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Are there situations where using a positive duct test pressure is advantageous?
The main disadvantage to positive duct test pressure is that it blows off register seals or makes them leak. An LBL study showed a wide dispersion of results when the same duct test was repeated many times. Depressurizing will allow the use of lower sticking masking materials that are less likely to pull off the paint. In some cases plastic food wrap will work.
The main advantage to positive duct test pressure is that the flex duct doesn't contract as much. In cases where the flex runs to the ceiling this contraction effect might cause problems.
Testing using depressurization will usually result in less leakage because gaps and seams will be pulled closed”