CMC · California Mechanical Code
What tables and limits govern vent connector lengths and vent diameters?
In plain terms: use Table 803.2.1 to check the maximum horizontal run allowed for each connector diameter and use the vent-sizing tables (for example Table 803.1.2(1) for Type B vents) to pick a diameter that meets appliance input. If your actual vent height or configuration isn’t covered by the tables (or you exceed the table length), you must reduce rated capacity per the code or get engineering calculations as required by § 803.2.26.
Last reviewed: July 6, 2026
What the code requires — 2-4 sentences
The California Mechanical Code requires you to size vent connectors and vents using the tabulated capacities and limits in the venting tables, and to obey the maximum vent connector horizontal lengths listed in Table 803.2.1 and the vent-sizing tables such as Table 803.1.2(1). Where a vent height or condition is not covered by the tables, engineering methods must be used (see § 803.2.26). The controlling tables and length limits are stated in § 803.2.1, the vent-sizing table references (including Table 803.1.2(1)), and the sizing-not-covered rule in § 803.2.26.
The most important rule: follow the tabulated vent capacities and the maximum connector lengths in Table 803.2.1; if your actual vent height or configuration is outside those table entries, you must use engineering calculations (§ 803.2.26).
Requirements in detail
Maximum connector length — what to use
The maximum permitted horizontal length for a vent connector is given in Table 803.2.1 and is equivalent to 18 inches per inch (18 mm/mm) of connector diameter (the table provides the specific maximum horizontal feet per diameter). See § 803.2.1 and the Table itself.
If a connector is longer than the maximum in Table 803.2.1, the connector capacity (the table Btu/h rating) must be reduced 10% for each additional multiple of the listed maximum length; reductions stack (10% per extra multiple). See § 803.2.2 for the adjustment method.
Which vent-sizing table to read
- For Type B double-wall connectors and vents, use the appropriate Type B vent tables (for example, Table 803.1.2(1) for many Type B vent situations). The tables have columns for FAN/FAN, FAN+NAT, NAT+NAT, FAN Max / NAT Max and FAN Min values; read the proper column for the appliance types being served. See § 803.2 and Table references.
When the tables cannot be used
- If the vent height is lower than 6 feet (1829 mm) or higher than the heights shown in the tables, or if the exact condition isn’t in the tables, engineering methods must be used to calculate capacities (this is the requirement in § 803.2.26).
Key sizing and upsizing limits
- Vent connectors shall not be upsized more than two table sizes above the appliance’s listed vent or flue-collar diameter; draft-hood-connected appliances cannot have a connector smaller than the draft hood outlet. See § 803.2.21 and related provisions.
Decision-relevant quick reference table
| Decision factor | Typical values / rule | Code Reference |
|---|---|---|
| Max horizontal length for 3" connector | 4½ ft | Table 803.2.1 / § 803.2.1 |
| Max horizontal length for 4" connector | 6 ft | Table 803.2.1 / § 803.2.1 |
| Max horizontal length for 6" connector | 9 ft | Table 803.2.1 / § 803.2.1 |
| General rule for max length per diameter | 18 in per in (18 mm per mm) | § 803.2.1 |
| Capacity reduction when connector > max | Reduce capacity 10% for each extra multiple | § 803.2.2 |
| When tables do not apply (vent height out of range) | Use engineering methods | § 803.2.26 |
| Which vent-sizing table for Type B double-wall | Table 803.1.2(1) (use FAN/NAT columns appropriately) | Table 803.1.2(1) / § 803.1.2(1) |
(Full connector-length table entries are published as Table 803.2.1 in the CMC; the table lists each diameter with its specific maximum horizontal feet.)
Elbows, offsets and manifold adjustments
The vent tables assume allowance for up to two 90° elbows in a connector; additional elbows require capacity reductions: 5% per extra ≤45° elbow and 10% per elbow >45° up to 90°, per § 803.2.6 and § 803.2.5.
Where connectors are manifolded before the vertical vent, apply a 10% reduction to the common-vent capacity and limit manifold length to 18 in per in of the common vent diameter. See § 803.2.3.
Exceptions & special cases
Draft-hood-equipped appliances: vent connectors must not be smaller than the draft hood outlet; also appliance minimum/maximum input limits in the tables apply differently for draft-hood vs fan-assisted appliances. See § 803.2.21 and the table columns (FAN/NAT).
Multiple connector/vent types: you may mix diameters and types within a run only if all sizes/types are permitted by the tables as though they were used for the entire length; otherwise use the larger required size. See § 803.2.22 and § 803.2.23.
Vents exposed outdoors or unusual installations: certain table series are only for vents not exposed to outdoors below the roof line; use the correct table series for the application (see § 803.2 note on Table applicability).
