11.4 Vent Termination, Clearances & Direct-Vent Systems
Key Takeaways
- IFGC 2018 Figure 503.6.5 sets gas vent height by roof pitch: 1.0 foot flat through 6/12, 1.5 feet over 7/12 to 8/12, 2.0 feet over 8/12 to 9/12, and 8.0 feet over 20/12 to 21/12.
- A gas vent over 12 inches in size, or standing less than 8 feet from a vertical wall, must terminate 2 feet above the roof penetration and 2 feet above any portion of the building within 10 feet horizontally (IFGC 2018 Section 503.6.5, Item 2).
- Chimneys for residential or low-heat appliances follow the 3-2-10 rule: 3 feet above the roof penetration, 2 feet above any building portion within 10 feet, and 5 feet above the highest connected draft hood outlet (IFGC 2018 Section 503.5.4).
- IFGC 2018 Table 503.8 gives direct-vent terminal clearance from any air opening as 6 inches under 10,000 Btu/h, 9 inches from 10,000 through 50,000 Btu/h, and 12 inches over 50,000 through 150,000 Btu/h.
- A mechanical draft vent other than direct-vent must terminate 4 feet below, 4 feet horizontally from, or 1 foot above any door, operable window or gravity air inlet, with the bottom of the terminal not less than 12 inches above finished ground level.
11.4 Vent Termination, Clearances & Direct-Vent Systems
Quick Answer: A gas vent 12 inches or less in size standing at least 8 feet from a vertical wall terminates at the height given in IFGC 2018 Figure 503.6.5 -- 1.0 foot above the roof for pitches flat through 6/12. Every other gas vent must terminate 2 feet above the roof penetration and 2 feet above any portion of the building within 10 feet horizontally.
Gas Appliances and Piping is 40 of the 100 scored questions on the Maryland Journey Plumber/Gas Fitter exam, and termination clearances are the most heavily tested slice of that domain because every answer is a hard, lookup-able number. The International Fuel Gas Code (IFGC) 2018 is an allowed open-book reference, so the exam rewards knowing which section holds the number rather than memorizing all of them. Input ratings below are in British thermal units per hour (Btu/h).
Roof Terminations: Two Different Paths
IFGC 2018 Section 503.6.5 splits gas vents into two cases, and picking the wrong one is the classic error.
Path 1 -- the roof-pitch table. A gas vent 12 inches or less in size that stands not less than 8 feet from a vertical wall or similar obstruction terminates at height H above the roof, measured to the lowest discharge opening of the listed cap:
| Roof slope | H (minimum), feet |
|---|---|
| Flat to 6/12 | 1.0 |
| Over 6/12 to 7/12 | 1.25 |
| Over 7/12 to 8/12 | 1.5 |
| Over 8/12 to 9/12 | 2.0 |
| Over 9/12 to 10/12 | 2.5 |
| Over 10/12 to 11/12 | 3.25 |
| Over 11/12 to 12/12 | 4.0 |
| Over 12/12 to 14/12 | 5.0 |
| Over 14/12 to 16/12 | 6.0 |
| Over 16/12 to 18/12 | 7.0 |
| Over 18/12 to 20/12 | 7.5 |
| Over 20/12 to 21/12 | 8.0 |
Read the rows as ranges, not as single pitches. A 9/12 roof sits at the top of the 'Over 8/12 to 9/12' band and needs 2.0 feet, not the 2.5 feet printed on the next line.
Path 2 -- the 2-2-10 rule. A gas vent that is over 12 inches in size, or that stands less than 8 feet from a vertical wall, loses the table entirely. It must terminate 2 feet above the highest point where it passes through the roof and 2 feet above any portion of a building within 10 feet horizontally. Both conditions apply and you build to the taller result.
Worked Example
A 5-inch Type B vent penetrates a 10/12 roof only 6 feet from a two-story wall. The vent is under 12 inches, but it is closer than 8 feet to a vertical wall, so Path 1 is unavailable. Under Path 2 it must rise 2 feet above the penetration and 2 feet above the top of any construction within 10 feet. If that wall rises 9 feet above the penetration point, the vent must extend 11 feet -- not the 2.5 feet the pitch table would have suggested.
