12.3 Type B, BW, L Vents & Plastic Venting

Once the category is right, the product has to be the listed type. 2024 IFGC Chapter 2 defines the four factory vents you will see on an M2 job:

• Type B vent — listed and labeled for appliances with draft hoods and other Category I appliances listed for use with Type B vents.
• Type BW vent — listed and labeled for use with wall furnaces.
• Type L vent — listed and labeled for appliances listed for use with Type L or Type B vents.
• Special gas vent — listed and labeled for listed Category II, III, and IV appliances.

A vent, in the same chapter, is a pipe or other conduit of factory-made components, listed and labeled for a specific type or class of appliance. Field-rolled galvanized pipe is not Type B just because it is round and shiny.

Type B, Type BW, and Type L

Listing standards — IFGC 502.1 and 503.6.1. Type B and BW vents are tested to UL 441. Type L vents are tested to UL 641. Vents for Category II and III appliances are tested to UL 1738. Plastic vents for Category IV appliances are not required to be listed and labeled when they are the plastic the appliance manufacturer specifies and they are installed to that manufacturer’s instructions — but 503.4.1 still requires the plastic to be labeled to the product standard the manufacturer names or listed to UL 1738.

Type B is the double-wall, air-insulated gas vent. The inner wall carries flue gas; the outer wall keeps the listed clearance, often 1 inch to combustibles when the listing says so. IFGC 503.6.14: screws, rivets, and other fasteners shall not penetrate the inner wall of double-wall gas vents except at the transition from an appliance draft hood, a flue collar, or a single-wall metal connector to the double-wall vent. A ring of sheet-metal screws through both walls “so it cannot slip” is a fail.

Type BW is the oval or rectangular wall-furnace vent. Table 503.4 and IFGC 608.2 send listed vented wall furnaces to Type B-W. 503.6.3: a Type B-W gas vent shall have a listed capacity not less than that of the listed vented wall furnace to which it is connected. 503.6.6: a Type B-W gas vent shall terminate not less than 12 feet (3658 mm) in vertical height above the bottom of the wall furnace. That 12-foot number is not the Type B 5-foot number. Mixing them is an easy exam trap.

Type L is the oil vent. IMC 802.1 tests Type L and pellet vents to UL 641. IMC Table 802.2: Type L serves oil-burning appliances listed and labeled for Type L and gas appliances listed and labeled for Type B. IFGC Table 503.4: listed combination gas-and-oil appliances use Type L or a chimney. Type L is not a wood-stove chimney and is not a Category IV plastic substitute.

Minimum heights and caps. IFGC 503.6.6 / IMC 802.6: a Type B or L vent shall terminate not less than 5 feet (1524 mm) in vertical height above the highest connected appliance draft hood or flue collar. IMC 802.5: Type L also terminates not less than 2 feet above the highest point of the roof penetration and not less than 2 feet higher than any portion of a building within 10 feet. IFGC 503.6.5 Item 2 uses that same 2 / 10 pair for gas vents over 12 inches in size or closer than 8 feet to a wall; smaller gas vents away from walls follow Figure 503.6.5’s roof-slope table (1.0 foot above a flat-to-6/12 roof, climbing as the slope steepens). 503.6.7: the vent extends through the flashing, roof jack, or thimble and terminates with a listed cap or listed roof assembly. IMC 802.4 requires a listed Type L cap. 503.6.5.1: decorative shrouds on gas vents only if listed for that venting system.

Offsets and support. IFGC 503.6.10.2: Type B and L vents sized under the 503.6.10.1 area shortcuts extend generally vertical with offsets not exceeding 45 degrees, except one 60-degree offset is permitted. Any angle greater than 45 degrees from vertical is considered horizontal. Total horizontal distance of the vent plus the connector serving draft-hood appliances shall be not greater than 75 percent of the vent’s vertical height. 503.6.12 / 502.6: support and space all portions per the manufacturer’s instructions and the design weight.

Inside a masonry chimney. IFGC 503.6.4: a gas vent installed within a masonry chimney is installed per the vent manufacturer and identified with a permanent label where the vent enters the chimney: “This gas vent is for appliances that burn gas. Do not connect to solid or liquid fuel-burning appliances or incinerators.” 503.6.13 requires the same label on the wall or ceiling in localities where solid and liquid fuels are used extensively. The space around that vent is not another flue (503.5.10).

Single-wall metal pipe — the concealed-space fail

Single-wall is still legal for some Category I appliances (Table 503.4), but 503.7 is a cage, not a suggestion.

Construction — 503.7.1. Galvanized sheet steel not less than 0.0304 inch (0.7 mm) thick, or other approved noncombustible corrosion-resistant material.

