11.3 Chimney & Vent Sizing Tables (Type B, Masonry & Common Venting)

11.3 Chimney & Vent Sizing Tables (Type B, Masonry & Common Venting)

Quick Answer: Sizing a Category I vent is a disciplined table lookup: identify FAN (fan-assisted) or NAT (draft-hood) appliance, count single appliance versus common vent, measure H (total vent height) and L (lateral) - plus R (connector rise) on a common vent - then choose the smallest diameter whose Max capacity equals or exceeds the appliance input and whose Min capacity does not exceed it. IFGC 2018 Section 504 sizes Category I appliances only.

Step 1 - Pick the Right Table

IFGC 2018 Chapter 5 contains thirteen sizing tables that look nearly identical at a glance. The exam does not test your memory of their numbers; it tests whether you land on the correct table.

Section 504.3.22 supplies the mixing rule: where single-wall and Type B double-wall connectors are both used in one system, the common vent must be sized from Table 504.3(2) or 504.3(4) - the single-wall tables - as though the entire system were single-wall.

Step 2 - Measure H, L and R

• H, total vent height. For a single appliance, the vertical distance from the draft hood outlet or flue collar to the vent outlet. For multiple appliances on one floor, Section 504.3.12 measures H from the highest draft hood outlet or flue collar up to the level of the common vent outlet.
• L, lateral. The horizontal run. Section 503.6.10.2 treats any angle greater than 45 degrees from vertical as horizontal, so a 60-degree offset counts entirely as lateral. Section 504.2.4 permits a "zero lateral" entry only for a straight vertical vent on a top-outlet draft hood or flue collar, and 504.2.3 forbids elbows in a zero-lateral configuration.
• R, connector rise (common vents only). Section 504.3.11 measures rise from that appliance's draft hood outlet or flue collar to the centerline where the vent gas streams come together - not to the ceiling and not to the vent cap.

Lateral length quietly destroys capacity. Holding H at 20 feet in Table 504.2(1) and reading down the 4-inch Type B column, in thousands of Btu/h:

Going from a straight vertical vent to a 20-foot lateral costs that 4-inch Type B vent 43 percent of its fan-assisted capacity - 202,000 down to 116,000 Btu/h - while the minimum input it can safely carry climbs from zero to 55,000 Btu/h.

Step 3 - Read FAN Min, FAN Max and NAT Max

Section 504.1 defines the column headings:

• FAN Min - the minimum input of a Category I fan-assisted appliance that may be attached to that vent or connector.
• FAN Max - the maximum input of a Category I fan-assisted appliance.
• NAT Max - the maximum input of a Category I draft-hood-equipped appliance. There is no NAT Min; a draft hood dilutes flue gas with room air and does not suffer the same starvation problem.
• FAN+FAN, FAN+NAT, NAT+NAT - combined input limits for a common vent, selected by what the pair of appliances actually is.
• NA - the configuration is not allowed, because it would form condensate, pressurize the venting system, or is geometrically impossible.

Why a minimum exists at all. A fan-assisted appliance pushes a modest volume of relatively cool flue gas. Put 40,000 Btu/h into a vent sized for 200,000 and the gas slows, surrenders heat to the vent wall, falls below its dew point and condenses inside a vent built of aluminum and galvanized steel that was never meant to sit in acidic water. Draft collapses at the same time. That is why the tables publish a floor as well as a ceiling, and why the instinct that "an oversized vent is a safe vent" is wrong.

Worked Example 1 - Single Appliance on a Type B Vent

A Category I fan-assisted furnace rated 100,000 Btu/h vents through a Type B double-wall gas vent. Total vent height H = 20 feet, lateral L = 5 feet, Type B connector. Table 504.2(1) governs, and inputs are read in thousands of Btu/h, so the target number is 100.

Read across the H = 20, L = 5 row:

Answer: a 4-inch Type B vent. Now change one word in the problem. If that same 100,000 Btu/h appliance were draft-hood equipped (NAT) instead of fan-assisted, you read the NAT Max column, where 4 inches tops out at 96 - less than 100 - so the identical geometry demands a 5-inch vent (NAT Max 160). Same input, same height, same lateral; different column, different answer. That single distinction produces more wrong answers on venting questions than any other.