Height entries between table rows: either interpolate or use the lower FAN/NAT Max and higher FAN Min values per § 803.1.17 / § 803.2.14 (interpolation is permitted).
Common mistakes
Using table values without checking maximum connector horizontal length. The venting tables assume the connector is at or below the Table 803.2.1 maximum; if the connector is longer, the capacity must be reduced (10% per multiple) — failing to do this under-sizes the connector.
Ignoring elbow/offset reductions. The tables include allowance for two 90° elbows; installers often forget to reduce capacity for additional elbows or offsets, which can make a selected vent diameter undersized.
Upsizing beyond allowed limits. Upsizing more than two table sizes above the appliance’s listed vent diameter without checking the code rules or appliance instructions is not permitted.
Using the wrong table series (exposed outdoors vs not-exposed tables). This yields incorrect capacity numbers — always confirm the table series applies to your vent exposure and construction.
Worked example — concrete scenario
Scenario: A draft-hood water heater (35,000 Btu/h) uses a single-wall metal vent connector with a connector rise of 2 ft, the vertical common vent is 30 ft high, and the connector horizontal run from appliance to vent is 4 ft. Determine minimum connector diameter.
Steps:
Confirm the connector horizontal length (4 ft) is equal to or less than the Table 803.2.1 maximum for the candidate diameters (e.g., 4" connector max = 6 ft) — so table values may be used without reduction. § 803.2.1.
Use the Type B vent connector capacity table appropriate for a draft-hood appliance (see Table 803.2(2) / Table 803.1.2(1) guidance) and read the NAT Max column at H = 30 ft and R = 2 ft to find the smallest diameter whose NAT Max ≥ 35,000 Btu/h. The code commentary example shows a 3 in connector rated 37,000 Btu/h but § 803.2.21 prohibits a 3 in connector in some cases; the example thus selects 4 in as the safe choice because it equals the draft hood outlet diameter and meets table capacity. (See the CMC example and § references.)
Result: the installer uses a 4 in single-wall metal vent connector in that example, because the horizontal length is within Table 803.2.1 limits and the table capacity and appliance-specific restrictions are satisfied.
Related provisions
- § 803.2.1 — Vent connector maximum length; Table 803.2.1 lists per-diameter maxima.
- § 803.2.2 — How to adjust capacities when connector exceeds Table 803.2.1 length.
- § 803.2.3 — Manifolded connectors: length limits and 10% capacity reduction.
- § 803.2.6 / § 803.2.5 — Elbow and offset capacity reductions (connectors and common vents).
- § 803.2.21 — Vent connector sizing limits, draft-hood minimum, and upsizing rules.
- § 803.2.22 / § 803.2.23 — Mixing connector and vent sizes/types; permitted combinations per tables.
- Table 803.2.1 — The published table of connector diameters vs maximum horizontal length (see Table 803.2.1).
- Table 803.1.2(1) — Vent-sizing tables for Type B double-wall vent scenarios (use the correct column for FAN/NAT combinations).
- § 803.2.26 — Sizing vents not covered by tables: use engineering methods.
Code references
Grounded in the retrieved California Mechanical Code — click a citation to read the verbatim passage:
CMC § 803.2 High relevance — show source text
Solution:
Table 803.2(2) should be used to size single-wall metal vent connectors attached to Type B vertical vents. In the vent connector capacity portion of Table 803.2(2), find the row associated with a 30 foot (9144 mm) vent height. For a 2 foot (610 mm) rise on the vent connector for the water heater, read the shaded columns for draft hood-equipped appliances to find that a 3 inch (76 mm) diameter vent connector has a capacity of 37 000 Btu/h (10.8 kW). Therefore, a 3 inch (76 mm) single-wall metal vent connector can be used with the water heater. For a draft hood-equipped furnace with a 3 foot (914 mm) rise, read across the appropriate row to find that a 5 inch (127 mm) diameter vent connector has a maximum capacity of 120 000 Btu/h (35 kW) (which is too small for the furnace), and a 6 inch (152 mm) diameter vent connector has a maximum vent capacity of 172 000 Btu/h (50 kW). Therefore, a 6 inch (152 mm) diameter vent connector should be used with the 150 000 Btu/h (44 kW) furnace. Because both vent connector horizontal lengths are less than the maximum lengths listed in Section 803.2.1, the table values can be used without adjustments.
