13.2 Vent Sizing, Common Venting & Draft

Once the connector is legal, the area has to be legal. 2024 IFGC 501.4 sends chimney and vent size to Sections 503 and 504. Section 504 is only for Category I draft-hood appliances, fan-assisted Category I appliances, and appliances listed for Type B. Category II, III, and IV vents are sized from the appliance manufacturer (503.6.10.3; 501.14), not from a 504 table you already had open for the water heater.

Draft-hood versus fan-assisted (induced-draft) Category I. A draft-hood (NAT) appliance dilutes flue gas with room air at the hood and needs enough buoyancy to keep the hood from spilling. A fan-assisted Category I appliance has an integral fan that pulls or pushes combustion products through the heat exchanger, then still dumps into a nonpositive Type B or chimney. The fan is not a Category III or IV power vent. It does mean the table has a FAN Min as well as a FAN Max: too large a vent on a small fan-assisted input cools the gases and condenses. 504.1 defines those columns. NA means the configuration is not allowed because of condensate, pressurization, or a geometric restraint — it does not mean “use the next size up and hope.”

Which table, and how you enter it

Single appliance — 504.2. Tables 504.2(1) through 504.2(6) apply under 504.2.1 through 504.2.17.

• Table 504.2(1): Type B double-wall gas vent, appliance connected with Type B (or directly to the vent).
• Table 504.2(2): Type B double-wall gas vent, single-wall metal connector.
• Tables 504.2(3)–(6) cover masonry and exterior-masonry cases, including the clay-tile exterior-chimney pair 504.2(3) + 504.2(6).

Enter with height H (vertical from the highest draft-hood outlet or flue collar to the vent termination) and lateral L (horizontal offset). 504.2.4: zero lateral applies only to a straight vertical vent on a top-outlet draft hood or flue collar. A 2-foot offset is not a zero-lateral row. Each table cell has FAN Min, FAN Max, and NAT Max in thousands of Btu/h. A draft-hood appliance uses NAT Max only. A fan-assisted appliance must sit between FAN Min and FAN Max. 504.2.6: for a multi-rate appliance, FAN Min must be less than the lowest input and FAN Max / NAT Max greater than the highest input.

Common vent — 504.3. Tables 504.3(1) through 504.3(7) apply under 504.3.1 through 504.3.28.

• Table 504.3(1): Type B common vent, Type B double-wall connectors.
• Table 504.3(2): Type B common vent, single-wall metal connectors.
• Later 504.3 tables cover masonry common vents and exterior masonry, including winter-design-temperature limits that can read “not recommended for any vent configurations.”

504.1 common-vent definitions you will be tested on:

• FAN+FAN — combined maximum input of two or more Category I fan-assisted appliances on the common vent.
• FAN+NAT — combined maximum input of one or more Category I fan-assisted appliances plus one or more Category I draft-hood appliances on the common vent.
• NAT+NAT — combined maximum input of two or more Category I draft-hood appliances on the common vent.

You do not pick NAT+NAT because “both appliances are Category I.” Category I is the family. FAN versus NAT is the draft type. A fan-assisted 80-percent furnace plus a draft-hood water heater is FAN+NAT. Using the NAT+NAT number for that pair overstates capacity and is the field fail this section is written to catch.

How to walk the common-vent table. (1) Identify each appliance as FAN or NAT from the nameplate, not from the sales brochure. (2) Measure connector rise R (vertical from the draft hood or flue collar to the common-vent connection) and common-vent height H. (3) Size each connector from the connector-capacity half of the table at that H and R — FAN Min / FAN Max / NAT Max for that connector diameter. (4) Add the inputs that actually enter the common vent and read FAN+FAN, FAN+NAT, or NAT+NAT on the common-vent half. (5) If a cell is NA, stop; do not interpolate an illegal configuration (504.2.15 / 504.3: extrapolation is prohibited).

504.3.2 / Table 504.3.2. Route the connector by the shortest practical path. Maximum connector horizontal length for table use is 1½ feet per inch of connector diameter (a 4-inch connector = 6 feet; a 6-inch connector = 9 feet), unless 504.3.3 is used. 504.3.3 allows a longer connector if FAN Max or NAT Max is reduced 10 percent for each additional multiple of that length (4-inch connector at 7–12 feet → 0.90 × table max; 13–18 feet → 0.80 × table max). Fan-assisted FAN Min on a long connector is then read from the matching single-appliance table as if the other appliance were gone.

Shortcuts, downsizing, and oversized masonry

503.6.10.1 / 503.5.5 area shortcuts still exist for listed draft-hood and Type B-listed appliances when you are not using the tables: a single draft-hood vent or chimney shall be not smaller than the draft-hood outlet and not greater than seven times that outlet area; two draft-hood appliances, not smaller than the larger outlet plus 50 percent of the smaller, and not greater than seven times the smallest outlet. Those shortcuts are not a license to skip FAN Min on a fan-assisted appliance. Mechanical-draft chimney systems are engineered (503.5.5 Item 4; 503.6.10.4).

You cannot casually downsize below the flue collar. 504.2.2 is the only table path to a vent smaller than the draft-hood outlet or flue collar, and every item must be true: total vent height H not less than 10 feet; outlets 12 inches or smaller reduced not more than one table size (larger than 12 inches, not more than two table sizes); FAN Max reduced 10 percent; draft-hood outlet greater than 4 inches — do not connect a 3-inch vent to a 4-inch draft hood (that last item does not apply to fan-assisted appliances). 504.2.10: a corrugated connector shall not be smaller than the listed categorized vent, flue collar, or draft-hood diameter. 504.2.11: do not increase a connector more than two sizes above that same diameter. 504.2.8: if the vertical vent is larger than the connector, use the vertical diameter for minimum capacity and the connector diameter for maximum capacity, and keep the vertical flow area not more than seven times the appliance categorized vent, flue-collar, or draft-hood area unless the system is engineered.

Oversized masonry is not “safer.” A cold 12-by-12 clay tile on a 4-inch water-heater draft hood exceeds the 7× cap, drops below dew point, and condenses. Condensate runs back, rusts the connector, and the hood spills CO into the mechanical room. 504.1 invented NA for that physics. 501.15.1 requires resizing when appliances are added or removed so condensation is controlled and draft is provided. Exterior masonry needs the 504.2(3)/(6) or 504.3 exterior tables and the winter-design-temperature check — some climates are simply “not recommended.”

504.2.7 corrugated liners. Size a listed corrugated metallic liner from Table 504.2(1) or 504.2(2) with maximum capacity cut 20 percent (and further cut for extra offsets under 504.2.3). That 20 percent already includes one long-radius 90-degree turn at the bottom. Join liner to Type B connector with a listed adapter.

Elbows. 504.2.3: a zero-lateral single-appliance row has no elbows. A lateral row already includes two 90s. Each extra elbow ≤45° cuts max capacity 5 percent; each extra elbow >45° through 90° cuts it 10 percent. 504.3.6 / 504.3.7 apply the same 5/10 percent cuts on common vents and extra connector elbows. 504.3.4: a manifold that combines connectors before the vertical common vent takes a 10 percent cut on the common-vent capacity, and the manifold length itself is capped at 1½ feet per inch of manifold diameter.

Multistory. 503.6.11: Category I appliances on more than one floor on a common vent are approved engineering methods, except the 503.6.11.2 Table 504.3(1) path when appliances sit in rooms accessed only from outdoors and each connector is not smaller than the draft hood or flue collar. Crawl spaces, basements, and attics count as floors.

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