Before writing the answer down, run the code checks:

1. 504.2.2 minimum size. A vent smaller than the draft hood outlet or flue collar is permitted only where H is at least 10 feet, the reduction is not more than one table size for outlets 12 inches or smaller, fan-assisted maximum capacity is taken at 0.90 x the table value, and the draft hood outlet is larger than 4 inches. You may never connect a 3-inch vent to a 4-inch draft hood outlet.
2. 504.2.8 area cap. The flow area of the vertical vent shall not exceed seven times the appliance categorized vent area, flue collar area or draft hood outlet area.
3. 504.2.3 elbows. A configuration with a lateral already includes two 90-degree elbows. Each additional elbow up to 45 degrees costs 5 percent of maximum capacity; each additional elbow over 45 and up to 90 degrees costs 10 percent. Add one more 90-degree elbow to this example and FAN Max becomes 0.90 x 143 = 128.7, or 128,700 Btu/h - still above 100,000, so the 4-inch vent survives.
4. 504.2.7 liners. A listed corrugated metallic liner in a masonry chimney is sized from this same Type B table with maximum capacity reduced 20 percent (0.80 x) and minimum capacity unchanged. Here 0.80 x 143 = 114.4, so a 4-inch liner still carries the 100,000 Btu/h furnace.

Worked Example 2 - Two Appliances on a Common Vent

A basement mechanical room holds a 40,000 Btu/h draft-hood (NAT) water heater with a 1-foot connector rise and a 90,000 Btu/h fan-assisted (FAN) furnace with a 2-foot connector rise. Both use Type B double-wall connectors into a Type B double-wall common vent. Total height from the highest flue collar to the common vent outlet is H = 20 feet. Table 504.3(1) governs.

Step A - size each connector individually. Read the VENT CONNECTOR CAPACITY block at H = 20.

• Water heater, R = 1 ft, NAT column: 3 in. NAT Max = 31, too small for 40; 4 in. NAT Max = 56. Use a 4-inch connector.
• Furnace, R = 2 ft, FAN columns: 3 in. gives Min 22 / Max 57, and 57 is below 90, so it fails; 4 in. gives Min 34 / Max 105, and 34 is at or below 90, which is at or below 105. Use a 4-inch connector.

Step B - size the common vent on the combined input. 40 + 90 = 130 thousand Btu/h. One fan-assisted appliance plus one draft-hood appliance means the FAN+NAT column. In the COMMON VENT CAPACITY block at H = 20: 4 in. FAN+NAT = 123; 5 in. FAN+NAT = 183. Because 130 exceeds 123, the 4-inch common vent fails and the answer is a 5-inch common vent.

Step C - check the companion rules.

• 504.3.8: the cross-sectional area of the common vent must be at least that of the largest connector. Five inches beats four. Pass.
• 504.3.2: each 4-inch connector is limited to 6 feet of horizontal run.
• 503.10.4: the smaller connector enters the common vent at the highest level consistent with headroom and clearance to combustibles, so the water heater ties in above the furnace, never below.
• 504.3.17: the largest section of vertical vent shall not exceed seven times the smallest appliance's categorized vent, flue collar or draft hood outlet area.

The trap here is the column, not the arithmetic. A candidate who reads FAN+FAN finds 136 at 4 inches, concludes that 130 fits, and installs an undersized common vent. The pair is one FAN and one NAT, so FAN+NAT is the only correct column.

