6.4 Direct-Vent Systems and Vent Terminations
Key Takeaways
For a through-wall direct-vent terminal, use 2024 IRC Figure and Table G2427.8 plus the appliance listing; the required clearance depends on input and opening type.
Do not apply a single 6/9/12-inch rule to every input: Table G2427.8 has a separate manufacturer-instruction and nondirect-vent requirement for input above 150,000 Btu/h.
Nondirect-vent terminal clearances differ from direct-vent clearances in Table G2427.8; inspect the system type before choosing a row.
Roof vent and chimney terminations are governed by the applicable G2427, Chapter 18, Chapter 10, and listing provisions; the 3-2-10 rule is not universal for all gas vents.
Terminations must avoid re-entry, blockage, condensate hazards, and damage while meeting the listed appliance and vent instructions.
6.4 Direct-Vent Systems and Vent Terminations
Quick Answer: Identify the appliance, venting method, and terminal location before applying a clearance. The 2024 IRC G2427.8 includes a figure and table for through-wall direct-vent and nondirect-vent terminals. Listed appliance instructions can add constraints. Roof vent rules are a separate check; a masonry chimney rule should not be applied automatically to a listed gas vent.
Through-wall direct vent
A direct-vent appliance draws combustion air from outdoors and exhausts flue gas outdoors through its listed system. Table G2427.8 distinguishes clearance to an openable door/window from other terminal relationships. For openable doors/windows, the direct-vent table has input-based rows: 6 inches for appliances at or below 10,000 Btu/h, 9 inches for above 10,000 through 50,000 Btu/h, and 12 inches for above 50,000 through 150,000 Btu/h. Above 150,000 Btu/h, follow the appliance instructions and the table's additional nondirect-vent minimum; do not extend the 12-inch row indefinitely. The table and figure also address grade, mechanical air intakes, walkways, and other openings. The combustion-air intake of a direct-vent appliance has an express table exception.
Nondirect-vent mechanical draft
A nondirect-vent terminal uses the separate table column. It cannot simply borrow the direct-vent window clearances. Check its relation to doors/windows, forced-air intakes, grade, and walkways with the actual G2427.8 table and the manufacturer instructions. For Category II and IV systems, also check whether condensate or vapor at a walkway or equipment location creates a prohibited hazard.
Roof terminations and inspection
For a roof termination, identify whether it is a masonry chimney, factory-built chimney, Type B/Type L vent, or another listed assembly. G2427.6, Chapter 18, Chapter 10 and the component listing contain different height and clearance methods. The familiar 3-foot-above-penetration and 2-feet-above-nearby-building rule applies to specified chimney situations; it is not a blanket rule for every gas vent. Verify actual roof geometry, nearby obstructions, support, listed cap and termination, and whether snow or vegetation can block the outlet.
| Installation | Primary 2024 IRC check |
|---|---|
| Through-wall direct vent | G2427.8 figure/table, appliance input, opening type, listing |
| Through-wall nondirect vent | Separate G2427.8 table column and listed instructions |
| Roof gas vent | G2427.6 and listed vent termination rules |
| Masonry/factory-built chimney | Chapter 10, Chapter 18 or Chapter 24 as applicable, and listing |
A through-wall direct-vent appliance has a 160,000 Btu/h input. Which clearance source applies to an openable window?
The 12-inch row for all appliances above 50,000 Btu/h
The appliance instructions and the over-150,000-Btu/h Table G2427.8 requirement
The local-exhaust M1504.3 rule alone
No clearance because it is direct vent
What should be determined before using the G2427.8 through-wall terminal table?
Only the vent pipe color
Whether the terminal is direct vent or nondirect vent, its appliance input, and the nearby opening type
Only the roof pitch
Only the size of the gas meter
Does the masonry chimney 3-2-10 roof-height rule automatically govern every listed gas vent?
Yes, without exception
No; use the vent-specific 2024 IRC roof rules and product listing
Yes, if the appliance has a draft hood
Only if the roof is flat
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