CMC · California Mechanical Code
How are common vents designed for appliances on more than one floor?
If you vent gas appliances on more than one floor with a single common vent, the California Mechanical Code requires the system be designed using engineering methods (or the multistory tables when the §802.6.3.2 conditions are met). Appliances must be in rooms separated from living space with independent combustion air, connector sizes cannot be smaller than the appliance flue collar, and each vertical segment is sized based on the total inputs entering that segment and the available height (H).
Last reviewed: July 6, 2026
What the code requires — 2-4 sentences
When a single vertical vent (a common vent) is used to serve Category I appliances on multiple floor levels, the multistory venting system must be designed and installed using engineering methods (it is not a simple “pick a size” rule) — see § 802.6.3. All appliances tied into that common vent must be located in rooms separated from occupiable space, and each room must have its own supply of combustion/ventilation/dilution air not taken from occupiable spaces — see § 802.6.3.1. Sizing of the individual connectors and of each vertical segment of the multistory vent follows the multistory sizing procedure (Table 803.2(1)) when the three conditions in § 802.6.3.2 apply; otherwise the system must be engineered.
The most important rule: design multistory common-vent systems using engineering methods; use Table 803.2(1) only when the specific multistory sizing conditions in § 802.6.3.2 are met.
Requirements in detail
Key defined terms (first‑use)
- Category I — appliance classification referenced by the code; multistory common vents addressed here apply to Category I appliances. § 802.6.3.
- common vent — the shared vertical venting element that serves appliances on more than one floor. § 802.6.3.
- multistory venting system — a venting system serving appliances on multiple floor levels; its segments are sized per the multistory rules. § 802.6.3.2.
- Type B double-wall gas vent — the vent construction type called out for sizing from the common-vent tables referenced by the multistory rules. § 802.6.3.2.
- available total height (H) — the vertical distance used to size each segment of a multistory vent (defined in § 802.6.3.2(1)).
How the multistory sizing procedure works (plain steps)
- Confirm the installation involves Category I appliances on more than one floor; crawl spaces, basements and attics count as floor levels. § 802.6.3.
- Verify each appliance connected to the common vent is in a room separated from occupiable space and that each such room has adequate combustion, ventilation and dilution air not drawn from occupiable space. § 802.6.3.1.
- If the appliances are listed for use with a Type B double‑wall gas vent, you may size connectors and common vertical segments using Table 803.2(1) — but only when the three conditions in § 802.6.3.2 are satisfied. Otherwise, design by engineering methods. § 802.6.3.2.
- For each segment of the multistory venting system:
- Determine the available total height (H) for that segment (vertical distance from the highest draft‑hood outlet or flue collar entering that segment up to the centerline of the next higher interconnection tee). § 802.6.3.2(1).
- Size each appliance’s connector from the appliance’s gas input and the connector’s available rise; connector size must not be smaller than the appliance draft hood outlet or flue collar. § 802.6.3.2(2).
- Size the common vertical vent segment (and the interconnection tee at its base) based on the total gas input rate entering that segment and the segment’s available total height (H), using the appropriate common‑vent table (Table 803.2(1)) unless engineering methods are required. § 802.6.3.2(3).
Decision‑relevant dimensions / values
| Decision item | Value / rule | Code Reference |
|---|---|---|
| Is multistory rule applicable? | Applies when a common vent serves Category I appliances on >1 floor; crawl spaces/basements/attics = floors. | § 802.6.3 |
| Appliance location / combustion air | Appliances must be in rooms separated from occupiable space; each room must have adequate combustion/ventilation/dilution air not from occupiable space. | § 802.6.3.1 |
| Use of Table 803.2(1) | Connectors & common segments may be sized from Table 803.2(1) for appliances listed for use with Type B vents — only if conditions (1)-(3) of § 802.6.3.2 are met. | § 802.6.3.2 |
| Definition of H (available total height) | H = vertical distance between the highest draft hood outlet/flue collar entering the segment and the centerline of the next higher interconnection tee. | § 802.6.3.2(1) |
| Connector sizing rule | Size connector from appliance input & available connector rise; connector size not smaller than draft hood outlet or flue collar. | § 802.6.3.2(2) |
| Sizing common vertical segment | Based on total appliance input entering the segment and H; select common vent and interconnection tee per table or engineering methods. | § 802.6.3.2(3) |
(If you need to pick an actual vent diameter for a numeric input and H, use the referenced tables (Table 803.2(1)) or an engineered calculation — the code text directs you to those tables or engineering design.)
Exceptions & special cases
- The code treats crawl spaces, basements and attics as floor levels, so they participate in multistory sizing just like living floors — § 802.6.3.
- Table‑based sizing (Table 803.2(1)) applies only where appliances are listed for use with Type B double‑wall gas vent and the three specific conditions in § 802.6.3.2 are met; when not met the venting system must be sized via engineering methods. § 802.6.3.2.
- The code requires the connector size to be at least the size of the appliance’s draft hood outlet or flue collar — you cannot down‑size a connector to “fit” even if table numbers would otherwise appear to allow it. § 802.6.3.2(2).
