Furnace vent pipe too close to combustibles. But then I already knew that. Type B Gas Vent requires a minimum of one-inch clearance to combustibles. The manufacture even stamps this into the vent. WARNING: Vent pipe air space clearances to combus-tibles are 1"4000 on all sides except on the horizontal sections , which require 2" clearances from the top of the pipe. NOTICE: Read these instruc- ... B vent clearance; If this is your first visit, be sure to check out the FAQ. Type B Gas Vent requires a minimum of one-inch clearance to combustibles. vent system. Here’s a guide: Table of Type B-Vent Rooftop Clearance Requirements. This is an area where I slide on the requirements - I don't report on it. A B-vent is the metal duct that is normally used to vent gas and propane appliances such as furnaces and water heaters. Furnace B-vent clearance from window. They are: 1. flue-gas temperature 2. heat loss in the vent 3. vent height 4. vent system capacity 5. restrictions to flow and 6. the ambient temperature. I know about the 1 inch rule. Single wall vents should have a minimum of 6 inches of clearance and double wall, sometimes called a “B” vent, should have 1 inch minimum clearance. All locking joints are to be securely snap-locked together. Durability is the result of high-quality materials and workmanship, with galvanized outer pipe and aluminum inner pipe providing protection against corrosion and external damage. Clearance Requirements from Vent Terminations to Building Openings for SV Models * All clearance requirements are in accordance with ANSI Z21.10.3 and the National Fuel Gas Code, ANSI Z223.1 and in Canada, in accordance with NSCNGPIC. When installed as a Type B Gas Vent, a minimum of one (1) inch airspace clearance to combustible material is required. Therefore, insulation, including fiberglass, should be a couple inches away from a B-vent to eliminate the hazard of a fire. The B vent is on the lower roof, 3 feet away and at least 2 feet below the other roof. This one-inch is a typical clearance for all brands of Type B Vent, but each section of vent pipe is marked with the clearance required by Underwriter Laboratories, so it should be checked. Vent caps 12" and smaller may terminate a distance above the roof if 8 feet or more away from a vertical surface as follows: The 1992 Vent Sizing Tables require that all Type B gas vents terminate above the roof with a listed cap or listed roof assembly in accordance with the manufacturer’s instructions. Does Type B Gas Vent have a fire rating? In 16 years I have never seen a resulting problem of B-vent … Specific Inspection Topics. Factors Affecting Vent System Design and Operation There are many variables dictating how well a vent system will operate. This one-inch is a typical clearance for all brands of Type B Vent, but each section of vent pipe is marked with the clearance required by Underwriter Laboratories, so it should be checked. Inspecting HVAC Systems. The minimum requirements for vent ducting are: Single wall, welded seam 18 gauge ducting or Class-A Double Wall Positive Pressure Grease Vent Ducting. Diedrich Manufacturing, Inc. recommends UL certified (CE for European Union) Class-A Positive Pressure Ducting due to its superiority over the Single Wall welded seam 18-gauge ducting. The B vent is in contact with the loose fill insulation. These vents can achieve temperatures of 300 degrees when gas appliances are operating. Type B Gas Vent is designed for negative pressure applications and flue gas temperatures not exceeding 400°F above ambient for Category I appliances. The problem with this is that the single wall pipe requires even greater clearances (6″). A proper vent with insulation baffle looks more like this: Insulation baffle around a B-Vent pipe in an attic Where the termination cap penetrates a combustible wall, 1" air space clearance is required. jfrederick (Joshua Frederick) ... B-vents within 8 feet of a vertical wall are required to extend above the next roof line. Metal vents that are too close to wood or combustionable materials may cause the wood to burst into flames. For Type B 2x4 and BW Vents see Section 2, page 10 for clearance to combustibles. Do not use com-mon vent systems.