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Duct Diligence

Posted by GBM Research

May 30, 2014 7:42:00 AM

The majority of heating systems are forced air, and distribution problems often lie in duct design and integration. What new technologies and research can make these air distribution systems more efficient?

EFFICIENT AIR DISTRIBUTION is critical in the delivery of high performance homes. Innovations—simple and cost effective to implement—have been developed and tested throughout the country. But the application of these strategies continues to be a challenge—especially ones that address less-than-optimal situations—such as flex duct and splitter box air distribution systems and ducts in unconditioned spaces. More focus needs to be put on dealing with these scenarios.

effective duct junction box

Flex duct dos and don’ts. The image shows the wrong way (red) and the right way (black) to design the flex duct junction box. It is important that the width of the box is twice the inlet diameter, for proper air flow to the outlets. (Image: DOE)

The Bottom Line

The building scientists at Building America used this session to discuss flexible duct junction boxes, and the impact of encapsulating (and/or burying) ducts, in cases where placing ducts in conditioned space isn’t possible or practical.

bury duct for energy efficiency

Deep Results: Burying ducts more fully in insulation yields major efficiency gain. (Images: DOE)

Key Expert Comments/Recommendations

Flexible Duct Junction Boxes. Building America (IBACOS) has conducted extensive evaluation of duct junction boxes, and has found that in common configurations, duct junction boxes can create air flow instability and irregular air flows from outlets. The following recommendations prove to reduce these problems and create better, more consistent air flows to rooms:

  • Use a diffuse fitting at the entrance to recover velocity pressures and prevent air swirling.
  • Use a straight approach and a straight exit.
  • Design exit openings on the sides—no top or bottom exits.
  • Make exit openings at least two diameters from the entrance—the most important bit is the distance between the entrance and the first exit.
  • Make the box as small as possible, while complying with regulations.

Loose-Fill Insulation Type

Deeply Buried

Fully Buried

Partially Buried

Fiberglass

R-25

R-13

R -9

Cellulose

R-31

R-15

R-9

Buried (and Encapsulated) Ducts. Why bury and/or encapsulate our ducts? Because duct thermal losses can range from 10-25% and although putting ducts in conditioned space is the best current solution, in some cases it may be impractical, or increase envelope loads. In humid climates, burying ducts can cause condensation issues—but not when they’re encapsulated first with 1.5 inches of spray foam. Building America has extensively studied this practice and provides three key recommendations:

  • For new construction, planning is critical. The HVAC contractor must be on board to design and install a simple, low-profile duct system that can be encapsulated and buried.
  • In existing homes, coordination is often required between the HVAC contractor and insulator: ducts must often be reconfigured to be streamlined and closer to the ceiling.
  • Finally, there are situations where this approach is impractical: too many obstructions in attics, shallow roofs with insufficient attic space, etc.

Expert Tip:

Encapsulating/Burying Duct Retrofit

Flex and Rigid Ducts

Best Practice. In a buried duct installation, a combination of flex and rigid ducts has been shown to work effectively.

  • In a retrofit scenario, quality control is critical to avoid some of the challenges.
  • Existing flues must be protected.
  • Electrical wiring is sometimes permanently spray-foamed in place.
  • Supply plenum boxes were inadequately spray-foamed.
  • Exposed underside of ducts must be detailed properly.
  • Ductwork must be well sealed.

Installation Costs


Derived from responses by Robert Beach (IBACOS) and Robb Aldrich (Steven Winter Associates, Inc., Consortium for Advanced Residential Buildings).

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