Minimum height above the appliance (Section 503.6.6). Independently of the roof geometry, a Type B or Type L gas vent must terminate not less than 5 feet in vertical height above the highest connected appliance draft hood or flue collar, and a Type B-W vent serving a wall furnace must terminate not less than 12 feet above the bottom of the wall furnace. Short single-story vents routinely fail this rule even when the roof height checks out.
A 6-inch Type B gas vent penetrates a roof with a 9/12 pitch, 15 feet from the nearest vertical wall. Under IFGC 2018 Figure 503.6.5, what is the minimum height from the roof surface to the lowest discharge opening?
Chimneys Terminate Differently Than Gas Vents
This is the single most confused pair in the venting chapter. IFGC 2018 Section 503.5.4 governs chimneys -- masonry, metal and factory-built -- and not gas vents:
| Termination rule | Chimney (Section 503.5.4) | Gas vent (Sections 503.6.5, 503.6.6) |
|---|---|---|
| Height above roof penetration | 3 feet minimum | Per Figure 503.6.5, or 2 feet under Path 2 |
| Above any building portion within 10 feet | 2 feet | 2 feet (Path 2 only) |
| Above highest connected draft hood outlet or flue collar | 5 feet | 5 feet for Type B and Type L |
| Medium-heat appliances | 10 feet above any building within 25 feet | Not applicable |
So a chimney follows 3-2-10; a listed gas vent follows the pitch table or 2-2-10. Candidates who memorize a single '3 feet above the roof' rule lose points on every gas-vent item.
Caps, Shrouds, Wall Penetrations and Enclosed Spaces
- Roof terminations (503.6.7): a gas vent must extend through the roof flashing, roof jack or roof thimble and terminate with a listed cap or listed roof assembly. Section 503.7.11 states the parallel rule for single-wall metal pipe -- the vent cap or roof assembly must have a venting capacity not less than that of the pipe to which it is attached -- so an undersized decorative cap chokes the system.
- Decorative shrouds (503.6.5.1): prohibited at a gas vent termination unless the shroud is listed for that specific venting system and installed per the manufacturer's instructions. Site-built chase covers wrapped around a vent cap are a routine violation.
- Forced air inlets (503.6.8): a gas vent must terminate not less than 3 feet above any forced air inlet located within 10 feet.
- Exterior wall penetrations (503.6.9): a gas vent extending through an exterior wall may not terminate adjacent to the wall or below eaves or parapets. The only exceptions are direct-vent appliances under Section 503.2.3 and mechanical draft systems under Section 503.3.3. A natural-draft Category I vent cannot simply be elbowed out through a sidewall.
- Never terminate inside the building. Section 501.2 requires every appliance to discharge the products of combustion to the outdoors, and Section 503.3 requires the venting system to convey all flue and vent gases outdoors. Stopping a vent in an attic, crawl space, soffit, garage or any other enclosed space is a violation and a carbon monoxide hazard no matter how well that space is ventilated.
A masonry chimney serves a residential gas boiler and passes through a flat roof. A parapet wall 4 feet away rises 4 feet above the roof deck. What minimum chimney height above the roof deck satisfies IFGC 2018 Section 503.5.4?
Direct-Vent (Sealed Combustion) Terminations
A direct-vent appliance draws all combustion air from the outdoors through a sealed pipe and discharges all flue gases directly outdoors, so the combustion chamber never communicates with the room. Because the appliance is sealed and the exhaust leaves under fan pressure, IFGC 2018 permits far tighter clearances than an open-combustion vent gets. Section 503.2.3 sends you to Section 503.8, Item 3 and Table 503.8:
| Direct-vent appliance input rating (Btu/h) | Through-the-wall terminal clearance from any air opening into the building |
|---|---|
| Less than 10,000 | 6 inches |
| 10,000 through 50,000 | 9 inches |
| Over 50,000 through 150,000 | 12 inches |
| Over 150,000 | Per the manufacturer's instructions, and not less than the Section 503.8 Item 2 clearances |
The bottom of the direct-vent terminal and its air intake must be not less than 12 inches above finished ground level -- and in Maryland practice above the expected snow accumulation, because a buried terminal trips the appliance pressure switch and locks out the burner.