In the common vent capacity portion of Table 803.2(2), find the row associated with a 30 foot (9144 mm) vent height and read over to the NAT + NAT portion of the 6 inch (152 mm) diameter column to find a maximum combined capacity of 257 000 Btu/h (75 kW). Since the two appliances total only 185 000 Btu/h (54 kW), a 6 inch (152 mm) common vent can be used. [NFPA 54:F.2.1]
F 102.2 Example 5(a): Common Venting a Draft Hood-Equipped Water Heater with a Fan-Assisted Furnace into a Type B Vent. In this case, a 35 000 Btu/h (10.3 kW) input draft hood-equipped water heater with a 4 inch (102 mm) diameter draft hood outlet, 2 feet (610 mm) of connector rise, and 4 feet (1219 mm) of horizontal length is to be common vented with a 100 000 Btu/h (29 kW) fanassisted furnace with a 4 inch (102 mm) diameter flue collar, 3 feet (914 mm) of connector rise, and 6 feet (1829 mm) of horizontal length. The common vent consists of a 30 foot (9144 mm) height of Type B vent. What are the recommended vent diameters for each connector and the common vent? The installer would like to use a single-wall metal vent connector. (See Figure F 102.2)
Solution:
Water Heater Vent Connector Diameter. Since the water heater vent connector horizontal length of 4 feet (1219 mm) is less than the maximum value listed in Table 803.2(2), the venting table values can be used without adjustments.
CMC § 510.2.1 High relevance — show source text
Therefore, a 3 inch (76 mm) single-wall metal vent connector can be used with the water heater. For a draft hood-equipped furnace with a 3 foot (914 mm) rise, read across the appropriate row to find that a 5 inch (127 mm) diameter vent connector has a maximum capacity of 120 000 Btu/h (35 kW) (which is too small for the furnace) and a 6 inch (152 mm) diameter vent connector has a maximum vent capacity of 172 000 Btu/h (50 kW). Therefore, a 6 inch (152 mm) diameter vent connector should be used with the 150 000 Btu/h (44 kW) furnace. Because both vent connector, horizontal lengths are less than the maximum lengths listed in Section 510.2.1; the table values can be used without adjustments.
In the common vent capacity portion of Table 510.2(2), find the row associated with a 30 foot (9144 mm) vent height and read over to the NAT + NAT portion of the 6 inch (152 mm) diameter column to find a maximum combined capacity of 257 000 Btu/h (75 kW). Since the two appliances total only 185 000 Btu/h (54 kW), a 6 inch (152 mm) common vent can be used. [NFPA 54:F.2.1]
For SI units: 1 foot = 304.8 mm, 1000 British thermal units per hour = 0.293 kW
FIGURE G 101.6
COMMON VENTING TWO DRAFT HOOD-EQUIPPED
APPLIANCES – EXAMPLE 4
[NFPA 54: FIGURE F.2.1]
G 101.7 Example 5(a): Common Venting a Draft Hood-Equipped Water Heater with a Fan-Assisted Furnace into a Type B Vent. In this case, a 35 000 Btu/h (10.3 kW) input draft hood-equipped water heater with a 4 inch (102 mm) diameter draft hood outlet, 2 feet (610 mm) of connector rise, and 4 feet (1219 mm) of horizontal length is to be common vented with a 100 000 Btu/h (29 kW) fanassisted furnace with a 4 inch (102 mm) diameter flue collar, 3 feet (914 mm) of connector rise, and 6 feet (1829 mm) of horizontal length. The common vent consists of a 30 foot (9144 mm) height of Type B vent. What are the recommended vent diameters for each connector and the common vent? The installer would like to use a single-wall metal vent connector. (See Figure G 101.7)
Solution:
Water Heater Vent Connector Diameter. Since the water heater vent connector, horizontal length of 4 feet (1219 mm) is less than the maximum value listed in Table 510.2(2), the venting table values can be used without adjustments. Using the Vent Connector Capacity portion of Table 510.2(2), read down the Total Vent Height ( H ) column to 30 feet (9144 mm) and read across the 2 feet (610 mm) Connector Rise ( R ) row to the first Btu/h rating in the NAT Max column that is equal to or greater than the water heater input rating. The table shows that a 3 inch (76 mm) vent connector has a maximum input rating of 37 000 Btu/h (10.8 kW).
CMC § 803.2 High relevance — show source text
(4) The input rating of each space-heating appliance is greater than the minimum input rating given by Table 803.2(7) (for NAT+NAT) or Table 803.2(9) (for FAN+NAT).
(5) The vent connector sizing is in accordance with Table 803.2(3). [NFPA 54:13.2.22] 803.2.21 Vent Connector Sizing. Vent connectors shall not be increased more than two sizes greater than the listed appliance categorized vent diameter, flue collar diameter, or draft hood outlet diameter. Vent connectors for draft hood-equipped appliances shall not be smaller than the draft hood outlet diameter. Where a vent connector size(s) determined from the tables for a fanassisted appliance(s) is smaller than the flue collar diameter, the use of the smaller size(s) shall be permitted, provided that the installation complies with all of the following conditions:
(1) Vent connectors for fan-assisted appliance flue collars 12 inches (300 mm) in diameter or smaller are not reduced by more than one table size [e.g., 12 inches to 10 inches (300 mm to 250 mm) is a onesize reduction] and those larger than 12 inches (300 mm) in diameter are not reduced more than two table sizes [e.g., 24 inches to 20 inches (600 mm to 500 mm) is a two-size reduction].