Common mistakes
- Treating attic/crawl space appliances as “not a floor” and omitting them from the multistory calculation (the code explicitly includes them). § 802.6.3.
- Sizing a connector smaller than the appliance’s draft hood outlet or flue collar — the connector must be at least that size. § 802.6.3.2(2).
- Applying single‑story common‑vent tables instead of the multistory tables (Table 803.2(1)) or engineering methods required by § 802.6.3.2.
- Forgetting the requirement that appliances be in rooms separated from occupiable space with independent combustion air supplies — a compliance/clearance and safety requirement in § 802.6.3.1.
Worked example — concrete scenario (procedure with numbers)
Scenario: Two Category I appliances — a furnace at 150,000 Btu/h on the first floor and a water heater at 35,000 Btu/h on the second floor. The vertical distance from the highest draft‑hood/flue‑collar of the first‑floor segment to the centerline of the interconnection tee above is H = 30 ft. Both appliances are listed for use with Type B double‑wall gas vent.
Procedure (how the code tells you to proceed):
- Confirm both appliances are in rooms separated from occupiable space and each room has adequate combustion air (per § 802.6.3.1).
- For each connector, determine required connector size from the appliance input and available connector rise; ensure the connector is not smaller than the appliance draft‑hood outlet/flue collar (per § 802.6.3.2(2)).
- For the common vertical segment that will carry both inputs upward from the first interconnection, add the appliance inputs that enter that segment (here 150,000 + 35,000 = 185,000 Btu/h) and use H = 30 ft to pick a common‑vent size from Table 803.2(1) for Type B vents, or perform an engineered calculation if table conditions are not met. § 802.6.3.2(3).
Note: Because the code text points you to Table 803.2(1) (multistory tables) for the final diameter selection, you must consult that table (or an engineered calculation). Example worked solutions using the identical NFPA-based tables are provided in the code commentary/examples (see a similar worked example using Table logic in the code guidance), which shows selecting a 6‑inch vent for comparable totals at H = 30 ft for Type B vents — use that table or engineering when applying to your numeric case.
Related provisions (useful cross references)
- § 803.2.15 — Multistory Type B vents required (vertical common vents in multistory systems shall be Type B double‑wall and shall have a listed vent cap).
- § 803.2.16 — Multistory vent offsets and capacity (limits on offsets and capacity reductions).
- § 803.2.13 — Multistory vent height definition (clarifies H measurement across sections).
- § 803.2.7 — Common vent minimum size (cross‑sectional area >= largest connector).
- § 802.6.2.2 — Vent offsets for Type B/L vents (offset angle / horizontal distance limits used elsewhere in vent sizing).
(These related sections explain vent construction, offsets, table usage and sizing adjustments that commonly interact with multistory designs.)
Code references
Grounded in the retrieved California Mechanical Code — click a citation to read the verbatim passage:
CMC § 0.79 High relevance — show source text
Any angle greater than 45 degrees (0.79 rad) from the vertical is considered horizontal. The total horizontal distance of a vent plus the horizontal vent connector serving draft hood-equipped appliances shall not be greater than 75 percent of the vertical height of the vent. [NFPA 54:12.7.4.2] 802.6.2.3 Category II, Category III, and Cate- gory IV Appliances. The sizing of gas vents for Category II, Category III, and Category IV appliances shall be in accordance with the appliance manufacturers’ instructions. The sizing of plastic pipe specified by the appliance manufacturer as a venting material for Category II, III, and IV appliances shall be in accordance with the appliance manufacturers’ instructions. [NFPA 54:12.7.4.3] 802.6.2.4 Sizing. Chimney venting systems using mechanical draft shall be sized in accordance with engineering methods. [NFPA 54:12.7.4.4] 802.6.3 Gas Vents Serving Appliances on More than One Floor. Where a common vent is installed in a multistory installation to vent Category I appliances located on more than one floor level, the venting system shall be designed and installed in accordance with engineering methods.
Crawl spaces, basements, and attics shall be considered as floor levels. [NFPA 54:12.7.5.1] 802.6.3.1 Occupiable Space. All appliances connected to the common vent shall be located in rooms separated from occupiable space. Each of these rooms shall have provisions for an adequate supply of combustion, ventilation, and dilution air that is not supplied from occupiable space. [NFPA 54:12.7.5.2] (See Figure 802.6.3.1) 802.6.3.2 Multistory Venting System. The size of the connectors and common segments of multistory venting systems for appliances listed for use with a Type B double-wall gas vent shall be in accordance with Table 803.2(1), provided all of the following apply: (1) The available total height ( H ) for each segment of a multistory venting system is the vertical distance between the level of the highest draft hood outlet or flue collar on that floor and the centerline of the next highest interconnection tee. (2) The size of the connector for a segment is determined from the appliance’s gas input rate and available connector rise and shall not be smaller
than the draft hood outlet or flue collar size.
