2021 IRC Sections G2425-G2427
Once an appliance has the combustion air it needs, the flue gases it produces still have to leave the house safely, and that is the job of the vent. This lesson covers how appliances are classified for venting purposes, the vent materials that classification allows, and the clearances that keep a vent termination away from roof surfaces, walls, grade, and building openings. Vent connectors, common venting of several appliances, and masonry chimney use are a related but separate topic taken up in the next lesson; this lesson stays with the vent itself and where it ends.
Gas appliances are sorted into four venting categories that describe how their vent behaves, and the category drives every other choice in this lesson. Category I covers the ordinary draft-hood-equipped furnace or water heater: it operates with a nonpositive (negative or neutral) vent static pressure and a vent gas temperature that avoids excessive condensate. Category III runs at a positive vent static pressure but, like Category I, has a vent gas temperature that avoids excessive condensate; because the vent is pressurized, its joints must be sealed against leakage. Category IV is the high-efficiency condensing appliance: positive vent pressure combined with vent gas temperatures low enough that condensation is expected, so the venting system has to be airtight and watertight to match. Category II, a nonpositive-pressure, condensing combination, describes very few real appliances and is rarely encountered in the field. The category an appliance is listed for is not a matter of judgment; it comes from the appliance's own listing and nameplate, and Table G2427.4 uses it to say which vent types are allowed: Category I appliances may use Type B gas vent, a chimney, single-wall metal pipe, a listed chimney lining system, or a listed special gas vent, while Category II, III, and IV appliances use the vent specified or furnished by the appliance manufacturer.
Type B vent is a factory-built, double-wall metal vent listed to UL 441 for listed gas appliances with a draft hood and other Category I appliances listed for use with it; it is not approved for oil-fired, wood-burning, or coal-fired equipment. It is the vent most GCHS staff will see on an ordinary natural-draft furnace or water heater. It is installed per the manufacturer's instructions (G2427.6.2), and its required clearance to combustibles comes from its listing and is marked on the vent rather than set by a single blanket number in the code text.
Type L vent is also double-wall, but the code defines it as a vent listed for appliances that are listed for use with Type L or Type B vents, and vents for listed combination gas-and-oil appliances are listed to UL 641 (G2427.6.1). That is why GCHS commonly sees it paired with an oil-fired furnace or boiler in older housing stock. Its clearance to combustibles likewise comes from its listing and label. Category IV condensing appliances are vented in a different material entirely: where plastic pipe is used, the appliance must be listed for it and its instructions must identify the specific plastic piping material (G2427.4.1), with joints made per the appliance manufacturer's instructions (G2427.4.1.1). This is one of the few places in the mechanical code where the manufacturer's listed instructions, not a single code table, control the material, joint method, slope, and support spacing, because those details vary from one condensing appliance model to the next; substituting ordinary plumbing-grade PVC without checking the appliance's own venting instructions is a common and serious field error.
Single-wall metal pipe is also a permitted vent for Category I appliances, but only within tight limits (G2427.7). It is galvanized sheet steel not less than 0.0304 inch thick, or other approved noncombustible, corrosion-resistant material, and it may be used only for runs directly from the space where the appliance is located through the roof or an exterior wall. It may not originate in an unoccupied attic or concealed space, and it may not pass through any attic, inside wall, concealed space, or floor. Where it does terminate, it must be at least 5 feet in vertical height above the highest connected draft hood outlet or flue collar and follow the roof clearances below.
A gas vent terminates according to G2427.6.4. A vent that is 12 inches or less in size, has a listed cap, and is located at least 8 feet from a vertical wall or similar obstruction terminates above the roof at the height shown in Figure G2427.6.4, which is a graphic keyed to roof pitch, so that height is read from the figure for the actual roof slope. A vent over 12 inches in size, or one located less than 8 feet from a vertical wall or similar obstruction, must terminate not less than 2 feet above the highest point where it passes through the roof and not less than 2 feet above any portion of a building within 10 feet horizontally. Whichever rule applies, a Type B or L vent must also terminate at least 5 feet in vertical height above the highest connected appliance draft hood or flue collar (G2427.6.5), and at least 3 feet above any forced air inlet within 10 feet (G2427.6.7). It must end in a listed cap or listed roof assembly (G2427.6.6), and a decorative shroud is allowed only where it is listed for that vent (G2427.6.4.1). Treating "2 feet" as a universal answer without checking which rule the vent falls under is a common estimating mistake.
