Polyvinyl Duct (PVS) Spiral Pipe for Underground Systems

About 4×1 PVS for Underground Duct Systems

About Our Underground Ductwork Systems

Sheet Metal Connectors, Inc. PVS duct spiral pipe is formed from a coil of metal into a rigid PVC tube with a 4-ply spiral lockseam. It has a smooth interior for low friction loss with the grooved seam entirely on the outside. 

By combining the strength of steel and the chemical inertness of plastic, PVS (also known as PCD) ductwork for HVAC is both lightweight and corrosion- and weather-resistant. PVS duct spiral pipe will not rust, chip, crack, peel, corrode or produce odors. Although light in weight, polyvinyl-coated ductwork is strong and will not collapse under concrete if buried and graded properly.

PVS underground ductwork has a resistance to crushing approximately 2.5 times that of longitudinal lockseam. Optional corrugations and culvert grooves are available which increase the rigidity of the pipe by approximately 300%. Pipe sections can be joined together by an E-Z Flange with Barrel Clamp, E-Z Flange Jr. with Barrel Clamp, or Standard Spiral Pipe Connector.


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PVS Product Pages
PVS Installation Guide – Underground Ducts

Single-Wall Materials


Underground Ductwork Specifications & Benefits

The use of PVS Ductwork eliminates the need for encasing in concrete, propping up, or tying down. Since only a shallow trench is required, PVS duct will not float. And because no additional concrete is required, savings can be considerable.

One person can easily lift and carry a 10’ length of 20” diameter PVS around the jobsite, while two people can lift and position a 20’ length of 20” diameter duct. Although light in weight, PVS ductwork is strong and will not collapse under concrete when used underground and buried properly. PVS ductwork is specified by more engineers and architects than any other underground HVAC duct system.

How PVS Is Made

The PVS duct spiral pipe manufacturing process begins with (1) prime, hot-dipped, G-90 galvanized steel that is cleaned and fire-treated. Then (2) a special epoxy primer is bonded to both sides of the metal. Finally (3) a 4-mil polyvinyl coating is heat-fused to one side (4 x 1) or both sides (4 x 4).


PVS ductwork is a rugged, durable material that will take ordinary shipping and handling without damage. Polyvinyl-coated ductwork needs no protection in the earth from sand, concrete or plaster. It is not affected by the minerals and salts found in normal backfilling materials. PVS ductwork has also passed thousands of accelerated salt spray and humidity cabinet tests with no loss of adhesion or corrosion resistance. It can be cut and formed with regular sheet metal equipment and tools with no harm to the coating. Even the largest diameter ducts are easily cut with portable metal-cutting saber saws or circular saws.

Working With PVS 

PVS material can be cut and formed with regular sheet metal equipment and tools with no harm to the polyvinyl coating. Even the largest diameter ducts are easily cut with portable metal-cutting reciprocating saws or circular saws.

PVS Connections

pvc-ezfjwbcE-Z Flange Jr. with Barrel Clamp – 6” to 24” Diameter
The E-Z flange jr. is a 5/8” flange turned out 90° on each end of the spiral pipe and fittings. The installer applies a gasket on one flange, mates the two flanges together, and attaches the barrel clamp. For field cuts, a 5/8” flanged sleeve is available, cut the spiral pipe to the measured length and apply the sleeve.
*For dimensional data please refer to the E-Z Flange Specification Manual.

pvc-ezfwbcE-Z Flange with Barrel Clamp – 26” to 96” Diameter
The E-Z flange with barrel clamp can be factory installed or shipped loose for field installation. A set consists of two E-Z flanges and one barrel clamp. For field installation, the installer attaches the E-Z flange to the pipe and fittings. Next, the installer applies the gasket to one flange, mates the two flanges together and attaches the barrel clamp. SMC also installs E-Z flanges. Flanges are fastened and internally sealed on all ends of the spiral pipe and fittings.

*For dimensional data please refer to the E-Z Flange Specification Manual.

Standard Spiral Pipe Connector
pvc-sspcPipe to Pipe connections are made by using a fitting size coupling that slips inside the mating pipe sections, as shown. A stop bead runs around the middle of the coupling to center the coupling in the connection. Secure the connection by installing sheet metal screws through the outer shell of the duct, 1/2 inch from the stop bead.

c-pvc_seamsPVC Underground Fittings
Sheet Metal Connectors, Inc. PVC fittings and connections are fabricated in 1 x 4 or 4 x 4 material to protect against corrosion from soil. SMC PVC fittings are manufactured with an elbow lock seam or standing seam. SMC fabricates PVC fittings in most sizes and configurations using our state-of-the-art equipment. All joints and connections should be sealed in accordance with SMC’s recommendations to assure an airtight system.
*The PVC fitting dimensional data listed in the PVC Specification Manual is for slip-fit type connections only. For E-Z Flange PVC fitting dimensional data refer to the E-Z Flange Specification Manual.