(3) The size of the common vertical vent segment, and of the interconnection tee at the base of that segment, is based on the total appliance’s gas input rate entering that segment and its available total height. [NFPA 54:12.7.5.3]
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CHIMNEYS AND VENTS
FIGURE 802.6.3.1
PLAN VIEW OF PRACTICAL SEPARATION METHOD
FOR MULTISTORY GAS VENTING
[NFPA 54: FIGURE A.12.7.5.2]
802.6.4 Support of Gas Vents. Gas vents shall be supported and spaced in accordance with the manufacturer’s installation instructions. [NFPA 54:12.7.6]
CMC § 803.2 High relevance — show source text
** The size of the connectors and common segments of multistory venting systems for appliances listed for use with a Type B double-wall gas vent shall be in accordance with Table 803.2(1), provided all of the following apply: (1) The available total height ( H ) for each segment of a multistory venting system is the vertical distance between the level of the highest draft hood outlet or flue collar on that floor and the centerline of the next highest interconnection tee. (2) The size of the connector for a segment is determined from the appliance’s gas input rate and available connector rise and shall not be smaller
than the draft hood outlet or flue collar size.
(3) The size of the common vertical vent segment, and of the interconnection tee at the base of that segment, is based on the total appliance’s gas input rate entering that segment and its available total height. [NFPA 54:12.7.5.3]
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CHIMNEYS AND VENTS
FIGURE 802.6.3.1
PLAN VIEW OF PRACTICAL SEPARATION METHOD
FOR MULTISTORY GAS VENTING
[NFPA 54: FIGURE A.12.7.5.2]
802.6.4 Support of Gas Vents. Gas vents shall be supported and spaced in accordance with the manufacturer’s installation instructions. [NFPA 54:12.7.6]
802.6.5 Marking. In those localities where solid and liquid fuels are used extensively, gas vents shall be permanently identified by a label attached to the wall or ceiling at a point where the vent connector enters the gas vent. The label shall read: “This gas vent is for appliances that burn gas. Do not connect to solid or liquid fuel-burning appliances or incinerators.” The Authority Having Jurisdiction shall determine whether its area constitutes such a locality. [NFPA 54:12.7.7] 802.7 Single-Wall Metal Pipe. Single-wall metal pipe shall be constructed of galvanized sheet steel not less than 0.0304 of an inch (0.7722 mm) thick or of other approved, noncombustible, corrosion-resistant material. [NFPA 54:12.8.1]
802.7.1 Cold Climate. Uninsulated single-wall metal pipe shall not be used outdoors for venting appliances in regions where the 99 percent winter design temperature is below 32°F (0°C). [NFPA 54:12.8.2]
802.7.2 Termination. The termination of single-wall metal pipe shall meet the following requirements:
(1) Single-wall metal pipe shall terminate at least 5 feet (1524 mm) in vertical height above the highest connected appliance draft hood outlet or flue collar.
(2) Single-wall metal pipe shall extend at least 2 feet (610 mm) above the highest point where it passes through a roof of a building and at least 2 feet (610 mm) higher than any portion of a building within a horizontal distance of 10 feet (3048 mm).
(3) An approved cap or roof assembly shall be attached to the terminus of a single-wall metal pipe. [NFPA 54:12.8.3]
CMC § 12.7.4.1 High relevance — show source text
(4) For sizing a gas vent connected to two appliances with draft hoods, the effective area of the vent shall be not less than the area of the larger draft hood outlet plus 50 percent of the area of the smaller draft hood outlet or greater than seven times the smaller draft hood outlet area.
(5) Engineering methods. [NFPA 54:12.7.4.1] 509.6.2.2 Vent Offsets. Type B and Type L vents sized in accordance with Section 509.6.2.1(3) or Section 509.6.2.1(4) shall extend in a generally vertical direction with offsets not exceeding 45 degrees (0.79 rad), except that a vent system having not more than one 60 degree (1.05 rad) offset shall be permitted. Any angle greater than 45 degrees (0.79 rad) from the vertical is considered horizontal. The total horizontal distance of a vent plus the horizontal vent connector serving draft hood-equipped appliances shall not be greater than 75 percent of the vertical height of the vent. [NFPA 54:12.7.4.2] 509.6.2.3 Category II, Category III, and Cate- gory IV Appliances. The sizing of gas vents for Category II, Category III, and Category IV appliances shall be in accordance with the appliance manufacturers’ instructions. The sizing of plastic pipe specified by the appliance manufacturer as a venting material for Category II, III, and IV appliances shall be in accordance with the appliance manufacturers’ instructions. [NFPA 54:12.7.4.3] 509.6.2.4 Sizing. Chimney venting systems using mechanical draft shall be sized in accordance with engineering methods. [NFPA 54:12.7.4.4] 509.6.3 Gas Vents Serving Appliances on More than One Floor. Where a common vent is installed in a multistory installation to vent Category I appliances located on more than one floor level, the venting system shall be designed and installed in accordance with engineering methods. Crawl spaces, basements, and attics shall be considered as floor levels. [NFPA 54:12.7.5.1]
509.6.3.1 Occupiable Space. All appliances connected to the common vent shall be located in rooms separated from occupiable space. Each of these rooms shall have provisions for an adequate supply of combustion, ventilation, and dilution air that is not supplied from occupiable space. [NFPA 54:12.7.5.2] (See Figure 509.6.3.1)
FIGURE 509.6.3.1
PLAN VIEW OF PRACTICAL SEPARATION
METHOD FOR MULTISTORY GAS VENTING
[NFPA 54: FIGURE A.12.7.5.2]
509.6.3.2 Multistory Venting System. The size of the connectors and common segments of multistory venting systems for appliances listed for use with a Type B double-wall gas vent shall be in accordance with Table 510.2(1), provided all of the following apply:
(1) The available total height ( H ) for each segment of a multistory venting system is the vertical distance between the level of the highest draft hood outlet or flue collar on that floor and the centerline of the next highest interconnection
tee.
