, for instance — which is the figure most relevant to energy loss and to any program requirement tied to it.

When this probably isn't a duct-sealing problem

  • A comfort complaint in one room with a tight duct system is more likely a balancing, sizing or return-air issue than a leakage issue.
  • High energy bills with low measured leakage point toward equipment efficiency, runtime or envelope factors instead.
  • Musty odours are not confirmed by a leakage test and may need separate moisture or filtration investigation.
  • A passing leakage result does not rule out restriction problems such as undersized returns or clogged filters.

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:

  • Leakage numbers are low but the system still short cycles or fails to reach set temperature.
  • Unusual noises, odours of burning, or refrigerant-related symptoms accompany the airflow complaint.
  • Static pressure readings suggest equipment or coil restriction rather than duct leakage.
  • The equipment itself is aging and its performance, not the ducts, appears to be the limiting factor.

Why this matters in Southeast Louisiana

In Southeast Louisiana's long cooling season, ducts routed through hot attics turn even modest leakage-to-outside numbers into a real, ongoing energy cost, which is why the outside-leakage distinction matters more here than in a home with all ducts inside conditioned space.

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Frequently asked questions

Is a duct leakage test the same as a home energy audit?
No. A duct leakage test is one specific measurement, focused entirely on the duct system. A full home energy audit may include duct testing as one component alongside insulation checks, air sealing of the envelope, and appliance review.
Do I need a duct leakage test if my ducts look fine?
Visible condition and measured leakage are not the same thing. Ducts can look intact while losing significant air through seams, boots and connections that are not visible without pressurisation, so a visual check alone cannot confirm a leakage rate.
What does 25 Pascals mean in this context?
25 Pascals is a standardised reference pressure used industry-wide so that leakage results from different systems and different times can be compared on equal footing. It is not related to normal operating pressure in a running duct system.
Can a duct leakage test tell me if my ducts are sized correctly?
No, sizing and leakage are separate questions. A tight duct system can still be undersized or poorly designed, and a leaky system can otherwise be sized adequately. Sizing is evaluated through airflow and load calculations, not the leakage test itself.

Find out what your ducts are actually doing

A duct leakage test gives you a specific, comparable number for your system instead of an assumption. Request an evaluation to see where your ducts stand.

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