2021 IFGC Chapter 5 and IMC Chapter 8
Burning fuel makes flue gas, and the code cares about two things: what carries it out, and where it ends up. This lesson follows 2021 IFGC Chapter 5 for gas appliances and then looks at IMC Chapter 8, which covers oil-fired and solid fuel appliances (IMC Section 801.1 sends gas appliances to the fuel gas code). The heart of the lesson is termination: how high a gravity vent must rise, and how far a through-the-wall terminal must stay from windows, grade, air inlets and neighbors. Lesson 1 explains why you confirm which mechanical and fuel gas editions your city has adopted before you quote any of these numbers.
Every appliance must connect to a venting system unless a listed exception applies, such as a listed direct-vent unit (503.2). Table 503.4 matches vent to appliance. A listed Category I appliance, or one with a draft hood, may use a Type B gas vent, a chimney, single-wall metal pipe, a listed chimney lining system or a listed special gas vent. Vented wall furnaces use Type B-W vent. Category II, III and IV appliances use whatever the manufacturer specifies or furnishes. A listed combination gas and oil appliance may use Type L vent or a chimney, and unlisted appliances, appliances that could be converted to solid fuel, and appliances listed for chimney use only need a chimney. The category numbers come from the appliance listing, so read the data plate rather than guessing. Where plastic pipe is used, the appliance must be listed for it, the instructions must name the material, the pipe must carry the required label, and any primer must be a contrasting color (503.4.1, 503.4.1.1). Category II and IV appliances and other condensing appliances must have condensate collected and drained (503.9).
A chimney serving a residential-type or low-heat appliance rises at least 3 feet above where it passes through the roof and at least 2 feet higher than any part of the building within 10 feet horizontally. A chimney for a medium-heat appliance rises at least 10 feet higher than any building portion within 25 feet, and every chimney rises at least 5 feet above the highest draft hood outlet or flue collar connected to it (503.5.4). A gas vent has three tests (503.6). A vent 12 inches or less in size and at least 8 feet from a vertical wall ends above the roof as Figure 503.6.5 shows. A larger vent, or one closer than 8 feet to a wall or similar obstruction, ends at least 2 feet above the highest point where it passes through the roof and at least 2 feet above any part of a building within 10 feet horizontally. A Type B or L vent also ends at least 5 feet above the highest connected draft hood or flue collar, and a Type B-W vent at least 12 feet above the bottom of the wall furnace (503.6.6). The vent goes through roof flashing and finishes with a listed cap (503.6.7), and it stays at least 3 feet above any forced-air inlet within 10 feet (503.6.8).
Worked example. A boiler draft hood sits 8 feet above the floor of a one-story community building, and the vent passes through a roof 12 feet above the floor. A taller part of the building rises to 15 feet, 7 feet from the vent. Because that wall is closer than 8 feet, the 2 foot tests of 503.6.5 apply, along with the 5 foot draft hood test of 503.6.6. The height tests are: hood, 8 + 5 = 13 feet; roof, 12 + 2 = 14 feet; taller part within 10 feet, 15 + 2 = 17 feet. The vent must reach at least 17 feet.
A connector joins the appliance to the vent unless the vent attaches directly (503.10.1). It slopes up toward the vent at least 1/4 inch per foot with no dips or sags (503.10.8). The maximum horizontal length of a single-wall connector is 75 percent of the vent or chimney height, and of a Type B double-wall connector 100 percent (503.10.9). Joined connectors use a tee or wye (503.10.7), the connector may not attach to a fireplace flue unless that flue opening is permanently sealed (503.10.13), and its whole length needs ready access for inspection, cleaning and replacement (503.10.12). Clearance to combustibles comes from Table 503.10.5: for residential and low-heat appliances not otherwise listed, single-wall connector needs 18 inches, Type L needs 9 inches, and Type B is not permitted; for medium-heat appliances, single-wall needs 36 inches; and the listing controls where it says something different. Connector example: on a 20 foot vent, a single-wall connector can run 15 feet horizontally and a Type B connector 20 feet. A 12 foot connector must climb at least 12 x 1/4 = 3 inches.
Direct-vent appliances and appliances with integral vents end at a sidewall terminal, and Table 503.8 sets the clearances. It does not apply to the combustion air intake of a direct-vent appliance. The table gives one column for direct-vent terminals and one for nondirect-vent terminals; rows with a single entry apply to both.
| Clearance from the terminal to | Minimum (Table 503.8) |
|---|---|
| Finished grade, veranda, porch, deck or balcony | 12 in |
| Openable window or door, direct-vent | 6 in to 10,000 Btu/h; 9 in to 50,000; 12 in to 150,000; above 150,000 per the maker and no less than the nondirect-vent clearance |
| Openable window or door, nondirect-vent | 4 ft below or beside; 1 ft above |
| Mechanical air supply inlet | 10 ft horizontally, or 3 ft above the inlet |
| Service regulator vent outlet | 3 ft up to 2 psi gas; 10 ft above 2 psi |
| Paved sidewalk or driveway on public property | 7 ft above, and not over public walkways where condensate or vapor is a nuisance |
| Underside of porch, deck or balcony | 12 in if open on at least two sides; prohibited if open on only one |
Fixed windows, soffits and building corners carry no table clearance unless the maker requires one, and a nonmechanical air inlet or another appliance's combustion air inlet follows the window row. Example: a 120,000 Btu/h direct-vent boiler needs 12 inches to an openable window, while a 45,000 Btu/h unit needs 9 inches. A nondirect-vent unit needs 4 feet from the same window unless it ends 1 foot above it.