CMC § 12.7.4.4 High relevance — show source text
[NFPA 54:12.7.4.4] 509.6.3 Gas Vents Serving Appliances on More than One Floor. Where a common vent is installed in a multistory installation to vent Category I appliances located on more than one floor level, the venting system shall be designed and installed in accordance with engineering methods. Crawl spaces, basements, and attics shall be considered as floor levels. [NFPA 54:12.7.5.1]
509.6.3.1 Occupiable Space. All appliances connected to the common vent shall be located in rooms separated from occupiable space. Each of these rooms shall have provisions for an adequate supply of combustion, ventilation, and dilution air that is not supplied from occupiable space. [NFPA 54:12.7.5.2] (See Figure 509.6.3.1)
FIGURE 509.6.3.1
PLAN VIEW OF PRACTICAL SEPARATION
METHOD FOR MULTISTORY GAS VENTING
[NFPA 54: FIGURE A.12.7.5.2]
509.6.3.2 Multistory Venting System. The size of the connectors and common segments of multistory venting systems for appliances listed for use with a Type B double-wall gas vent shall be in accordance with Table 510.2(1), provided all of the following apply:
(1) The available total height ( H ) for each segment of a multistory venting system is the vertical distance between the level of the highest draft hood outlet or flue collar on that floor and the centerline of the next highest interconnection
tee.
(2) The size of the connector for a segment is determined from the appliance’s gas input rate and available connector rise and shall not be smaller
than the draft hood outlet or flue collar size.
(3) The size of the common vertical vent segment, and of the interconnection tee at the base of that segment, is based on the total appliance’s gas input rate entering that segment and its available total height. [NFPA 54:12.7.5.3]
509.6.4 Support of Gas Vents. Gas vents shall be supported and spaced in accordance with the manufacturer’s installation instructions. [NFPA 54:12.7.6]
509.6.5 Marking. In those localities where solid and liquid fuels are used extensively, gas vents shall be per
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WATER HEATERS
manently identified by a label attached to the wall or ceiling at a point where the vent connector enters the gas vent. The label shall read: “This gas vent is for appliances that burn gas. Do not connect to solid or liquid fuel-burning appliances or incinerators.” The Authority Having Jurisdiction shall determine whether its area constitutes such a locality. [NFPA 54:12.7.7] 509.7 Single-Wall Metal Pipe. Single-wall metal pipe shall be constructed of galvanized sheet steel not less than 0.0304 of an inch (0.7722 mm) thick or of other approved, noncombustible, corrosion-resistant material. [NFPA 54:12.8.1]
CMC § 802.6.2 High relevance — show source text
installation instructions.
802.6.2 Size of Gas Vents. Venting systems shall be sized and constructed in accordance with Section 802.6.2.1 through Section 802.6.2.3 and the appliance manufacturer’s instructions. [NFPA 54:12.7.4] 802.6.2.1 Category I Appliances. The sizing of natural draft venting systems serving one or more listed appliances equipped with a draft hood or appliances listed for use with a Type B gas vent, installed in a single story of a building, shall be in accordance with one of the following: (1) The provisions of Section 803.0. (2) Vents serving fan-assisted combustion system appliances, or combinations of fan-assisted combustion system and draft hood-equipped appliances, shall be sized in accordance with Section 803.0 or other engineering methods. (3) For sizing an individual gas vent for a single, draft hood-equipped appliance, the effective area of the vent connector and the gas vent shall be not less than the area of the appliance draft hood outlet or greater than seven times the draft hood outlet area.
(4) For sizing a gas vent connected to two appliances with draft hoods, the effective area of the vent shall be not less than the area of the larger draft hood outlet plus 50 percent of the area of the smaller draft hood outlet or greater than seven times the smaller draft hood outlet area.
