10.5 Appliance Venting, Chimneys & Combustion Air

Appliance Venting, Chimneys & Combustion Air

Content Area C of the Florida trade blueprint is Natural Gas Piping at 15 percent, and it names two sub-topics. Sections 10.1 through 10.3 covered the second — natural gas piping systems, their materials, sizing and testing. This section covers the first: Natural Gas Venting System, with the combustion-air knowledge the outline attaches to the piping sub-topic.

The governing document is the Florida Building Code - Fuel Gas (FFGC), 2023 edition, which is on the Plumbing Contractor reference list as R269. Venting is Chapter 5, Chimneys and Vents; combustion air is Section 304.

Get the concept straight before the numbers: combustion air goes in, flue gas goes out, and both are sized from the same appliance input rating in Btu/h. Starve the appliance of combustion air and it produces carbon monoxide; fail the vent and it delivers that carbon monoxide to the occupants.

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1. Appliance Categories — The Sorting Rule

Everything in Chapter 5 depends on which of four categories the appliance falls into. The two variables are vent pressure and whether the flue gas condenses:

FFGC Table 503.4, Type of Venting System to Be Used, then maps appliance to vent:

• Listed Category I appliances, listed appliances with a draft hood, and appliances listed for use with Type B gas vent → Type B gas vent (503.6), chimney (503.5), single-wall metal pipe (503.7), a listed chimney lining system for gas venting (503.5.3), or a special gas vent listed for these appliances (503.4.2).
• Listed vented wall furnaces → Type B-W gas vent (Sections 503.6 and 608).
• Category II, III and IV appliances → as specified or furnished by manufacturers of listed appliances (Sections 503.4.1 and 503.4.2).
• Unlisted appliances, appliances convertible to solid fuel, unlisted combination gas-and-oil appliances, and combination gas-and-solid-fuel appliances → chimney (503.5).
• Listed combination gas-and-oil-burning appliances → Type L vent (503.6) or chimney.
• Direct-vent appliances → Section 503.2.3; appliances with integral vents → Section 503.2.4.

Vent listing standards (FFGC 502.1): Type B and BW vents are tested to UL 441; Type L vents to UL 641; vents for Category II and III appliances to UL 1738.

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2. Combustion Air (FFGC Section 304)

Section 304.1 requires air for combustion, ventilation and dilution of flue gases to be provided by one of the methods in Sections 304.5 through 304.9. Where the requirements of 304.5 are not met, outdoor air is introduced by one of the methods in 304.6 through 304.9. Direct-vent appliances, gas appliances of other than natural draft design, vented gas appliances not designated Category I, and appliances with power burners are instead provided with combustion air in accordance with the appliance manufacturer's instructions.

Indoor Combustion Air (304.5)

• 304.5.1 Standard Method: the minimum required volume shall be 50 cubic feet per 1,000 Btu/h (4.8 m3/kW) of the appliance input rating.
• 304.5.2 Known Air-Infiltration-Rate Method: where the infiltration rate of the structure is known, volume is computed from Equations 3-1 and 3-2, using a different equation for fan-assisted appliances. Where the air infiltration rate is known to be less than 0.40 ACH, Section 304.5.2 shall be used, and an infiltration rate greater than 0.60 ACH shall not be used in the equations.
• The total required volume is the sum of the volumes required for all appliances in the space, and rooms communicating directly with the appliance space through openings not furnished with doors — and through combustion air openings sized per 304.5.3 — count toward it.

Worked example. A utility closet holds a 100,000 Btu/h furnace and a 40,000 Btu/h gas water heater — 140,000 Btu/h total input. $V_{required} = \frac{140,000}{1,000} \times 50 = \mathbf{7,000\text{ cubic feet}}$ A closet 4 feet by 5 feet with an 8-foot ceiling holds 160 cubic feet, so it cannot qualify on its own. The designer either borrows volume from adjoining rooms through openings sized under 304.5.3, or brings in outdoor air under 304.6.

