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Duct Tightness Testing
Duct Test Failure & How to Pass

Most HVAC contractors we serve have figured out how to seal ductwork.  However, the first time the ductwork is tested, the duct test usually fails—and it has nothing to do with the ductwork! 

Primary Reasons for Duct Test Failure

Leaky Cabinets. Don’t be surprised to find a leaky cabinet!  Manufacturers are finally getting the memo from the field that their air handler units are too air leaky.  We’re beginning to see new cabinet styles.

Gaps Around Boots. The primary remaining leakage site is around register boots, where they go through wall, floor, and ceiling coverings: drywall or wood subfloors.  The physics is simple.  Supply air hits the supply grille and bounces back through the gap around the boot in the wall, floor, or ceiling openings.

Air Sealing Gaps Before Installing Interior Coverings

For new construction, it’s best to use register boots with flanges around openings.  When drywall is installed, sealant is applied to the flanges.

Air Sealing Gaps After Installing Interior Coverings

Otherwise, holes are closed by with foam, foil tape, or caulk.

Low-Expansion Foam. It’s faster, easier, and infinitely more effective use low-expansion foam on the unconditioned side of the register boot.  Within an hour, the duct test can be redone.

Foil Tape & Mastic. Metal foil HVAC tape can close the gap, but should be regarded as temporary until coated with mastic.  The trick is to close the gap while staying under the trim.

Foil tape is considered temporary because the adhesive will eventually dry and fail.  The solution should last as long as the air distribution system does.  Tape can be applied while we’re testing on-site and masticed after we leave.

Sealant. The other way seal the wall, floor, or ceiling coverings is by applying a bead of caulk in the gap between the boot and covering.  Home InSight doesn’t recommend this method, but acknowledges it may be the most practical way to seal the holes. 

First, until caulk “dries”, it usually falls out due to gravity—especially in ceilings. It also makes a mess!

Then there is the issue of cure time.  Several hours are needed before retesting can occur to avoid damaging the seal with pressurized air flow. We'll have to come back another day.

Finally, most people are clueless when it comes to proper sealant practices.  The product doesn’t work right when gaps are too small.  Even if the sealant was effective under test, the caulk may crack with seasonal changes when applied incorrectly. 

Just say “no” to using sealant unless it’s the only choice!


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