(2) The fan-assisted appliance(s) is common vented with a draft hood-equipped appliance(s).
(3) The vent connector has a smooth interior wall.
[NFPA 54:13.2.24]
803.2.22 Multiple Vent and Connector Sizes. All combinations of pipe sizes, single-wall metal pipe, and double-wall metal pipe shall be allowed within any connector run(s) or within the common vent, provided ALL of the appropriate tables permit ALL of the desired sizes and types of pipe, as if they were used for the entire length of the subject connector or vent. Where singlewall and Type B double-wall metal pipes are used for vent connectors within the same venting system, the common vent shall be sized using Table 803.2(2) or Table 803.2(4) as appropriate. [NFPA 54:13.2.25]
2025 CALIFORNIA MECHANICAL CODE 169
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CHIMNEYS AND VENTS
803.2.23 Multiple Connector and Vent Sizes. Where Table 803.1.2(1) through Table 803.2(9) permits more than one diameter of pipe to be used for a connector or vent, all the permitted sizes shall be permitted to be used. [NFPA 54:13.2.26]
803.2.24 Interpolation. Interpolation shall be permitted in calculating capacities for vent dimensions that fall between table entries. [NFPA 54:13.2.27]
803.2.25 Extrapolation. Extrapolation beyond the table entries shall not be permitted. [NFPA 54:13.2.28]
803.2.26 Sizing Vents Not Covered by Tables. For vent heights lower than 6 feet (1829 mm) and higher than shown in the tables, engineering methods shall be used to calculate vent capacities. [NFPA 54:13.2.29]
CMC § 13.2.2 High relevance — show source text
[NFPA 54: TABLE 13.2.2]
CONNECTOR DIAMETER
(inches)MAXIMUM CONNECTOR
HORIZONTAL LENGTH
(feet)
341⁄2
46
571⁄2
69 7 101⁄2
812
9131⁄2
1015
1218
1421 16 24
1827
2030
2233
2436 For SI units: 1 inch = 25.4 mm, 1 foot = 304.8 mm
803.2.2 Vent Connector Exceeding Maximum Length. The vent connector shall be routed to the vent utilizing the shortest possible route. Connectors with longer horizontal lengths than those listed in Table 803.2.1 are permitted under the following conditions:
(1) The maximum capacity (FAN Max or NAT Max) of the vent connector shall be reduced 10 percent for each additional multiple of the length listed in Table 803.2.1. For example, the maximum length listed for a 4 inch (100 mm) connector is 6 feet (1829 mm). With a connector length greater than 6 feet (1829 mm) but not exceeding 12 feet (3658 mm), the maximum capacity must be reduced by 10 percent (0.90
TABLE 803.2.1
VENT CONNECTOR MAXIMUM LENGTH
[NFPA 54: TABLE 13.2.2]
For SI units: 1 inch = 25.4 mm, 1 foot = 304.8 mm
2025 CALIFORNIA MECHANICAL CODE 167
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CHIMNEYS AND VENTS
maximum common vent capacity listed in the venting tables shall be reduced by 5 percent. For each elbow greater than 45 degrees (0.79 rad) up to and including 90 degrees (1.57 rad), the maximum common vent capacity listed in the venting tables shall be reduced by 10 percent. [NFPA 54:13.2.6]
803.2.6 Elbows in Connectors. The vent connector capacities listed in the common vent sizing tables include allowance for two 90 degree elbows. For each additional elbow up to and including 45 degrees, the maximum vent connector capacity listed in the venting tables shall be reduced by 5 percent. For each elbow greater than 45 degrees up to and including 90 degrees, the maximum vent connector capacity listed in the venting tables shall be reduced by 10 percent. [NFPA 54:13.2.7]
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]
CMC § 510.2 High relevance — show source text
Solution:
Water Heater Vent Connector Diameter. Since the water heater vent connector, horizontal length of 4 feet (1219 mm) is less than the maximum value listed in Table 510.2(2), the venting table values can be used without adjustments. Using the Vent Connector Capacity portion of Table 510.2(2), read down the Total Vent Height ( H ) column to 30 feet (9144 mm) and read across the 2 feet (610 mm) Connector Rise ( R ) row to the first Btu/h rating in the NAT Max column that is equal to or greater than the water heater input rating. The table shows that a 3 inch (76 mm) vent connector has a maximum input rating of 37 000 Btu/h (10.8 kW). Although this rating is greater than the water heater input rating, a 3 inch (76 mm) vent connector is prohibited by Section 510.2.18. A 4 inch (102 mm) vent connector has a maximum input rating of 67 000 Btu/h (19.6 kW) and is equal to the draft hood outlet diameter. A 4 inch (102 mm) vent connector is selected. Since the water heater is equipped with a draft hood, there are no minimum input rating restrictions.