Indoor Openings Between Spaces (304.5.3)

• 304.5.3.1 Combining spaces on the same story: each opening shall have a minimum free area of 1 square inch per 1,000 Btu/h of the total input of all appliances in the space, but not less than 100 square inches. One permanent opening shall commence within 12 inches of the top and one within 12 inches of the bottom of the enclosure, and the minimum dimension of air openings shall be not less than 3 inches.
• 304.5.3.2 Combining spaces in different stories: the volumes are considered communicating where connected by permanent openings in doors or floors having a total minimum free area of 2 square inches per 1,000 Btu/h of total input.

Outdoor Combustion Air (304.6)

The minimum dimension of any air opening is not less than 3 inches.

• 304.6.1 Two-Permanent-Openings Method: two openings, one commencing within 12 inches of the top and one within 12 inches of the bottom of the enclosure.
  • Communicating directly with the outdoors, or through vertical ducts: each opening needs 1 square inch of free area per 4,000 Btu/h of total input.
  • Communicating with the outdoors through horizontal ducts: each opening needs 1 square inch per 2,000 Btu/h — twice the area, because a horizontal duct does not get buoyancy help.
• 304.6.2 One-Permanent-Opening Method: a single opening commencing within 12 inches of the top of the enclosure, with the appliance having clearances of not less than 1 inch from the sides and back and 6 inches from the front. The opening needs 1 square inch per 3,000 Btu/h of total input and not less than the sum of the areas of all vent connectors in the space.
• 304.9 Mechanical Combustion Air Supply: where all combustion air comes from a mechanical supply, it shall be supplied from the outdoors at not less than 0.35 cubic feet per minute per 1,000 Btu/h of total input, each appliance shall be interlocked with the supply so main burners cannot operate when the supply is not running, and makeup air shall be provided where exhaust fans are installed.

Worked example continued. Taking the same 140,000 Btu/h closet to outdoor air through two openings communicating directly with the outdoors: $A_{each} = \frac{140,000}{4,000} = \mathbf{35\text{ square inches of free area, top and bottom}}$ If those openings instead run through horizontal ducts: $A_{each} = \frac{140,000}{2,000} = \mathbf{70\text{ square inches each}}$

Louvers, Grilles and Ducts (304.10, 304.11)

• Opening size is based on net free area. Where the free area of a design is unknown, assume wood louvers 25 percent free area and metal louvers and grilles 75 percent free area. Screens shall have a mesh not smaller than 1/4 inch.
• Nonmotorized louvers and grilles shall be fixed in the open position; motorized louvers shall be interlocked with the appliance, proven full open before main burner ignition and during operation, with means to prevent ignition if they fail to open and to shut down the burner if they close.
• Combustion air ducts shall be galvanized steel or equivalent, shall serve a single enclosure, shall not serve both upper and lower openings where both are used, shall not be screened where terminating in an attic, and horizontal upper combustion air ducts shall not slope downward toward the source of combustion air.
• Exterior combustion air intake openings shall have their lowest side located not less than 12 inches vertically above the adjoining finished ground level — a Florida-relevant rule given yard flooding and landscape mulch.

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3. Vent and Chimney Terminations

Type B and L Gas Vents

• FFGC 503.6.6 Minimum Height: a Type B or L gas vent shall terminate not less than 5 feet in vertical height above the highest connected appliance draft hood or flue collar; a Type B-W gas vent shall terminate not less than 12 feet in vertical height above the bottom of the wall furnace.
• FFGC 503.6.5 Gas Vent Terminations: a gas vent 12 inches or less in size and located not less than 8 feet from a vertical wall terminates above the roof per Figure 503.6.5, whose height H depends on roof slope — 1.0 foot for flat through 6/12, 1.25 feet over 6/12 through 7/12, 1.5 feet over 7/12 through 8/12, 2.0 feet over 8/12 through 9/12, and rising to 8.0 feet over 20/12 through 21/12. A gas vent over 12 inches in size, or located less than 8 feet from a vertical wall or similar obstruction, shall 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.
• 503.6.7 Roof Terminations: gas vents shall extend through the roof flashing, roof jack or roof thimble and terminate with a listed cap or listed roof assembly.
• 503.6.5.1: decorative shrouds shall not be installed at the termination of gas vents except where listed for the specific venting system.