(5) Engineering methods. [NFPA 54:12.7.4.1]
802.6.2.2 Vent Offsets. Type B and Type L vents sized in accordance with Section 802.6.2.1(3) or Section 802.6.2.1(4) shall extend in a generally vertical direction with offsets not exceeding 45 degrees (0.79 rad) except that a vent system having not more than one 60 degree (1 rad) offset shall be permitted. Any angle greater than 45 degrees (0.79 rad) from the vertical is considered horizontal. The total horizontal distance of a vent plus the horizontal vent connector serving draft hood-equipped appliances shall not be greater than 75 percent of the vertical height of the vent. [NFPA 54:12.7.4.2] 802.6.2.3 Category II, Category III, and Cate- gory IV Appliances. The sizing of gas vents for Category II, Category III, and Category IV appliances shall be in accordance with the appliance manufacturers’ instructions. The sizing of plastic pipe specified by the appliance manufacturer as a venting material for Category II, III, and IV appliances shall be in accordance with the appliance manufacturers’ instructions. [NFPA 54:12.7.4.3] 802.6.2.4 Sizing. Chimney venting systems using mechanical draft shall be sized in accordance with engineering methods. [NFPA 54:12.7.4.4] 802.6.3 Gas Vents Serving Appliances on More than One Floor. Where a common vent is installed in a multistory installation to vent Category I appliances located on more than one floor level, the venting system shall be designed and installed in accordance with engineering methods.
Crawl spaces, basements, and attics shall be considered as floor levels. [NFPA 54:12.7.5.1] **802.6.3.1 Occupiable Space.
CMC § 510.2.8 High relevance — show source text
510.2.8 Tee and Wye Fittings. Tee and wye fittings connected to a common gas vent shall be considered as part of the common gas vent and constructed of materials consistent with that of the common gas vent. [NFPA 54:13.2.9] 510.2.9 Size of Fittings. At the point where tee or wye fittings connect to a common gas vent, the opening size
of the fitting shall be equal to the size of the common vent. Such fittings shall not be prohibited from having reduced size openings at the point of connection of appliance gas vent connectors. [NFPA 54:13.2.10] 510.2.10 High-Altitude Installations. Sea level input ratings shall be used when determining maximum capacity for high-altitude installation. Actual input (derated for altitude) shall be used for determining minimum capacity for high-altitude installation. [NFPA 54:13.2.11]
510.2.11 Vent Connector Rise. The vent connector rise ( R ) for each appliance shall be measured from the draft hood outlet or flue collar to the centerline where the vent gas streams come together. {NFPA 54:13.2.12} 510.2.12 Vent Height. The available total height ( H ) for multiple appliances on the same floor shall be measured from the highest draft hood outlet or flue collar up to the level of the outlet of the common vent. [NFPA 54:13.2.13]
510.2.13 Multistory Vent Height. Where appliances are located on more than one floor, the available total height ( H ) for each segment of the system shall be the vertical distance between the highest draft hood outlet or flue collar entering that segment and the centerline of the next higher interconnection tee. [NFPA 54:13.2.14] 510.2.14 Size of Vents for Multistory Installa- tions. The size of the lowest connector and of the vertical vent leading to the lowest interconnection of a multistory system shall be in accordance with Table 510.1.2(1) or Table 510.1.2(2) for available total height ( H ) up to the lowest interconnection. [NFPA 54:13.2.15] 510.2.15 Multistory Type B Vents Required. Where used in multistory systems, vertical common vents shall be Type B double wall and shall be installed with a listed vent cap. [NFPA 54:13.2.16] 510.2.16 Multistory Vent Offsets and Capacity. Offsets in multistory common vent systems shall be limited to a single offset in each system, and systems with an offset shall comply with all of the following:
(1) The offset angle shall not exceed 45 degrees (0.79 rad) from vertical.
(2) The horizontal length of the offset shall not exceed 18 inches per inch (18 mm/mm) of common vent diameter of the segment in which the offset is located.
(3) For the segment of the common vertical vent containing the offset, the common vent capacity listed in the common venting tables shall be reduced by 20 percent (0.80 x maximum common vent capacity).
(4) A multistory common vent shall not be reduced in size above the offset. [NFPA 54:13.2.17]
CMC § 803.0 High relevance — show source text
material and sized in accordance with Section 803.0 or engineering methods. [NFPA 54:12.11.3.5]
As an alternative method applicable only where there are two draft hood-equipped appliances, the effective area of the common vent connector or vent manifold and all junction fittings shall be not less than the area of the larger vent connector plus 50 percent of the area of the smaller flue collar outlet.
[NFPA 54:12.11.3.6]
802.10.2.4 Size Increase. Where the size of a
vent connector is increased to overcome installation limitations and obtain connector capacity equal to the appliance input, the size increase shall be made at the appliance draft hood outlet. [NFPA 54:12.11.3.7]
802.10.3 Two or More Appliances Connected to a Single Vent. Where two or more openings are provided into one chimney flue or vent, either of the following shall apply:
(1) The openings shall be at different levels.
(2) The connectors shall be attached to the vertical portion of the chimney or vent at an angle of 45 degrees or less relative to the vertical. [NFPA 54:12.11.4.1]
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CHIMNEYS AND VENTS
appliances, from the appliance outlet to the junction with the common vent or another connector, shall be 100 percent of the height of the chimney or vent.