For SI units: 1 foot = 304.8 mm, 1000 British thermal units per hour = 0.293 kW
FIGURE G 101.7
COMMON VENTING A DRAFT HOOD-EQUIPPED WATER
HEATER WITH A FAN-ASSISTED FURNACE INTO A TYPE B DOUBLE-WALL COMMON VENT – EXAMPLE 5(a)
[NFPA 54: FIGURE F.2.2]
430 2025 CALIFORNIA PLUMBING CODE
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APPENDIX G
Furnace Vent Connector Diameter. Using the Vent Connector Capacity portion of Table 510.2(2), read down the Total Vent Height ( H ) column to 30 feet (9144 mm) and across the 3 feet (914 mm) Connector Rise ( R ) row. Because the furnace has a fan-assisted combustion system, find the first FAN Max column with a Btu/h rating greater than the furnace input rating. The 4 inch (102 mm) vent connector has a maximum input rating of 119 000 Btu/h (34.9 kW) and a minimum input rating of 85 000 Btu/h (24.9 kW).
The 100 000 Btu/h (29 kW) furnace in this example falls within this range, so a 4 inch (102 mm) connector is adequate. Because the furnace vent connector horizontal length of 6 feet (1829 mm) is less than the maximum value listed in Section 510.2.1; the venting table values can be used without adjustment. If the furnace had an input rating of 80 000 Btu/h (23.4 kW), a Type B vent connector would be needed in order to meet the minimum capacity limit.
CMC § 102.2 High relevance — show source text
F 102.2 Example 5(a): Common Venting a Draft Hood-Equipped Water Heater with a Fan-Assisted Furnace into a Type B Vent. In this case, a 35 000 Btu/h (10.3 kW) input draft hood-equipped water heater with a 4 inch (102 mm) diameter draft hood outlet, 2 feet (610 mm) of connector rise, and 4 feet (1219 mm) of horizontal length is to be common vented with a 100 000 Btu/h (29 kW) fanassisted furnace with a 4 inch (102 mm) diameter flue collar, 3 feet (914 mm) of connector rise, and 6 feet (1829 mm) of horizontal length. The common vent consists of a 30 foot (9144 mm) height of Type B vent. What are the recommended vent diameters for each connector and the common vent? The installer would like to use a single-wall metal vent connector. (See Figure F 102.2)
Solution:
Water Heater Vent Connector Diameter. Since the water heater vent connector horizontal length of 4 feet (1219 mm) is less than the maximum value listed in Table 803.2(2), the venting table values can be used without adjustments. Using the Vent Connector Capacity portion of Table 803.2(2), read down the Total Vent Height ( H ) column to 30 feet (9144 mm) and read across the 2 feet (610 mm) Connector Rise ( R ) row to the first Btu/h rating in the NAT Max column that is equal to or greater than the water heater input rating. The table shows that a 3 inch (76 mm) vent connector has a maximum input rating of 37 000 Btu/h (10.8 kW). Although this rating is greater than the water heater input rating, a 3 inch (76 mm) vent connector is prohibited by Section 803.2.21. A 4 inch (102 mm) vent connector has a maximum input rating of 67 000 Btu/h (19.6 kW) and is equal to the draft hood outlet diameter. A 4 inch (102 mm) vent connector is selected. Since the water heater is equipped with a draft hood, there are no minimum input rating restrictions.
Furnace Vent Connector Diameter. Using the Vent Connector Capacity portion of Table 803.2(2), read down the Total Vent Height ( H ) column to 30 feet (9144 mm) and across the 3 feet (914 mm) Connector Rise ( R ) row. Because the furnace has a fan-assisted combustion system, find the first FAN Max column with a Btu/h rating greater than the furnace input rating. The 4 inch (102 mm) vent connector has a maximum input rating of 119 000 Btu/h (34.9 kW) and a minimum input rating of 85 000 Btu/h (24.9 kW).
The 100 000 Btu/h (29 kW) furnace in this example falls within this range, so a 4 inch (102 mm) connector is adequate. Because the furnace vent connector horizontal length of 6 feet (1829 mm) is less than the maximum value listed in Section 803.2.1, the venting table values can be used without adjustment. If the furnace had an input rating of 80 000 Btu/h (23.4 kW), a Type B vent connector would be needed in order to meet the minimum capacity limit.
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CMC § 803.2 High relevance — show source text
803.2 Additional Requirements to Multiple-Appliance Vent. Venting Table 803.2(1) through Table 803.2(9) shall not be used where obstructions are installed in the venting system. The installation of vents serving listed appliances with vent dampers shall be in accordance with the appliance manufacturer’s instructions, or in accordance with the following:
(1) The maximum capacity of the vent connector shall be determined using the NAT Max column.
