Duct Leakage Testing

Duct Leakage Testing for new IECC construction requirements and Boulder SmartRegs.

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Duct Leakage Testing:

Duct Leakage testing has become a requirement in many jurisdictions and counties throughout Colorado. The purpose of duct leakage testing is to evaluate how well a home’s duct system can hold air. The idea behind it is that the more air that leaks out of the duct system, the more potential energy loss can occur.

Because of this, a pressure test is performed on the duct system itself to determine if the duct system is meeting the standards of today’s building codes.

Most CO municipalities have now adopted the 2018 IECC or the 2021 IECC. The 2018 IECC code is stated as follows:

Ducts shall be pressure tested to determine air leakage by either of the following:

  • Rough-in test: Total leakage measured with a pressure differential of 0.1 inch w.g. (25 Pa) across the system including manufacturer’s air handler enclosure – All registers taped or otherwise sealed
  • Post-construction test: Total leakage measured with a pressure differential of 0.1 inch w.g. (25 Pa) across the entire system including manufacturer’s air handler enclosure – All registers taped or otherwise sealed
  • Exception: Duct air leakage test not required where ducts and air handlers are entirely within the building thermal envelope.

The 2021 IECC is similar to the 2018 IECC except that it also requires duct leakage testing within the building thermal envelope. 

How a Duct Leakage Test Works:

First, you need to determine if a duct leakage test is required. The only time duct testing is required is when parts of the HVAC system are outside of the conditioned envelope. This includes areas like unconditioned attics, garages, and crawlspaces. If any supply or return duct chases, flexible lines or air handler units are in these areas, by IECC 2015 code, a duct leakage test is required. If a crawlspace is conditioned, then no test is required.

A duct leakage test (duct testing) can be performed at rough-in before drywalling, or post-construction. In both cases, the requirements are the same. The system will need to achieve a leakage rate of less than 4 CFM (cubic feet per minute) per 100 square feet of the home. Here is an example:

40 CFM moving through the ducts when put under pressure.

1000 sqft house.

38CFM / 1000sqft = 0.38 x 100 sqft = 3.8 CFM

This is commonly referred to as a 4% leakage rate.

Duct Testing Process:

Testing the ducts requires sealing all the openings of a duct system. This includes all supply and return registers. We do this by using large HVAC tape to cover and seal the openings. This process can take some time as we try to create the best seal possible. Our fan is then sealed to the air handler or a large return opening.

The fan is run to depressurize the duct system to -25 pascals. The amount of air moving through our fan is then measured. The number generated under 25 pascals of pressure is known as CFM@25. This CFM number is the number used in the calculation.

Sealing a register during a Duct Leakage Test

Testing Preparation:

After performing hundreds of tests, we have come up with a list to help you easily past our test.

  • Ensure your HVAC contractor is aware the test will need to be done before they put the system together. This usually ensures they use an adequate amount of sealing product during their installation.
  • Flexible ducts tend to leak around the zip ties. Minimizing these junctions, especially in attics, can be very helpful.
  • Use extra “pookie” around the edges of chase boxes and corners.
  • Heavily seal around the air handler. We commonly see poorly sealed ducts around the back of the furnace.
  • Have an airtight system for the filter slot. Have some type of weather-stripped door that will make a tight seal when the door is closed.
  • Plan to do the test during the rough-in so any problem areas can still be accessed.

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**Discounts are available for multiple units. Call for larger projects.

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Make your building process simpler with a duct leakage test from Scott Home Inspection and Scott Energy Services.