Edition note: the pre-2018 IFGC stated these tiers as prose with only three steps (6 inches, 9 inches, and 12 inches for anything over 50,000 Btu/h). The 2018 edition converted them into Table 503.8 and added the over-150,000 Btu/h row. Cite the table when you answer.
Mechanical Draft (Power Vent) Terminations
For a mechanical draft venting system -- a fan-assisted or power-vented appliance that is not direct-vent -- Section 503.8 controls:
- 4 feet below, 4 feet horizontally from, or 1 foot above any door, operable window or gravity air inlet into any building (Item 2).
- Bottom of the vent terminal not less than 12 inches above finished ground level (Item 2).
- Not less than 3 feet above any forced-air inlet located within 10 feet (Item 1). Two exceptions: the combustion air intake of a direct-vent appliance, and the integral outdoor air inlet and flue gas discharge of a listed outdoor appliance.
- Section 503.3.3, Item 6: exit terminals of mechanical draft systems must be not less than 7 feet above finished ground level where located adjacent to public walkways.
- Section 503.8, Item 5: a Category IV vent that discharges flue gases perpendicular to the wall must be not less than 10 feet horizontally from an operable opening in an adjacent building, unless the terminal is 2 feet or more above, or 25 feet or more below, that opening.
Meters, regulators and air intakes. The IFGC does not tabulate a vent-to-meter distance, so the appliance instructions and NFPA 54 govern -- and Maryland adopts NFPA 54 by reference at COMAR 09.20.01. NFPA 54 requires a line pressure regulator vent to terminate at least 3 feet from a source of ignition, and a vent terminal is a source of ignition. The termination diagram reproduced in nearly every appliance manual carries the same 3 feet to a service regulator vent outlet and 3 feet to each side of the centerline extended above a meter or regulator assembly within 15 feet. Separately, a combustion air intake opening on the exterior of a building must have its lowest side not less than 12 inches above finished grade (Section 304.11, Item 8).
Condensate on Category II and Category IV Appliances
Section 503.9 requires provisions to collect and dispose of condensate from venting systems serving Category II and Category IV appliances and non-categorized condensing appliances. Section 503.8, Item 4 bars those through-the-wall vents from terminating over public walkways, or anywhere condensate or vapor could create a nuisance or hazard or damage regulators, relief valves and other equipment. The drain itself is Section 307.2: liquid combustion byproducts must discharge to an approved plumbing fixture or disposal area per the manufacturer, through corrosion-resistant piping not smaller than the appliance drain connection, sloped not less than 1/8 inch per foot (1 percent). Section 307.3 sets a minimum 3/4-inch internal diameter; Section 307.5 requires an auxiliary drain pan for Category IV condensing appliances where a stoppage would damage a building component; and Section 307.6 requires a condensate pump in an attic or crawl space to be wired so the appliance cannot run when the pump fails. Flue gas condensate is acidic, roughly pH 3 to 5, so where the manufacturer or the local authority requires it a limestone or marble-chip neutralizer is installed upstream of the drain to protect metallic waste piping.
Exam Trap: Candidates grab the 4-foot and 1-foot door-and-window numbers for every sidewall termination. Those belong to Section 503.8, Item 2, which applies to mechanical draft systems excluding direct-vent appliances. A 40,000 Btu/h direct-vent water heater terminating 12 inches from an openable window is compliant, because Table 503.8 requires only 9 inches at that input -- yet the identical 12 inches would fail for a power-vented, non-direct-vent unit.
A 45,000 Btu/h direct-vent tankless water heater terminates through a sidewall. Under IFGC 2018 Table 503.8 and Section 503.8, which pair of dimensions is required?