(2) The maximum capacity of the vertical vent or chimney shall be determined using the FAN + NAT column when the second appliance is a fan-assisted appliance, or the NAT + NAT column when the second appliance is equipped with a draft hood.
(3) The minimum capacity shall be determined as if the appliance were a fan-assisted appliance, as follows:
(a) The minimum capacity of the vent connector shall be determined using the FAN Min column.
(b) The FAN + FAN column shall be used when the second appliance is a fan-assisted appliance, and the FAN + NAT column shall be used when the second appliance is equipped with a draft hood, to determine whether the vertical vent or chimney configuration is not permitted (NA). Where the vent configuration is NA, the vent configuration shall not be permitted and an alternative venting configuration shall be utilized. [NFPA 54:13.2.1]
803.2.1 Vent Connector Maximum Length. The maximum vent connector horizontal length shall be 18 inches per inch (18 mm/mm) of connector diameter as shown in Table 803.2.1, or as permitted by Section 803.2.2. [NFPA 54:13.2.2]
TABLE 803.2.1
VENT CONNECTOR MAXIMUM LENGTH
[NFPA 54: TABLE 13.2.2]
CONNECTOR DIAMETER
(inches)MAXIMUM CONNECTOR
HORIZONTAL LENGTH
(feet)
341⁄2
46
571⁄2
69 7 101⁄2
812
9131⁄2
1015
1218
1421 16 24
1827
2030
2233
2436 For SI units: 1 inch = 25.4 mm, 1 foot = 304.8 mm
803.2.2 Vent Connector Exceeding Maximum Length. The vent connector shall be routed to the vent utilizing the shortest possible route. Connectors with longer horizontal lengths than those listed in Table 803.2.1 are permitted under the following conditions:
(1) The maximum capacity (FAN Max or NAT Max) of the vent connector shall be reduced 10 percent for each additional multiple of the length listed in Table 803.2.1. For example, the maximum length listed for a 4 inch (100 mm) connector is 6 feet (1829 mm). With a connector length greater than 6 feet (1829 mm) but not exceeding 12 feet (3658 mm), the maximum capacity must be reduced by 10 percent (0.90
TABLE 803.2.1
VENT CONNECTOR MAXIMUM LENGTH
CMC § 803.2.21. High relevance — show source text
Although this rating is greater than the water heater input rating, a 3 inch (76 mm) vent connector is prohibited by Section 803.2.21. A 4 inch (102 mm) vent connector has a maximum input rating of 67 000 Btu/h (19.6 kW) and is equal to the draft hood outlet diameter. A 4 inch (102 mm) vent connector is selected. Since the water heater is equipped with a draft hood, there are no minimum input rating restrictions.
Furnace Vent Connector Diameter. Using the Vent Connector Capacity portion of Table 803.2(2), read down the Total Vent Height ( H ) column to 30 feet (9144 mm) and across the 3 feet (914 mm) Connector Rise ( R ) row. Because the furnace has a fan-assisted combustion system, find the first FAN Max column with a Btu/h rating greater than the furnace input rating. The 4 inch (102 mm) vent connector has a maximum input rating of 119 000 Btu/h (34.9 kW) and a minimum input rating of 85 000 Btu/h (24.9 kW).
The 100 000 Btu/h (29 kW) furnace in this example falls within this range, so a 4 inch (102 mm) connector is adequate. Because the furnace vent connector horizontal length of 6 feet (1829 mm) is less than the maximum value listed in Section 803.2.1, the venting table values can be used without adjustment. If the furnace had an input rating of 80 000 Btu/h (23.4 kW), a Type B vent connector would be needed in order to meet the minimum capacity limit.
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» FIGURE F 102.1 COMMON VENTING TWO DRAFT HOOD-EQUIPPED APPLIANCES – EXAMPLE 4
[NFPA 54: FIGURE F.2.1]
540 2025 CALIFORNIA MECHANICAL CODE
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APPENDIX F
»
»
Common Vent Diameter. The total input to the common vent is 135 000 Btu/h (40 kW). Using the Common Vent Capacity portion of Table 803.2(2), read down the Total Vent Height ( H ) column to 30 feet (9144 mm) and across this row to find the smallest vent diameter in the FAN + NAT column that has a Btu/h rating equal to or greater than 135 000 Btu/h (40 kW). The 4 inch (102 mm) common vent has a capacity of 132 000 Btu/h (39 kW) and the 5 inch (127 mm) common vent has a capacity of 202 000 Btu/h (59 kW). Therefore, the 5 inch (127 mm) common vent should be used in this example.
