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Plastic Duct Fittings for Installation

Evaluation of Plastic Duct Fittings For Underground Installation

Client:

General Plastics 3500 N. Harrison Shawnee, OK 74801

Contact:

John Inda, President Telephone: 405-275-3171 Facsimile: 405-275-8114

Scope:

General Plastics manufactures plastic duct fittings for installation in heating, ventilating, and air conditioning (HVAC) systems. JB Engineering and Code Consulting, P.C. was retained to evaluate the plastic duct fittings for use in an underground duct system.

Abstract:

Underground HVAC ducts systems have been installed with sheet metal ducts. The ducts are either encased in concrete or lined with an exterior coating. While the ducts are protected from direct exposure to the ground, there have been failures of underground duct systems. The failures have resulted in ducts allowing the leakage of water or the entrance of vermin. The failed duct system can also result in mold and bacteria growth.
  To reduce the possibility of failure, underground ducts have been installed using plastic pipe with solvent cement and mastic joints. General Plastics has developed a full range of duct fittings for underground duct systems using plastic duct.

Product Description:

The General Plastic duct fittings are manufactured with either high density polyethylene (HDPE) or polyvinyl chloride (PVC). There is a full range of duct fittings in sizes 4 inch through 24 inch, including tees, wyes, elbows, 45E elbows, register boxes, saddle boots, saddle connections, reducers, couplings, end caps, and starting collars. Plastic plenum boxes are also available for underground installation. The HDPE fittings are black in color, identified by the trade name Black Max®. The PVC fittings are white in color. The PVC plenum boxes are gray in color.

Test Reports: JB Engineering and Code Consulting, P.C. had the General Plastic duct fittings tested by SGS U.S. Testing Company, Inc., Tulsa, Oklahoma. The test report numbers and dates are, as follows:

Report No. 109962-1, dated 7/9/98 by SGS U.S. Testing Company, Inc.
Report No. 109962-2, dated 7/9/98 by SGS U.S. Testing Company, Inc.
Report No. 109962-3, dated 7/9/98 by SGS U.S. Testing Company, Inc.
Report No. 109962-4, dated 7/9/98 by SGS U.S. Testing Company, Inc.

The first test was an external loading test conducted in accordance with Section 6.3.2 of ASTM D2665. This test requires pipe fittings to withstand a load of 750 lbf/ft without failure. Fittings of every size were tested. All of the fittings passed the test without any signs of failure.

The second and third test were cell classification tests for rating the raw material used to produce the fittings. The PVC had a cell classification of 12344A. The HDPE had a cell classification of 33663.

The fourth test was a pressure tightness test. A 6 inch duct assembly was joined in accordance with the manufacturer's installation instructions. The assembly was subjected to a 4 foot head of pressure (1.7 psi) on the exterior. There was no leakage present in the assembly.

Engineering

Analysis: Plastic duct and fittings are well suited for underground installation. The material is resistant to corrosion and rusting. The duct will provide a long, usable life as a part of the HVAC system.

The joining method of plastic duct and fittings is watertight and airtight for the normal pressures anticipated in an HVAC system. The duct will provide service without outside leakage when completely exposed to ground water on the exterior of the duct system. The duct is buried below the slab, hence, it would not be subjected to a ground water pressure that would approach the tested 4 feet of head pressure.
Report Number: 07G0516E1
May 16, 2007

Both PVC and HDPE are acceptable materials for temperatures up to 150E F. Standard furnaces and air handlers for HVAC systems limit the outlet temperature to less than 150E F. This would be required for any underground plastic duct system.

The smooth interior of the plastic duct has minimal friction resistance to air flow. The system can be sized using the round duct sizing charts and programs. The General Plastic duct fittings have a pattern and smooth interior that is equivalent to round duct fittings. The sizing charts and programs for the round duct fittings can be used when evaluating the plastic duct fitting for sizing the system.

In a comparative analysis, PVC plastic ducts are the same materials as PVC plastic pipe used below ground in plumbing systems. PVC plastic pipe has proven to be an acceptable material for underground piping systems. The material has been installed in the United States since the 1950's. Widespread use of PVC plastic pipe below grade dates back to the late 1960's. PVC plastic duct and fittings installed below grade should experience the same longevity as PVC plastic pipe used in plumbing systems.

 

Since the plastic duct and fitting systems are installed below grade, they are not directly exposed to a fire within the building. Therefore, the fire characteristics of the duct and fittings would not be applicable to underground duct systems.

Code Analysis: The Mechanical Codes regulate the installation of duct systems. The use of General Plastics duct fittings installed in an underground plastic duct system were analyzed for compliance to the various Mechanical Codes.

ICC International Mechanical Code: Section 603.8.3 regulates plastic underground duct systems. Plastic ducts are required to be constructed with PVC having a minimum pipe stiffness of 8 psi at 5percent deflection when tested in accordance with ASTM D2412. Plastic duct fittings are required to be either PVC or high density polyethylene. The General Plastics plastic ducts and fittings conform to the requirements of the International Mechanical Code.

ICC International Residential Code: Section M1601.1.2 permits the use of plastic underground duct systems. The plastic duct must have a cell classification of 12454-B (or higher). The duct and fittings must meet the external loading properties of ASTM D2412. The external load test in ASTM D2412 is for pipe stiffness of plastic pipe. The General Plastic duct and fittings conform to the requirements of the International Residential Code.
Report Number: 07G0516E1
May 16, 2007

IAPMO Uniform Mechanical Code: The Uniform Mechanical Code does not directly regulate plastic underground ducts. Section 604.1 requires metal ducts to be installed a minimum of 4 inches above grade. Metal ducts in or below grade are required to be encased in a minimum of 2 inches of concrete.

Section 105.0 permits the use of alternative material that is equivalent to the material permitted by the code. Plastic ducts perform the same as metal ducts when installed underground. Plastic ducts exceed the requirements of metal ducts encased in concrete since they are water and air tight, will not corrode, and provide adequate strength without concrete encasement.

Installation

Requirements: The General Plastic duct fittings must be installed in accordance with the manufacturer's installation instructions and the following:

The plastic ducts and fitting shall be joined in accordance with the manufacturer's instructions.
The furnace or air handler shall limit the temperature of the air in the plastic duct system to a maximum of 150E F.
Summary: General Plastic duct and fittings conform to the requirements of the ICC International Residential Code and the ICC International Mechanical Code for underground plastic duct systems. General Plastic duct and fittings meet the intent of the IAPMO Uniform Mechanical Code for underground plastic duct system as an alternative material. The code official should permit the use of General Plastic duct fittings for all underground HVAC plastic duct systems.

Certification: This report was prepared by Julius Ballanco, P.E., President, JB Engineering and Code Consulting, P.C., registered as a Professional Engineer in the State of Indiana, license number PE60900631. JB Engineering and Code Consulting, P.C. is a Registered Engineering Professional Corporation in the State of Indiana, license number PC50910000.

Respectfully submitted,

Julius Ballanco, P.E. President

FIND US

General Plastics, Inc

3500 North Harrison,

Shawnee, OK 74804

Phone. 888-275-3171

Email. sales@general-plastics.com