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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.
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.
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: 
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 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: 
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: 
Flex duct runs should be as short and direct as possible without at the same time causing inappropriately sharp bends or kinks.- Sources:  Illustration adapted from .
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.
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.
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.
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.
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.
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: 
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.
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).
Hopefully needless to say,
flex duct should never be buried underground nor set into concrete slabs. 
SLAB DUCTWORK - catalogs the functional and environmental problems found when HVAC air ducts are routed in or below floor slabs
- Sources:  Illustration adapted from .
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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My house is one story on a slab, 1600sq ft. I am converting froms electric baseboard to a gas forced hot air system. All ducts are located in the attic. Since the return ducts will also be in the ceiling how many returns should I have the contractor install? How much efficency will I loose because the returns cannot be located close to the floor? - firstname.lastname@example.org 5/29/11
I agree that you're asking an important question about proper supply and return duct design, but I cannot answer it by email and with so little information. I'd ask the contractor to go over the design with you and to show you how the return air and supply air will be both adequate and balanced. Also pay attention to the locations of the supply and return registers - you don't want the return too close to the supply or you'll have a short circuit in the system that will prevent good operation.
I also agree that pulling cool air from close to floor level is preferable to pulling it from a ceiling, but I don't know an actual efficiency number. In part that's because you need to consider the effect of good (versus poor) location and distance between supply and return outlet and inlet.
how long I can use the fex duct work?
A home inspector say can not used mor the 12 feet is that true?
Angel, I think you are asking what is the maximum recommended length for flexduct in an HVAC system.
Flexible air duct systems should be sized and designed in accordance with ACCA Manual D for residential duct systems or other ICC referenced engineering standards. An engineering answer to the question of HVAC duct design length restrictions for a specific installation would have to consider the resulting air flow through the system, determined by a number of factors including:
Adding to all that complexity are other factors that can snarl up the delivery of air flow by a heater or air conditioner into a room, such as
OK so those are a bunch of factors that tell us how well a duct system will work.
But there are more: assuming you are describing using flex-duct to deliver air to a single supply register in some building room, is the flex duct routed directly to the room from the supply plenum, or did the designer include a larger diameter trunk line that brings conditioned air to the general area of the building, from which you are using a smaller diameter flex duct to get from the trunk line over to the actual air supply register?
In addition to duct design calculators and worksheets provided by duct manufacturers and HVAC industry associations, people often also do a "sanity check" on the duct design by applying a rule of thumb - much as you are asking about in your question:
The routing and length of flexible duct, the numbers of degrees of each bend and the amount of sag allowed between support joints will have serious effects on system performance due to the increased resistance each introduces. Use the minimum length of flexible duct to make connections. Do not install excess lengths of ducts to allow for possible future relocations of air terminal devices.
Flexible air duct trunk lines should not be limited in length. It is recommended as a guide that flexible air duct branch takeoffs (run-outs), flexible air ducts direct to boots from the plenum and flexible ducts in radial duct systems be limited in length to 25 feet (7620 mm). The preferred duct geometry is a “Trunk and Branch” system (see ACCA Manual D Figures 1-4 and 10-7).
... Terminal devices [air supply registers] should be supported independently of the flexible duct.
- using the International Mechanical Code & the Georgia state construction codes  as an example.
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