Summary: In this example, the installer can use a 4 inch (102 mm) diameter, single-wall metal vent connector for the water heater and a 4 inch (102 mm) diameter, single-wall metal vent connector for the furnace. The common vent should be a 5 inch (127 mm) diameter Type B vent. [NFPA 54:F.2.2]
For SI units: 1 foot = 304.8 mm, 1000 British thermal units per hour = 0.293 kW
CMC § 510.2 High relevance — show source text
(5) The vent connector sizing is in accordance with Table 510.2(3). [NFPA 54:13.2.22] 510.2.21 Vent Connector Sizing. Vent connectors shall not be increased more than two sizes greater than the listed appliance categorized vent diameter, flue collar diameter, or draft hood outlet diameter. Vent connectors for draft hood-equipped appliances shall not be smaller than the draft hood outlet diameter. Where a vent connector size(s) determined from the tables for a fan
assisted appliance(s) is smaller than the flue collar diameter, the use of the smaller size(s) shall be permitted, provided that the installation complies with all of the following conditions:
(1) Vent connectors for fan-assisted appliance flue collars 12 inches (300 mm) in diameter or smaller are not reduced by more than one table size [e.g., 12 inches to 10 inches (300 mm to 250 mm) is a onesize reduction], and those larger than 12 inches (300 mm) in diameter are not reduced more than two table sizes [e.g., 24 inches to 20 inches (600 mm to 500 mm) is a two-size reduction].
(2) The fan-assisted appliance(s) is common vented with a draft hood-equipped appliance(s).
(3) The vent connector has a smooth interior wall.
[NFPA 54:13.2.24] 510.2.22 Combination of Pipe Types and Sizes. All combinations of pipe sizes, single-wall metal pipe, and double-wall metal pipe shall be allowed within any connector run(s) or within the common vent, provided ALL of the appropriate tables permit ALL of the desired sizes and types of pipe, as if they were used for the entire length of the subject connector or vent. Where singlewall and Type B double-wall metal pipes are used for vent connectors within the same venting system, the common vent shall be sized using Table 510.2(2) or Table 510.2(4) as appropriate. [NFPA 54:13.2.25] 510.2.23 Multiple Connector and Vent Sizes. Where Table 510.2(1) through Table 510.2(9) permits more than one diameter of pipe to be used for a connector or vent, all the permitted sizes shall be permitted to be used. [NFPA 54:13.2.26] 510.2.24 Interpolation. Interpolation shall be permitted in calculating capacities for vent dimensions that fall between table entries. [NFPA 54:13.2.27] 510.2.25 Extrapolation. Extrapolation beyond the table entries shall not be permitted. [NFPA 54:13.2.28] 510.2.26 Engineering Methods. For vent heights lower than 6 feet (1829 mm) and higher than shown in the tables, engineering methods shall be used to calculate vent capacities. [NFPA 54:13.2.29] 510.2.27 Height Entries. Where the actual height of a vent falls between entries in the height column of the applicable table in Table 510.2(1) through Table 510.2(9), either of the following shall be used:
(1) Interpolation.
(2) The lower appliance input rating shown in the table entries, for FAN Max and NAT Max column values; and the higher appliance input rating for the FAN Min column values. [NFPA 54:13.2.30]
CMC § 510.2.1 High relevance — show source text
TABLE 510.2.1
VENT CONNECTOR MAXIMUM LENGTH
[NFPA 54: TABLE 13.2.2]
CONNECTOR DIAMETER
(inches)MAXIMUM CONNECTOR
HORIZONTAL LENGTH
(feet)
341⁄2
46
571⁄2 6 9
7101⁄2
812
9131⁄2
1015
1218
1421 16 24
1827
2030
2233
2436 For SI units: 1 inch = 25.4 mm, 1 foot = 304.8 mm
510.2.2 Vent Connector Exceeding Maximum Length. The vent connector shall be routed to the vent utilizing the shortest possible route. Connectors with longer horizontal lengths than those listed in Table 510.2.1 are permitted under the following conditions:
(1) The maximum capacity (FAN Max or NAT Max) of the vent connector shall be reduced 10 percent for each additional multiple of the length listed in Table 510.2.1. For example, the maximum length listed for a 4 inch (100 mm) connector is 6 feet (1829 mm). With a connector length greater than 6 feet (1829 mm) but not exceeding 12 feet (3658 mm), the maximum capacity must be reduced by 10 percent (0.90 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 shall be reduced by 20 percent (0.80 x maximum vent capacity).
TABLE 510.2.1
VENT CONNECTOR MAXIMUM LENGTH
[NFPA 54: TABLE 13.2.2]
For SI units: 1 inch = 25.4 mm, 1 foot = 304.8 mm
106 2025 CALIFORNIA PLUMBING CODE
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WATER HEATERS
(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 510.1.2(1) shall be used. For single-wall connectors, Table 510.1.2(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 § 803.2.20 High relevance — show source text
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).
