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Duct Sealing & Indoor Air Quality

posted Dec 30, 2014, 11:35 AM by Stephen Mogowski   [ updated Dec 30, 2014, 11:37 AM ]
Quality duct design can create a comfortable environment for occupants.  Software programs Wrightsoft and Elite Software that utilize ACCA Manual D help create designs that will provide the recommended air flow for a given room or area based on variables like insulation, glazing specs and areas, room size, orientation, etc.

But once the plans are stamped and ground is broken, the focus shifts from planning to reality.  And the reality is the importance of excellent installation practices has never been more important.

In Phoenix, the majority of custom residential builders have shifted to conditioned attic systems (insulation applied to the roof deck).  This installation strategy reduces the heat gain on the duct system but does not diminish the importance of duct sealing with respect to indoor air quality.

Indoor air quality, the oft forgotten benefit of a tight duct system, can suffer drastically if the system isn't air tight.  Proper practices include sealing all transitions (i.e. Y's, S-Drives, etc) with mastic (duct sealant) or metal tape/sealant combination.  Flex to collar connections should be secured with a zip tie and sealant.  Collars to plenums should be secured with sheet metal screws and followed with sealant to prevent any leaks.

Remember, leaks on the return side will result in filter bypass and can expedite coil clogging-- which is a direct performance issue that will result in higher energy bills for the occupants, reduced air quality and potentially comfort complaints and service calls from a system freezing up.

Leaks on the supply side directly reduce the amount of air flow to that was engineered at conception.  It can also pressurize the attic system which (depending on the design) could create problems.  Furthermore, kinked/smashed/poorly routed ducts can reduce the recommended amount of supply air too.

Once the HVAC contractor finishes their installation, the drywall contractors will cut out a hole for each supply and return can.  Unfortunately, no one is perfect and the gaps around each supply boot and return can contribute to duct leakage if left unsealed.  Caulk is the easiest and most durable choice of sealant for this application.

For our firm, if we are performing a duct leakage test at final-- we require the contractor to seal these junctions before we test.  We typically see about 2-3% of CFA (conditioned floor area) leakage from this item.  When you consider the 2012 IECC only allows for 4% total for the entire system, you recognize this is a major contributor of duct leakage.

Creating a tight duct system combined with routine filter change-outs and a well-maintained home can lead to better health for our tenants. 

Caulk and duct sealant is cheap.  Quality is tough to come by.  If you're a HVAC contractor, it is time for you to take the next steps... because code is starting to require it.

  • Design with Manual D
  • Seal all seams and transitions
  • Contact HERS Rater for testing (if required or desired)
  • Follow up with caulk around all supply and return cans
  • At start up, seal all non-gasketed air handler cabinet doors with caulk or metal tape
  • Give homeowners a 3-pack of the filters you want them to use with your system and tell them where to buy replacements