Cold climate — 503.7.2. Uninsulated single-wall metal pipe shall not be used outdoors for venting appliances in regions where the 99-percent winter design temperature is below 32°F (0°C).

Use and path — 503.7.4 and 503.7.6. Single-wall metal pipe shall be used only for runs directly from the space in which the appliance is located through the roof or exterior wall to the outdoor atmosphere. It shall not originate in any unoccupied attic or concealed space and shall not pass through any attic, inside wall, concealed space, or floor. 503.10.14 repeats the connector version: single-wall metal pipe connectors shall not pass through any wall, floor, or ceiling except as 503.7.4 permits. IMC 803.2 is the non-gas twin: connectors stay in the appliance room except for the listed pass-through; a connector passing through an unheated space shall not be single-wall.

Clearances — 503.7.8 and Table 503.10.5. Listed draft-hood and Type B-listed appliances: 6 inches from single-wall metal pipe to combustibles (unless the listing is different). Residential appliances listed for Type L: 9 inches for single-wall. Residential and low-heat appliances other than the listed rows: 18 inches. Medium-heat: 36 inches. Type B connector material on a listed draft-hood appliance uses the listed Type B clearance, not the 6-inch single-wall number.

Field scenario. A Category I unit heater in a stockroom is connected with single-wall galvanized pipe that punches the lay-in ceiling and runs in the concealed plenum to a masonry chimney in the next bay. The run is short, sloped, and 18 inches from storage racks. The installation fails 503.7.6 and 503.10.14 the moment the pipe enters the concealed ceiling. IFGC 503.3.6 would still require a listed special gas vent, a sealed Category III/IV vent, no joints in the ceiling space, or a sealed enclosure if this were a legal vent type in an above-ceiling air-handling space — single-wall is none of those. Use Type B (or a listed chimney connector) that is allowed in the concealed space, with the insulation shield and fireblocking the listing requires.

Plastic vents, special gas vent, enclosure, fireblocking

Plastic — 503.4.1. Where plastic piping is used to vent an appliance, the appliance shall be listed for use with such venting materials and the manufacturer’s installation instructions shall identify the specific plastic piping material. The plastic shall be labeled in accordance with the product standards the manufacturer specifies, or listed and labeled to UL 1738. “Schedule 40 PVC from the plumbing rack” is legal only when that furnace’s manual names that material, cell class, and cement system. IMC 801.20: plastic pipe and fittings used to vent appliances shall be installed in accordance with the appliance manufacturer’s installation instructions.

Joints — 503.4.1.1. Plastic pipe and fittings follow the appliance manufacturer’s instructions. UL 1738-listed plastic follows the vent manufacturer’s instructions. Where a primer is required, it shall be of a contrasting color. Pink or purple primer on purple CPVC is how you verify the joint was primed; a clear wipe with no contrast is a 503.4.1.1 fail when primer is required.

Condensate, slope, and support. Category IV vents are wet by definition. Collect and drain condensate per IMC 307.1 (corrosion-resistant piping, not smaller than the appliance drain, slope not less than 1/8 in 12). Horizontal vent slope, hanger spacing, and termination kits come from the appliance or UL 1738 instructions — do not invent a universal plastic-vent slope that 503.4.1 does not publish. 502.6 still requires every portion of a vent to be supported for its weight; a long PVC run sagging into a trap is both a listing fail and a condensate blockage.

Special gas vent — 503.4.2. Special gas vent shall be listed and labeled in accordance with UL 1738 and installed in accordance with the special-gas-vent manufacturer’s instructions. That is the stainless or polypropylene positive-pressure system for Category II, III, and IV when the appliance specifies it instead of (or in addition to) plastic.

Enclosure, shields, and fireblocking. 502.4: vents through insulated assemblies get a No. 26 gage (0.0187 inch) steel insulation shield holding the listed clearance, terminating not less than 2 inches above attic insulation. 502.7: in concealed locations, a vent through holes or notches less than 1½ inches from the edge of a stud, joist, or rafter gets No. 16 gage (0.0575 inch) steel shield plates extending not less than 4 inches beyond the sole plate, top plate, or member. 503.3.5: vents shall not extend into or pass through a fabricated air duct or furnace plenum. 503.3.6: an above-ceiling air-handling space is legal only with a listed special gas vent, a Category III/IV or other positive-pressure vent with sealed joints, a run that has no fittings or joints in that space, or a sealed conduit/enclosure that separates the vent from the ceiling space. IBC 2113.20 and IFGC 501.15.4.1 still require noncombustible fireblocking at floor and ceiling penetrations. Type B is allowed in concealed construction when those firestops, shields, and listed clearances are in place. Single-wall is not.

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