Underground Duct
 PVS (Polyvinyl Steel) coated underground HVAC duct (also known as PCD, PVC coated, and PVCS) is UL® listed and specified more often by architects, engineers, and contractors than any other underground air delivery system because it offers both the strength of steel and the inertness of plastic. These attributes make PVS ideal for in underground as well as corrosive fume exhaust applications, such as in the plating industry.
How PVS is manufactured
PVS is manufactured by a three step process: 1) hot-dipped, galvanized G- 60 steel is cleaned and fire treated; 2) a special epoxy primer is baked onto both sides of the sheet; 3) Finally, a 4 mil. polyvinyl chloride coating is heat fused onto one side (4 x 1) for underground HVAC or onto both sides (4 x 4) for chemical fume exhaust applications. The result is a tough, corrosion resistant surface that will not crack, chip, peel, or rust.
• Placing ductwork underground results in a more aesthetically pleasing interior. Acoustical aesthetics is also improved since most or all of the "rumble" associated with interior duct is significantly reduced or eliminated.
• The space between the ceiling and roof can be reduced, making it easier to install electrical and fire suppression systems.
• Installing duct underground allows the air delivery system to be designed for optimum efficiency because ducts do not have to run through, or parallel to roof supports. In addition, runs inside interior or exterior walls can often be eliminated.
• Underground duct is a cost effective solution when air needs to be supplied to adjacent or contiguous buildings because a central unit can serve all locations.
• PVS duct requires no protection from concrete or the minerals and salts found in backfill.
• It is strong enough to walk on and will carry moderate soil loads; yet it can be cut or modified on the jobsite with circular or saber saws fitted with metal cutting blades. |
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Installation
Although PVS duct has been used successfully in underground applications for over 30 years, successful results depend on correct installation procedures.
Engineering Considerations:
• It is always recommended that duct systems– whether above or below grade?be designed by a qualified engineer and installed by a qualified contractor. When a concrete slab will not cover underground duct, special consideration must be given to potential future loading from heavy equipment. If such loading is expected, PVS duct can be incased in concrete, in which case the duct will need to be securely tied down to prevent "floating." When a concrete slab will cover the duct, loading is not as critical, but it is recommended that the duct not be buried deeper than 2.5 times its diameter. At depths greater than 2.5 times the diameter, additional measures must be taken to insure the duct does not collapse. Such measures include the use of painted angle-iron flange connectors for added stiffness at joints, special reinforcing around the duct, or (on ducts 36" or larger) internal reinforcement. Consult a qualified engineer when such measures are required or when there is any concern about loading.
Preparing the sub grade:
• PVS duct can be placed directly on the soil with no special precautions to protect the duct. However, drainage needs to be considered because standing water may eventually find its way into the duct causing mold and odor problems. The grade should always be sloped back to the utility room, and never place PVS duct at or below the water table.
• It is recommended that the duct be placed on 2" to 6" of pea gravel or other material that will permit easy drainage, especially when soil conditions are marginal. (Although not required to protect the duct, a vapor barrier should be placed under the duct and below the entire slab to prevent moisture from percolating through the concrete.) |