[NFPA 54:12.11.9.2]
802.10.9 Support. A vent connector shall be supported for the design and weight of the material employed to maintain clearances and prevent physical damage and separation of joints. [NFPA 54:12.11.10]
802.10.10 Chimney Connection. Where entering a flue in a masonry or metal chimney, the vent connector shall be installed above the extreme bottom to avoid stop page. Where a thimble or slip joint is used to facilitate removal of the connector, the connector shall be firmly attached to or inserted into the thimble or slip joint to prevent the connector from falling out. Means shall be employed to prevent the connector from entering so far as to restrict the space between its end and the opposite wall of the chimney flue. [NFPA 54:12.11.11.1 – 12.11.11.3] 802.10.11 Inspection. The entire length of a vent connector shall be readily accessible for inspection, cleaning, and replacement. [NFPA 54:12.11.12]
802.10.12 Fireplaces. A vent connector shall not be connected to a chimney flue serving a fireplace unless the fireplace flue opening is permanently sealed. [NFPA 54:12.11.13]
802.10.13 Passage Through Ceilings, Floors, or Walls. A vent connector shall not pass through a ceiling, floor, or fire-resistance-rated wall. A single-wall metal pipe connector shall not pass through an interior wall.
Exceptions:
(1) Vent connectors made of listed Type B or Type L vent material and serving listed appliances with draft hoods and other appliances listed for use with Type B gas vents that pass through walls or partitions constructed of combustible material shall be installed
with not less than the listed clearance to combustible
CMC § 803.2.13 High relevance — show source text
803.2.13 Multistory Vent Height. Where appliances are located on more than one floor, the available total height ( H ) for each segment of the system shall be the vertical distance between the highest draft hood outlet or flue collar entering that segment and the centerline of the next higher interconnection tee. [NFPA 54:13.2.14] 803.2.14 Size of Vents for Multistory Installa- tions. The size of the lowest connector and of the vertical vent leading to the lowest interconnection of a multistory system shall be in accordance with Table 803.1.2(1) or Table 803.1.2(2) for available total height ( H ) up to the lowest interconnection. [NFPA 54:13.2.15]
»
x maximum vent connector capacity). With a connector length greater than 12 feet (3658 mm) but not exceeding 18 feet (5486 mm), the maximum capacity must be reduced by 20 percent (0.80 x maximum vent capacity).
(2) For a connector serving a fan-assisted appliance, the minimum capacity (FAN Min) of the connector shall be determined by referring to the corresponding single appliance table. For Type B double-wall connectors, Table 803.1.2(1) shall be used. For single-wall connectors, Table 803.1.2(2) shall be used. The height ( H ) and lateral ( L ) shall be measured according to the procedures for a single-appliance vent, as if the other appliances were not present. [NFPA 54:13.2.3]
803.2.3 Vent Connector Manifolds. Where the vent connectors are combined prior to entering the vertical portion of the common vent to form a common vent manifold, the size of the common vent manifold and the common vent shall be determined by applying a 10 percent reduction (0.90 x maximum common vent capacity) to the common vent capacity part of the common vent tables. The length of the common vent manifold ( L M ) shall not exceed 18 inches per inch (18 mm/mm) of common vent diameter ( D ). [NFPA 54:13.2.4] (See Figure 803.2.3)
FIGURE 803.2.3
USE OF MANIFOLDED COMMON VENT CONNECTOR
[NFPA 54: FIGURE F.1(k)]
803.2.4 Vent Offsets. Where the common vertical vent is offset, the maximum capacity of the common vent shall be reduced in accordance with Section 803.2.5, and the horizontal length of the common vent offset shall not exceed 18 inches per inch (18 mm/mm) of common vent diameter ( D ). Where multiple offsets occur in a common vent, the total horizontal length of all offsets combined shall not exceed 18 inches per inch (18 mm/mm) of the common vent diameter. [NFPA 54:13.2.5]
803.2.5 Elbows in Vents. For each elbow up to and including 45 degrees (0.79 rad) in the common vent, the
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CHIMNEYS AND VENTS
CMC § 803.2.7 High relevance — show source text
803.2.7 Common Vent Minimum Size. The crosssectional area of the common vent shall be equal to or greater than the cross-sectional area of the largest connector. [NFPA 54:13.2.8]
803.2.8 Tee and Wye Fittings. Tee and wye fittings connected to a common gas vent shall be considered as part of the common gas vent and constructed of materials consistent with that of the common gas vent. [NFPA 54:13.2.9]
803.2.9 Size of Fittings. At the point where tee or wye fittings connect to a common gas vent, the opening size of the fitting shall be equal to the size of the common vent. Such fittings shall not be prohibited from having reduced size openings at the point of connection of appliance gas vent connectors. [NFPA 54:13.2.10]
803.2.10 High-Altitude Installations. Sea level input ratings shall be used when determining maximum capacity for high-altitude installation. Actual input (derated for altitude) shall be used for determining minimum capacity for high-altitude installation. [NFPA 54:13.2.11]
803.2.11 Vent Connector Rise. The vent connector rise ( R ) for each appliance shall be measured from the draft hood outlet or flue collar to the centerline where the vent gas streams come together. {NFPA 54:13.2.12}
803.2.12 Vent Height. The available total height ( H ) for multiple appliances on the same floor shall be measured from the highest draft hood outlet or flue collar up to the level of the outlet of the common vent. [NFPA 54:13.2.13]
803.2.13 Multistory Vent Height. Where appliances are located on more than one floor, the available total height ( H ) for each segment of the system shall be the vertical distance between the highest draft hood outlet or flue collar entering that segment and the centerline of the next higher interconnection tee. [NFPA 54:13.2.14] 803.2.14 Size of Vents for Multistory Installa- tions. The size of the lowest connector and of the vertical vent leading to the lowest interconnection of a multistory system shall be in accordance with Table 803.1.2(1) or Table 803.1.2(2) for available total height ( H ) up to the lowest interconnection. [NFPA 54:13.2.15]
»
x maximum vent connector capacity). With a connector length greater than 12 feet (3658 mm) but not exceeding 18 feet (5486 mm), the maximum capacity must be reduced by 20 percent (0.80 x maximum vent capacity).