The Common-Venting Rules That Decide Exam Questions

• Each connector is sized on its own appliance input and its own rise; the common vent is sized on the combined input. Never size the common vent from the larger appliance alone.
• Different floor levels. Section 503.6.11 permits a common vent for Category I appliances located on more than one floor only where the venting system is designed and installed by approved engineering methods, and crawl spaces, basements and attics all count as floor levels. Section 503.6.11.1 requires those appliances to sit in rooms separated from occupiable space, each with combustion, ventilation and dilution air that is not supplied from an occupiable space, and 504.3.15 requires the vertical common vent to be Type B double wall with a listed vent cap. A basement water heater simply teed into a first-floor furnace's vent is not a compliant multistory system.
• Manifolds and offsets cost capacity. Where connectors combine before the vertical common vent, 504.3.4 applies 0.90 x the common vent capacity and limits the manifold length to 1 1/2 feet per inch of manifold diameter. Common-vent elbows cost 5 percent (up to 45 degrees) or 10 percent (over 45 and up to 90 degrees) each under 504.3.6, and a multistory offset costs 0.80 x under 504.3.16.
• Openings at different levels. Section 503.10.4.1 requires two or more openings into one flue or vent to be at different levels, or the connectors to attach to the vertical portion at 45 degrees or less from vertical.

Masonry Chimney Versus Type B, and the Exterior-Chimney Threshold

Tables 504.2(3), 504.2(4), 504.3(3) and 504.3(4) behave differently from the Type B tables: you read a connector diameter, then confirm that the chimney's internal area falls between the minimum and maximum area limits printed at the bottom of the table. Section 503.5.5 offers an alternate area method for draft-hood appliances - the connector and chimney flue areas shall be not less than the flue collar or draft hood outlet area and not more than seven times the draft hood outlet area, and for two draft-hood appliances the flue area shall be not less than the larger draft hood outlet plus 50 percent of the smaller.

The interior/exterior distinction is a hard gate. Sections 504.2.9 and 504.3.20 restrict Tables 504.2(1) through 504.2(5) and 504.3(1) through 504.3(5) to chimneys and vents not exposed to the outdoors below the roof line. A Type B vent or listed lining system passing through an unused masonry flue is not considered exposed, and neither is a Type B vent inside an unventilated chase insulated to R8. Anything else running up an outside wall is an exterior masonry chimney, must be clay-tile lined, and is sized from 504.2(3) combined with 504.2(6), or from the 504.3(6a)/(6b) and 504.3(7a)/(7b) pairs.

Those exterior tables impose a minimum input rating keyed to chimney internal area, vent height and the local 99-percent winter design temperature, and large blocks of them read NA. From Table 504.2(6): in the 5-to-16-degree-F design-temperature band, a 10-foot vent height on a 63-square-inch chimney demands a space-heating appliance of at least 289,000 Btu/h, and every internal area of 50 square inches or less is NA - not permitted in any size. Below a minus 11 degree F design temperature the table states that no vent configuration is recommended. Maryland's 99-percent heating design temperatures run roughly 4 degrees F in Garrett County to about 18 degrees F on the lower Eastern Shore, so Maryland work lands in the 5-to-16 or 17-to-26 degree bands - which is exactly why an 80,000 Btu/h residential furnace generally cannot be vented into an exterior masonry chimney in this state. An unlined exterior masonry chimney has no table at all: Section 503.5.3 requires the lining before Section 504 will size anything, and 503.5.6.3 requires an unsafe chimney to be repaired, lined, relined or replaced.

Rounding, Interpolation and Extrapolation

Round to the size whose capacity meets or exceeds the appliance input, never down. Section 504.2.14 permits interpolation for vent dimensions that fall between table entries; 504.2.15 flatly prohibits extrapolation beyond the entries; and 504.2.16 sends vent heights below 6 feet or above the tabulated range to engineering methods. Where the actual height falls between rows, 504.2.17 offers a conservative shortcut: use the lower appliance input rating for the FAN Max and NAT Max columns and the higher input rating for the FAN Min column. Applied to a 25-foot vent in Table 504.2(1), you read FAN Max and NAT Max from the 20-foot row and FAN Min from the 30-foot row.

Exam Trap: The FAN+FAN column is the most attractive wrong answer in the whole code. It carries the largest numbers in the common vent block, so it "works" for almost any combined input - and it applies only where every appliance on the vent is fan-assisted. One draft-hood water heater in the group forces FAN+NAT, and two draft-hood appliances force NAT+NAT, which is smaller still. At H = 20 feet on a 5-inch Type B common vent the three columns read 215, 183 and 160 thousand Btu/h - a 55,000 Btu/h spread that decides whether the vent you specify is legal. The companion trap is sizing the common vent from the larger appliance's input instead of the combined input of everything on it.