Chimneys

• FFGC 503.5.4 Chimney Termination: chimneys for residential-type or low-heat appliances shall extend not less than 3 feet above the highest point where they pass through a roof, and not less than 2 feet higher than any portion of a building within a horizontal distance of 10 feet. Chimneys for medium-heat appliances shall extend not less than 10 feet higher than any portion of any building within 25 feet. Every chimney shall extend not less than 5 feet above the highest connected appliance draft hood outlet or flue collar.
• FFGC 503.5.5 Size of Chimneys: the effective area of a chimney venting system serving listed appliances with draft hoods, Category I appliances and other appliances listed for Type B vents is determined by Section 504, or by these rules of thumb — for a single appliance with a draft hood, the effective areas of the connector and chimney flue shall be not less than the area of the appliance flue collar or draft hood outlet, nor greater than seven times the draft hood outlet area; for two appliances with draft hoods, the chimney flue shall be not less than the area of the larger draft hood outlet plus 50 percent of the area of the smaller, nor greater than seven times the smallest draft hood outlet area.
• 503.5.6: before replacing an appliance or connecting a vent connector to a chimney, the chimney passageway shall be examined to confirm it is clear and free of obstructions, and cleaned if previously used for solid or liquid fuel.

Through-the-Wall Terminals

FFGC Section 503.8 and Table 503.8 set clearances for through-the-wall direct-vent and nondirect-vent terminals, beginning with 12 inches of clearance above finished grade level, veranda, porch, deck or balcony, and a clearance to an openable window or door that varies with appliance input. The clearances in Table 503.8 do not apply to the combustion air intake of a direct-vent appliance.

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4. Sizing a Category I Vent (FFGC Section 504)

Category I venting is sized from the tables in Section 504, and the terminology is what the exam asks about:

The single-appliance tables, beginning with Table 504.2(1) for Type B double-wall gas vent, are entered with vent height H in feet and lateral length L in feet, and read across to the vent diameter columns to find the appliance input rating in thousands of Btu/h. A FAN Min column exists because a fan-assisted appliance in an oversized, tall vent can condense — too big is as wrong as too small.

Note the two key limitations the code attaches to Category I venting: a draft hood or barometric draft regulator shall be installed in the same room or enclosure as the appliance served (304.3), so no pressure difference develops between the regulator and the combustion air supply; and the remaining space around a chimney liner, gas vent, special gas vent or plastic piping installed within a chimney shall not be used to supply combustion air (304.11), except for listed direct-vent appliances in solid-fuel fireplaces.

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5. Protection and Support

• 502.4 Insulation shield: where vents pass through insulated assemblies, a shield of steel not less than 0.0187 inch (No. 26 gage) maintains the manufacturer's required clearance to insulation, and where a vent passes through an attic the shield shall terminate not less than 2 inches above the insulation and be secured against displacement.
• 502.7 Protection against physical damage: where a vent runs through holes or notches in studs, joists or rafters less than 1-1/2 inches from the nearest edge, it shall be protected by steel shield plates not less than 0.0575 inch (No. 16 gage) covering the notched or bored area and extending not less than 4 inches above sole plates, below top plates and to each side of the member.
• 502.7.1 Door swing: appliance and equipment vent terminals shall be located so that doors cannot swing within 12 inches horizontally of the vent terminal, and door stops or closers shall not be installed to obtain this clearance.
• 502.6 Support: all portions of vents shall be adequately supported for the design and weight of the materials employed.