All PVC fittings can be manufactured with:
E-Z Flange with Barrel Clamp – 26” to 96” Diameter
E-Z Flange Jr. with Barrel Clamp – 6” to 24” Diameter
Standard Spiral Slip Connector

Polyvinyl Duct (PVS) Products

Spiral pipe and fitting slip connector joints must be sealed to prevent air leakage and to keep dirt and sand out of the duct system. PVC Tape is recommended as it is ideal for this usage. It has superior adhesive qualities, even in extremely hot or extremely cold weather. Also as an insurance factor, PVC Duct Sealer No. 8 is available.

This is a flexible and self-curing air pressure sealant. This can be applied from one-gallon cans by brush application. It provides a positive sealing of joints and screw holes. PVC Duct Sealer No. 8 may also be obtained in a cartridge package. Approximate coverage is 375 linear feet per gallon at a 1/4” bead.
*PVC tape is not required for the PVC Spiral Pipe System



Trenching and Backfill

After the excavation has been made, no special bedding needs to be used for underground PVS Duct. It can rest right on the ground, or be set in sand or light aggregate. Pea gravel or sand (or the material taken from the trench, if equivalent) can be used to backfill. Spread the backfill material evenly around the duct making sure there are no gaps, and tamping in place is a recommended practice. No cement is needed to fill in around the duct, since the tamped fill holds the duct in place and the plastic coating prevents ground corrosion of the galvanized steel. This plastic coating is resistant to any minerals or salts that may be in the backfill soil. PVS Duct will not “float” when backfilled to within 2” of the top of the duct. Concrete is then poured so that all “tie-down” work is eliminated.

CAUTION: When backfilling or grading, care should be taken not to dump or push heavy loads directly on the duct, nor should heavy equipment be allowed to run over the duct. It can be crushed under thoughtless abuse.

Large Diameter Ducts

Special care should be taken with large-diameter ducts. Backfilling and tamping should be done without damage to the ducts. Temporary bracing can be used inside the ducts or angle rings can be permanently installed on the ducts to provide strength and rigidity. Rings should be primed and coated with PVS paint.

Polyvinyl Coated Ductwork Load Specifications

#1 Standard 4” – 8” 400 lbs./Linear Ft. 10” – 12” 600 lbs./Linear Ft.

#2 Corrugated 14” – 36” 1800lbs./Linear Ft.

*All ducts 14” or more in diameter are corrugated for extra strength.

Where deeper burial or heavier loading is required duct gauges can be increased. Also angle rings and/or special bracing can be used.

Specifications for PVS Underground Air Ducts

All ducts shall be PVS formed into spiral tubes. All couplings and fittings shall be factory fabricated of PVS. All joints and connections shall be screw fastened and taped in accordance with manufacturer’s recommendations to assure an air-tight system. Any damage to the coating shall be repaired with PVS touch-up paint or PVS aerosol spray.

All ducts and fittings shall be installed and back-filled according to manufacturer’s recommendations, using pea gravel, sand or equivalent with good compaction.

Polyvinyl Coated Ductwork Fume Exhaust Systems

Rectangular Duct Systems

Spiral Pipe Systems

PVS is an ideal material for many fume exhaust applications. Combining the strength of steel with the chemical inertness of plastic PCD offers completely integrated systems with diameters from 4” to 60” spiral pipe and all standard and special fittings that may be required. The assembly of the system is completed by the use of PCD No. 8 duct sealer and PVS tape or Hardcast tape.

Two Types Available

Polyvinyl-coated ductwork for fume exhaust is available in two styles, 4×4 and 4×1 REVERSE.

  • 4×4 has a 4 mil coating of polyvinyl chloride on both sides. It gives maximum interior and exterior protection.
  • 4×1 REVERSE, a more economical system, has the 4 mil coating on the inside and a grey wash coat on the outside for jobs where extra protection is not needed for the outside of the duct system.

Flat Sheets Available

Also available in gauges from 26. ga. to 18 ga. are flat sheets or wide coils for making hoods or forming rectangular duct systems.


Practical usage range is from -30º to 200º F.


PVS has been used with great success for many major exhaust systems. However, it is up to the user to determine the suitability of PVS for any particular job. Samples will be provided upon request.

Specifications for Fume Exhaust Duct Systems

Gauges shall be standard SMACNA gauges for low-pressure systems.

Round ducts shall be roll-formed spiral pipes. Fittings shall be factory fabricated with stainless steel rivets. All fitting seams shall be caulked on the inside with PVS No. 8 duct sealer.

All connecting joints are slip-fit. Apply No. 8 sealer to all raw edges and male ends. Secure joint connection by installing sheet metal screws with a minimum spacing around 6”. Apply No. 8 sealer over screws and seam of joint and wrap with tape (2).

Repair any damage to the coating with PVS touch-up paint or PVS aerosol spray.

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