(2) For a connector serving a fan-assisted appliance, the minimum capacity (FAN Min) of the connector shall be determined by referring to the corresponding single appliance table. For Type B double-wall connectors, Table 803.1.2(1) shall be used. For single-wall connectors, Table 803.1.2(2) shall be used. The height ( H ) and lateral ( L ) shall be measured according to the procedures for a single-appliance vent, as if the other appliances were not present. [NFPA 54:13.2.3]
CMC § 0.80 High relevance — show source text
(3) For the segment of the common vertical vent containing the offset, the common vent capacity listed in the common venting tables shall be reduced by 20 percent (0.80 x maximum common vent capacity).
(4) A multistory common vent shall not be reduced in size above the offset. [NFPA 54:13.2.17]
803.2.17 Vertical Vent Size Limitation. Where two
or more appliances are connected to a vertical vent or chimney, the flow area of the largest section of vertical vent or chimney shall not exceed seven times the smallest listed appliance categorized vent areas, flue collar area, or draft hood outlet area unless designed in accordance with engineering methods. [NFPA 54:13.2.18]
803.2.18 Two-Stage/Modulating Appliances. The minimum vent connector capacity (FAN Min) of appliances with more than one input rate shall be determined from the tables and shall be less than the lowest appliance input rating. The maximum vent connector capacity (FAN Max or NAT Max) shall be determined from the tables shall be greater than the highest appliance input rating. [NFPA 54:13.2.19]
803.2.19 Corrugated Metallic Chimney Liner Reduction. Listed corrugated metallic chimney liner systems in masonry chimneys shall be sized by using Table 803.2(1) or Table 803.2(2) for Type B vents, with the maximum capacity reduced by 20 percent (0.80 x maximum capacity) and the minimum capacity as shown in Table 803.2(1) or Table 803.2(2). Corrugated metallic liner systems installed with bends or offsets shall have their maximum capacity further reduced in accordance with Section 803.2.4 and Section 803.2.5. The 20 percent reduction for corrugated metallic chimney liner systems includes an allowance for one long radius 90 degree (1.57 rad) turn at the bottom of the liner. [NFPA 54:13.2.20]
803.2.20 Chimneys and Vents. Table 803.2(1) through Table 803.2(5) shall be used only for chimneys and vents not exposed to the outdoors below the roof line. A Type B vent or listed chimney lining system passing through an unused masonry chimney flue shall not be considered to be exposed to the outdoors. A Type B vent passing through an unventilated enclosure or chase
insulated to a value of not less than R8 shall not be con sidered to be exposed to the outdoors. Where vents extend outdoors above the roof more than 5 feet (1524 mm) higher than required by Table 802.6.1, and where vents terminate in accordance with Section 802.6.1(1)(b), the outdoor portion of the vent shall be enclosed as required by this section for vents not considered to be exposed to the outdoors, or such venting system shall be engineered. Table 803.2(6) through Table 803.2(9) shall be used for clay tile lined exterior masonry chimneys, provided all the following conditions are met:
(1) The vent connector is Type B double wall.
(2) At least one appliance is draft hood equipped.
(3) The combined appliance input rating is less than the maximum capacity given by Table 803.2(6) (for NAT+NAT) or Table 803.2(8) (for FAN+NAT).