Category III and IV appliances, and direct-vent appliances, often terminate through a sidewall, and those terminals follow Figure G2427.8 and Table G2427.8 instead of the roof-height rule. The bottom of the terminal is kept at least 12 inches above finished grade, veranda, porch, deck, or balcony, keeping it clear of snow, mulch, and splashing water. The clearance to an openable window or door depends on the terminal type. For a direct-vent terminal it steps up with the appliance's input rating: 6 inches for appliances up to 10,000 Btu/h, 9 inches above 10,000 and up to 50,000, and 12 inches above 50,000 and up to 150,000; above 150,000 the manufacturer's instructions govern, and never less than the nondirect-vent clearance. For a nondirect-vent terminal it is 4 feet below or to the side of the opening, or 1 foot above it. Where the terminal sits near a mechanical air supply inlet, such as another appliance's forced-air intake, it must be 10 feet away horizontally or 3 feet above the inlet, so the vent's own exhaust is not drawn back into it. Other rows of the table cover a service regulator vent outlet (3 feet at gas pressures up to 2 psi, 10 feet above 2 psi), a paved sidewalk or driveway on public property (7 feet, and not above public walkways where condensate or vapor could be a hazard), and the underside of a porch or deck (12 inches, only where the area beneath is open on at least two sides). The table's clearances do not apply to the combustion air intake of a direct-vent appliance. Where the code is less restrictive than the manufacturer's instructions, the instructions govern (G2408.1), so both are checked before a termination location is finalized.
Vent connectors, the horizontal or near-horizontal section of pipe between the appliance's draft hood and the vertical vent, have their own slope, length, and clearance rules, and several appliances sharing one common vent have their own sizing method entirely. Both are covered in the next lesson, along with the masonry chimney and the orphaned water heater problem that shows up when a furnace on a shared vent is removed during a rehab. This lesson's job is narrower: get the vent type right for the appliance's category, and get the termination far enough from the roof, walls, grade, and openings.
In the field
A common GCHS rehab scenario is replacing an aging natural-draft furnace with a high-efficiency condensing model as part of an energy retrofit. The new appliance's Category IV vent is a completely different animal from the old Category I Type B vent it replaces: different material, a sidewall termination instead of a roof termination, and its own clearance rules to the nearest window, the grade, and any nearby air intake. Treating the new vent as a drop-in replacement for the old one, routed the same way through the same roof penetration, is a frequent design mistake; the manufacturer's installation manual for the specific condensing model is the document that actually governs the vent run, not the routing the old furnace used.
Worked example: a 40,000 Btu/h direct-vent condensing water heater is being sidewall vented near a homeowner's bedroom window that opens. Table G2427.8 puts a direct-vent terminal at least 9 inches from an openable window at that input; a 120,000 Btu/h unit would need 12 inches. If the manufacturer's manual requires more, the manual governs. The terminal also has to be checked against the mechanical air intake clearance if a forced-air intake serves any structure within 10 feet, since that clearance is a separate row of the table and both must be satisfied at once.
1. An inspector finds a Category I furnace vented with single-wall galvanized pipe that leaves the furnace closet, runs up through the attic, and exits the roof. Under G2427.7, this is:
2. A Category I appliance venting system, as defined for gas appliances, operates with:
3. A fan-powered, positive-pressure vent connected to a furnace whose flue gas stays above the dew point and avoids condensation describes which category?
4. Type B vent is listed for use with:
5. Compared with Type B vent, Type L vent differs mainly because it:
6. A contractor installs a Category IV condensing furnace's vent using ordinary PVC pipe bought locally, without checking the furnace manufacturer's venting instructions. The concern is:
7. A Type B gas vent is over 12 inches in size. It must terminate not less than:
8. A gas vent has a listed cap 12 inches or less in size and sits at least 8 feet from any vertical wall. Its minimum height above the roof is taken from:
9. An installer needs the roof termination height for a Type B vent with a 6-inch listed cap that sits 12 feet from the nearest vertical wall, on a steep-slope roof. The correct approach is to:
10. A 40,000 Btu/h direct-vent appliance terminates through a sidewall near an openable window. Under Table G2427.8, the minimum clearance from the terminal to that window is:
11. A through-the-wall terminal for a nondirect-vent appliance is being located relative to an openable window. Under Table G2427.8, the minimum clearance is:
12. A through-the-wall vent terminal is within 10 feet horizontally of a mechanical air supply inlet. Table G2427.8 requires the terminal to be located not less than how far above the inlet?
13. Under Table G2427.8, the bottom of a through-the-wall vent terminal must be located above finished grade not less than:
14. A Type B gas vent serves a natural-draft water heater. Measured vertically, the vent must terminate not less than how far above the highest connected draft hood or flue collar?