Mechanical draft systems, forced or induced, must be listed to UL 378 and installed to the maker's instructions. Parts under positive pressure must not leak flue gas into the building, a natural-draft appliance may not connect to a positive-pressure part, and gas flow to the burner must stop when the draft system is not performing (503.3.3). A vent may not pass through a fabricated air duct or furnace plenum (503.3.5). Through an above-ceiling air-handling space it needs a listed special or positive-pressure vent with sealed joints, no joints in the space, or a sealed enclosure (503.3.6). Nothing that slows flue flow may be added to a connector, chimney or vent, apart from listed regulators, vent dampers and heat reclaimers (503.15). The annular space where any vent, including direct-vent, passes through an outside wall must be permanently sealed (503.16). A gas vent through an exterior wall does not end adjacent to the wall or below eaves or parapets, except direct-vent and mechanical draft systems (503.6.9).
Where the IMC governs, vents are listed and labeled (802.1), and a Type L vent ends with a listed cap at least 2 feet above the roof penetration and 2 feet above anything within 10 feet (802.4, 802.5). Vents rise 5 feet above the highest flue collar unless an exception applies (802.6). Connectors pitch at least 1/4 unit in 12 (803.10.5). Power exhausters end at least 10 feet from the lot line or adjacent buildings (804.3.3). Horizontal mechanical draft terminals stay at least 7 feet over a walkway, 3 feet above a forced-air inlet within 10 feet, and 4 feet below, 4 feet to the side or 1 foot above any door, window or gravity air inlet. They also stay at least 3 feet from an inside corner, 12 inches above grade, and out from above or within 3 feet of an oil tank vent or gas meter (804.3.4). Natural-draft integral vent terminals are 9 inches apart and forced-draft ones 12 inches apart (804.2.1).
In the field
A crew swaps an old gravity-vented boiler in a church annex for a sealed-combustion sidewall unit and drills the terminal under a covered entry that is open on one side only. That location is prohibited by Table 503.8, no matter the size of the appliance, because vapor collects under the cover. Also check the height: shrubs, mulch and new landscape beds have a way of raising finished grade, which eats into the 12 inch clearance. Measure from the finished grade, and look at what stands within 4 feet of the terminal.
1. A Type B gas vent serves a boiler in a Group R-2 apartment building. The vent is 6 feet from the wall of a taller part of the building. Under IFGC 503.6.5, how must it end relative to that taller part?
2. The draft hood of a boiler in a Group B clinic is 9 feet above the floor. The vent passes through a roof at 11 feet, and a taller roof at 15 feet is 6 feet away. What is the least height above the floor for the vent top?
3. A Type B-W gas vent serves a vented wall furnace in a Group R-2 rehab unit. Under IFGC 503.6.6, it must end at least how far above the bottom of the wall furnace?
4. A masonry chimney serves a residential-type gas appliance in a Group R-3 building. The chimney passes through the roof at 12 feet, and a taller portion of the building 8 feet away is 20 feet high. Under IFGC 503.5.4, what is the minimum chimney top height?
5. A chimney serves a medium-heat appliance in a Group F-1 plant. A building portion within 25 feet has a roof at 30 feet. Under IFGC 503.5.4, the chimney top must be at least:
6. A gas vent connector in a Group B office runs 12 feet horizontally to the vent. Under IFGC 503.10.8, what is the least rise it must have along its length?
7. A 24-foot-tall vent serves a low-heat appliance in a Group S-1 warehouse and uses a single-wall connector. Under IFGC 503.10.9, the maximum horizontal length of the connector is:
8. A direct-vent boiler terminal is being set on the wall of a Group A-3 community hall next to a landscaped bed. Under IFGC Table 503.8, the terminal must be at least how far above finished grade?
9. A 45,000 Btu/h direct-vent unit heater is being set on the wall of a Group B office. The nearest openable window is on the same wall. Under IFGC Table 503.8, the minimum clearance from the terminal to the window is:
10. A nondirect-vent terminal on a wall in a Group R-2 building is at the same level as an openable window. Under IFGC Table 503.8, how far must it be to the side of the window?
11. A contractor mounts a sidewall vent terminal under a covered porch of a Group R-2 building. The porch is open on only one side. Under IFGC Table 503.8, this location is:
12. A mechanical draft system serves a gas boiler in a Group B building. Under IFGC 503.3.3, which statement is correct?
13. An oil-fired boiler with a power exhauster is installed in a Group B building under the IMC. Under IMC 804.3.3, the vent termination must be at least how far from the lot line and adjacent buildings?
14. A crew installs a direct-vent water heater in a Group R-2 rehab and leaves a gap around the vent pipe where it passes through the outside wall. Under IFGC 503.16, the annular space must be: