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.) |