CMC § 25.4 High relevance — show source text
of
building|None unless otherwise specified by the appliance manufacturer|None unless otherwise specified by the appliance manufacturer| |G|Clearance to inside corner of
building|None unless otherwise specified by the appliance manufacturer|None unless otherwise specified by the appliance manufacturer| |H|Clearance to non-mechanical air
supply inlet to building and the
combustion air inlet to any other
appliance|Same clearance as specified for row B|Same clearance as specified for row B| |I|Clearance to a mechanical air
supply inlet|10 feet horizontally from inlet or 3 feet above inlet|10 feet horizontally from inlet or 3 feet above inlet| |J|Clearance above paved sidewalk
or paved driveway located on
public property or other areas
where condensate or vapor can
cause a nuisance or hazard|7 feet and not located above public walkways or other areas where
condensate or vapor can cause a nuisance or hazard|7 feet and not located above public walkways or other areas where
condensate or vapor can cause a nuisance or hazard| |K|Clearance to underside of
veranda, porch, deck, or balcony|12 inches where the area beneath the veranda, porch, deck, or balcony is
open on not less than two sides. The vent terminal is prohibited in this
location where only one side is open.|12 inches where the area beneath the veranda, porch, deck, or balcony is
open on not less than two sides. The vent terminal is prohibited in this
location where only one side is open.|For SI Units: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1000 British thermal units per hour = 0.293 kW
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FIGURE 802.8.1 « EXIT TERMINALS OF MECHANICAL DRAFT AND DIRECT VENT VENTING SYSTEMS
[NFPA 54: FIGURE 12.9.1]
802.10.2.1 Manifold. Where a single appliance having more than one draft hood outlet or flue collar is installed, the manifold shall be constructed according to the instructions of the appliance manufacturer. Where there are no instructions, the manifold shall be designed and constructed in accordance with engineering methods. As an alternative method, the effective area of the manifold shall equal the combined area of the flue collars or draft hood outlets, and the vent connectors shall have a minimum 1 foot (305 mm) rise. [NFPA 54:12.11.3.2]
802.10.2.2 Size. Where two or more appliances are connected to a common vent or chimney, each vent connector shall be sized in accordance with Section 803.0 or engineering methods. [NFPA 54:12.11.3.3]
As an alternative method applicable only where all of the appliances are draft hood-equipped, each vent connector shall have an effective area not less than the area of the draft hood outlet of the appliance to which it is connected. [NFPA 54:12.11.3.4] 802.10.2.3 Height. Where two or more appliances are vented through a common vent connector or vent manifold, the common vent connector or vent mani
fold shall be located at the highest level consistent with available headroom and clearance to combustible
CMC § 803.2.3 High relevance — show source text
FIGURE 803.2.3
USE OF MANIFOLDED COMMON VENT CONNECTOR
[NFPA 54: FIGURE F.1(k)]
803.2.4 Vent Offsets. Where the common vertical vent is offset, the maximum capacity of the common vent shall be reduced in accordance with Section 803.2.5, and the horizontal length of the common vent offset shall not exceed 18 inches per inch (18 mm/mm) of common vent diameter ( D ). Where multiple offsets occur in a common vent, the total horizontal length of all offsets combined shall not exceed 18 inches per inch (18 mm/mm) of the common vent diameter. [NFPA 54:13.2.5]
803.2.5 Elbows in Vents. For each elbow up to and including 45 degrees (0.79 rad) in the common vent, the
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CHIMNEYS AND VENTS
803.2.15 Multistory Type B Vents Required. Where used in multistory systems, vertical common vents shall be Type B double wall and shall be installed with a listed vent cap. [NFPA 54:13.2.16] 803.2.16 Multistory Vent Offsets and Capacity. Offsets in multistory common vent systems shall be limited to a single offset in each system, and systems with an offset shall comply with all of the following: (1) The offset angle shall not exceed 45 degrees (0.79 rad) from vertical. (2) The horizontal length of the offset shall not exceed 18 inches per inch (18 mm/mm) of common vent diameter of the segment in which the offset is located.
(3) For the segment of the common vertical vent containing the offset, the common vent capacity listed in the common venting tables shall be reduced by 20 percent (0.80 x maximum common vent capacity).
(4) A multistory common vent shall not be reduced in size above the offset. [NFPA 54:13.2.17]
803.2.17 Vertical Vent Size Limitation. Where two
or more appliances are connected to a vertical vent or chimney, the flow area of the largest section of vertical vent or chimney shall not exceed seven times the smallest listed appliance categorized vent areas, flue collar area, or draft hood outlet area unless designed in accordance with engineering methods. [NFPA 54:13.2.18]
803.2.18 Two-Stage/Modulating Appliances. The minimum vent connector capacity (FAN Min) of appliances with more than one input rate shall be determined from the tables and shall be less than the lowest appliance input rating. The maximum vent connector capacity (FAN Max or NAT Max) shall be determined from the tables shall be greater than the highest appliance input rating. [NFPA 54:13.2.19]
Frequently asked questions
Can I use the single‑story common vent tables for a two‑story installation?
No. The code requires multistory systems to be sized using the multistory procedure (engineering methods or Table 803.2(1) when the conditions in § 802.6.3.2 are met). § 802.6.3 and § 802.6.3.2.
Do attics or crawl spaces count as "floors" for this rule?
Yes — crawl spaces, basements, and attics shall be considered as floor levels for multistory venting. § 802.6.3.
My appliance room is adjacent to living space — is that allowed?
No. All appliances connected to the common vent shall be located in rooms separated from occupiable space, and each such room must have an adequate supply of combustion/ventilation/dilution air not drawn from occupiable space. § 802.6.3.1.
If Table 803.2(1) says a smaller vent is OK, can I reduce the connector that is smaller than the draft‑hood outlet?
No. The connector shall not be smaller than the draft hood outlet or flue collar size, regardless of table results. § 802.6.3.2(2).
What if I want an offset in the vertical common vent?
Offsets in multistory common vent systems are limited and may require capacity reductions — see the related multistory offset rules (e.g., § 803.2.16) for angle, horizontal length limits and required capacity reductions.
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