Metal-Fab 6" to 16" Diameter Corr/Guard Aluminum Eccentric Increaser

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Metal-Fab provides complete and comprehensive venting solutions to the HVAC, Hearth and Plumbing markets.  Founded in 1958 on the premise that offering better products and services would be the key to our success.

Metal-Fab continues to operate today as a privately held entity with the same core values that lead to success while creating steady, stable growth over the past six decades.

Customer needs are the highest priority at Metal-Fab that is why every customer receives a unique experience while choosing from a full line of high-quality venting products for commercial, residential and industrial applications. 

Cutting-edge manufacturing systems, innovative designs, patent products, unmatched product quality, and a focus for detail have resulted in a leadership position within the industries Metal-Fab serves.  Our constant product improvement process and experience in developing and engineer products for safe, efficient appliance operation sets Metal-Fab apart from the competition.


Corr/Guard® is tested and listed to UL 1738 for positive pressures up to 15" w.g., featuring a durable AL29-4C super ferritic stainless steel flue designed for extreme resistance to chloride ion pitting, crevice corrosion, stress corrosion cracking, and corrosion in moderately reducing environments.  FCS Corr/Guard® incorporates an integral joint sleeve for easy joint alignment and aids in shingle condensate across the joint, preventing excessive joint corrosion and leakage.  FCS products are manufactured with continuous laser-welded seams and casings, improving durability and reliability and increased efficiency.

Single and Double-Wall Positive Pressure Special Gas Vent engineered for venting today’s high-efficiency natural gas-burning or propane-fired appliances, categories II, III, and IV as defined by the Standard for Gas-Fired Central Furnaces, ANSI Z21.47 and NFPA 54.

Design Considerations

When venting gas appliances always consult the appliance installation instructions. Horizontal runs, elbows, offsets, vertical height, may be limited by appliance performance specifications.

Horizontal runs must slope a minimum of ¼": 12" (6mm-305mm). Neutral or negative pressure systems must slope upward – away from the appliance. The positive pressure within the vent may slope upward or downward. Provide condensate drainage where required.

Vertical runs are directional-specific and must be properly supported. Supports are required above every offset to prevent vertical loading on elbows.

Corrossion Resistance

Corr/Guard products are designed for use on today’s highly efficient condensing appliances. UNS S44735 (i.e. AL29-4C), a Superferritic Stainless Steel was the first steel to successfully exceed the requirements of UL 1738, “Standard For Venting Systems For Gas-Burning Appliances, Categories II, III, and IV." It is considered the benchmark for condensing flue materials.


      For use with high-efficiency category II, III, and IV natural gas-burning or propane-fired appliances including gas boilers, furnaces, booster heaters, pool heaters, unit and radiant tube heaters, and direct vent appliances.
      Click to see Appliance Vent Categories
      Negative pressure venting Positive pressure venting

      Category I

      • Combustion Efficiency
      • 83% or less
      • Use standard venting Masonry or Type B Vent

      Category III

      • Combustion Efficiency
      • 83% or less
      • Use only pressurizable vent as specified by manufacturer

      Category III

      • Combustion Efficiency
      • 83% or less
      • Use only pressurizable vent as specified by manufacturer

      Category IV

      • Combustion Efficiency
      • Over 87%
      • Use only pressurizable special condensing service vent as specified by manufacturer
      Listing Standard
      NFPA 54
      UL 1738
      Flue Material Specification:
      0.015, AL29-4C Stainless Steel
      Casing Material Specifications:
      0.018, Aluminized (Standard)
      Max Operating Temperature: 550º F (288 C)
      Max Operating Pressure: 15" w.g.
      Clearance to Combustibles ENCLOSED – VERTICAL @550ºF (288 C) 1"
      Clearance to Combustibles UNENCLOSED HORIZONTAL or VERTICAL @ 550º F (288 C) 1"
      Joint Connection: Sleeved – P077 Sealant Required
      Insulation: Ceramic Insulation
      Diameter: 6" to 16"


      WARNING: Proposition 65