Photograph of  this ugly duct routing risking water entry, mold, rodents, high operating cost. Heating & Air Conditioning Duct & Flexduct Routing & Support Specifications

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Air duct routing & support mistakes to avoid:

This HVAC duct design and repair article describes proper (and improper) Heating & Air Conditioning Duct Routing & Support such as duct routing and support details to prevent or fix loose, sagging, crimped, bent or otherwise defective heating or cooling ductwork.

Crimped, squashed, or sagging ductwork restricts cooling or heating airflow into a building. Air ducts routed in a concrete floor slab also invite many problems including air quality issues and collapsed ductwork that reduces airflow and increases system operating cost.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

HVAC Air Duct Routing & Support Errors & Guide to troubleshooting

Flex duct rests on ceiling joists - GA

Air conditioning duct system defects include a remarkably wide range of errors, from failure to supply cool air or failure to even circulate air in the building, to health hazards such as use of asbestos material in or on duct work, to very dangerous conditions such as drawing heating equipment combustion gases into the building cooling (or heating) air.

Article Contents

This article describes errors to avoid when installing ductwork in buildings.

Avoiding these errors will result in improved air flow, lower system operating cost, and may help in avoiding problems of mold or other contaminants in the heating or cooling duct system of a building.

The master document, of which this is a chapter, describes the inspection of residential air conditioning systems (A/C systems) to inform home buyers, owners, and home inspectors of common cooling system defects and repairs.

Sketches adapted from GA recommended construction code & from: [13][14][15][16]

HVAC Ducts in ground contact

Photograph of  duct work in ground contact in a crawl space

When heating and cooling ducts are placed in ground contact, such as in the crawl space shown in the photo, not only is the contact going to conduct heat or cooling away from the duct before the air reaches its destination.

Furthermore there is a significant risk of water leaks or accumulation of condensation in the duct in damp or flooding areas (risking a mold or health concern) or rodent entry.

Heating or cooling air ducts should be supported away from ground contact such as in crawl spaces.

Sharp Bends in Air Conditioning or Heating Duct Work Reduce Airflow

Photograph of  duct work in ground contact in a crawl space

Sharp bends in ductwork restrict airflow (and violate ASHRAE or SMACNA guidelines for duct installations).

Restrictions in airflow through duct systems increase the heating or cooling system operating cost and reduce the comfort of building occupants.

This photograph shows flex-duct in an attic making a too-tight 180 degree turn, crimping and restricting airflow in the duct system.

Bends in flex duct should not be acute and should not be less than one duct diameter in radius - Sources: [13][14][15][16]

Excessive or Unnecessary Duct Lengths Increase Cooling or Heating Cost

Photograph of excessive flex duct length

Excessive length of ducts is often found where flex-duct is installed by an amateur. If you see a length of flex-duct snaking across an area with multiple unnecessary twists and turns, the combination of length and unnecessary bends reduces airflow, with the costs just cited above.

This photograph shows unnecessary lengths of small-diameter flex duct left by the installer. The small diameter of these ducts also tells us that we're looking at a high-velocity air conditioning system that uses a combination of small-diameter ducts and higher air velocity to deliver cooling air to the conditioned space.

Flex duct runs should be as short and direct as possible without at the same time causing inappropriately sharp bends or kinks.- Sources: [13][14][15][16]

HVAC air duct kinks & sharp bends restrict airflow

Photograph of excessive flex duct length

Flex duct runs should be as short and direct as possible without at the same time causing inappropriately sharp bends or kinks.- Sources: [13][14][15][16] Illustration adapted from [13].

The smoothness of the duct interior (flex duct is more resistant to airflow than a solid duct surface) is affected by the degree of flexduct extension. ACCA Manual D includes a friction chart that takes this into consideration (Appendix 2, chart 7).

Incidentally, when installing a length of flexduct, you should not just leave extra duct length "squashed" and in the run. Duct calculations assume that you have removed excess length of flexduct and that the installed length of flex-duct has been "fully extended".Why? For duct interior smoothness and better airflow. Fully extended flex duct will significantly redue the friction losses in the sysem.

Flexible Ductwork Support Specifications

Photograph of pinched flex duct

Pinched or Crimped Air Ducts Reduce Heating or Cooling Airflow

Other common duct routing errors include sharp bends in duct work, mismatched sizes of duct work among sections, flex duct which has become crimped or pinched to restrict air flow such as in the photo at left, and of course ducts which have become disconnected.

We discuss and illustrate disconnected heating or air conditioning duct defects at DUCT CONNECTIONS.

We show the interior of crimped or squashed flexduct at DUCT DAMAGE, MECHANICAL.

The photo at left violates several of the flex-duct installation guidelines we cite just below:

Flexible duct shall be supported at manufacturer’s recommended intervals, but at no greater distance than 5 feet (1524 mm). Maximum permissible sag is ½ inch per foot of spacing between supports.

Photograph of pinched flex duct

Hanger or saddle material in contact with the flexible duct should be of sufficient width (minimum 1 ½” (38 mm)) to prevent any restriction of the internal diameter of the duct when the weight of the supported section rests on the hanger. Individual ducts should be separately supported.

Flex duct rests on ceiling joists - GA

Flexible ducts may rest on ceiling joists or truss supports. Maximum spacing between supports should not exceed the maximum spacing per manufacturer’s installation instructions.

Flex duct rests on ceiling joists - GA

Support the duct between a metal connection and a bend by allowing the duct to extend straight for a few inches before making the bend.

Vertical flexduct notes

Vertically installed duct should be stabilized by support straps at a maximum of 6 feet (1829 mm) on center.

Also, for cool air distribution from a blower assembly located below the area served, the height of ductwork determines how much cold air the blower has to push "up" - don't use flex-duct for vertical air supply risers past more than two floors in a building.

- Sources: [13][14][15][16]

HVAC Ducts in Concrete Floor Slabs are trouble

Our photo (below left) illustrates how you may spot an air duct routed through the building floor slab. In this case the furnace return air plenum was also located below the concrete floor. The air duct system in this building had been subject to periodic flooding, as illustrated in our second photo (below right).

A description of the health and functional problems that may be traced to air ducts that were routed in a concrete floor slab as well as our advice on how to properly abandon and seal in-slab air ducts are found at DUCT in CONCRETE FLOOR.

Water and rodents in air duct © D Friedman at Water and rodents in air duct © D Friedman at

There we describe concerns with ductwork run in floor slabs in the article above, including risks of air duct collapse that interferes with air flow through the system, water leaks into the in-slab duct system (not a problem unique to transite ducts), and rodent or insect infestations or even mold contamination. Odor complaints may be traced to the duct system because of these problems (DUCT & AIR HANDLER ODORS).

Below, reader-contributed photographs of problems in spiral metal ductwork routed in a concrete floor slab illustrate rust, flooding history (below left) and ductwork collapse (below right).

Water and rodents in air duct © D Friedman at

We discuss and illustrate disconnected heating or air conditioning duct defects at DUCT CONNECTIONS. We also show the interior of crimped or squashed flexduct at DUCT DAMAGE, MECHANICAL.

Water and rodents in air duct © D Friedman at

Hopefully needless to say,

flex duct should never be buried underground nor set into concrete slabs. [13]

SLAB DUCTWORK - catalogs the functional and environmental problems found when HVAC air ducts are routed in or below floor slabs

Flex Duct Connection & Splicing Specifications

Flex duct splice connection - GA Building Standards et als

- Sources: [13][14][15][16] Illustration adapted from [13].


Continue reading at FLEX DUCT SOURCES & HISTORY or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


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