(4) The input rating of each space-heating appliance is greater than the minimum input rating given by Table 803.2(7) (for NAT+NAT) or Table 803.2(9) (for FAN+NAT).
(5) The vent connector sizing is in accordance with Table 803.2(3). [NFPA 54:13.2.22] 803.2.21 Vent Connector Sizing. Vent connectors shall not be increased more than two sizes greater than the listed appliance categorized vent diameter, flue collar diameter, or draft hood outlet diameter. Vent connectors for draft hood-equipped appliances shall not be smaller than the draft hood outlet diameter. Where a vent connector size(s) determined from the tables for a fanassisted appliance(s) is smaller than the flue collar diameter, the use of the smaller size(s) shall be permitted, provided that the installation complies with all of the following conditions:
(1) Vent connectors for fan-assisted appliance flue collars 12 inches (300 mm) in diameter or smaller are not reduced by more than one table size [e.g., 12 inches to 10 inches (300 mm to 250 mm) is a onesize reduction] and those larger than 12 inches (300 mm) in diameter are not reduced more than two table sizes [e.g., 24 inches to 20 inches (600 mm to 500 mm) is a two-size reduction].
(2) The fan-assisted appliance(s) is common vented with a draft hood-equipped appliance(s).
(3) The vent connector has a smooth interior wall.
[NFPA 54:13.2.24]
California Mechanical Code High relevance — show source text
CONNECTOR CAPACITY**|VENT CONNECTOR CAPACITY|VENT CONNECTOR CAPACITY|VENT CONNECTOR CAPACITY| |||**SINGLE-WALL METAL VENT CONNECTOR DIAMETER– ****D (inch)**|**SINGLE-WALL METAL VENT CONNECTOR DIAMETER– ****D (inch)**|**SINGLE-WALL METAL VENT CONNECTOR DIAMETER– ****D (inch)**|**SINGLE-WALL METAL VENT CONNECTOR DIAMETER– ****D (inch)**|**SINGLE-WALL METAL VENT CONNECTOR DIAMETER– ****D (inch)**|**SINGLE-WALL METAL VENT CONNECTOR DIAMETER– ****D (inch)**|**SINGLE-WALL METAL VENT CONNECTOR DIAMETER– ****D (inch)**|**SINGLE-WALL METAL VENT CONNECTOR DIAMETER– ****D (inch)**|**SINGLE-WALL METAL VENT CONNECTOR DIAMETER– ****D (inch)**|**SINGLE-WALL METAL VENT CONNECTOR DIAMETER– ****D (inch)**|**SINGLE-WALL METAL VENT CONNECTOR DIAMETER– ****D (inch)**|**SINGLE-WALL METAL VENT CONNECTOR DIAMETER– ****D (inch)**|**SINGLE-WALL METAL VENT CONNECTOR DIAMETER– ****D (inch)**|**SINGLE-WALL METAL VENT CONNECTOR DIAMETER– ****D (inch)**|**SINGLE-WALL METAL VENT CONNECTOR DIAMETER– ****D (inch)**|**SINGLE-WALL METAL VENT CONNECTOR DIAMETER– ****D (inch)**| |||3|3|3|3|4|4|4|5|5|5|6|6|6|7|7|7| |||APPLIANCE INPUT RATING LIMITS IN THOUSANDS OF BTU PER HOUR|APPLIANCE INPUT RATING LIMITS IN THOUSANDS OF BTU PER HOUR|APPLIANCE INPUT RATING LIMITS IN THOUSANDS OF BTU PER HOUR|APPLIANCE INPUT RATING LIMITS IN THOUSANDS OF BTU PER HOUR|APPLIANCE INPUT RATING LIMITS IN THOUSANDS OF BTU PER HOUR|APPLIANCE INPUT RATING LIMITS IN THOUSANDS OF BTU PER HOUR|APPLIANCE INPUT RATING LIMITS IN THOUSANDS OF BTU PER HOUR|APPLIANCE INPUT RATING LIMITS IN THOUSANDS OF BTU PER HOUR|APPLIANCE INPUT RATING LIMITS IN THOUSANDS OF BTU PER HOUR|APPLIANCE INPUT RATING LIMITS IN THOUSANDS OF BTU PER HOUR|APPLIANCE INPUT RATING LIMITS IN THOUSANDS OF BTU PER HOUR|APPLIANCE INPUT RATING LIMITS IN THOUSANDS OF BTU PER HOUR|APPLIANCE INPUT RATING LIMITS IN THOUSANDS OF BTU PER HOUR|APPLIANCE INPUT RATING LIMITS IN THOUSANDS OF BTU PER HOUR|**APPLIANCE INPUT RATING LIMITS IN THOUSANDS OF BTU PER
Frequently asked questions
What is the single most important table to check for connector horizontal length?
Check Table 803.2.1 and the text of § 803.2.1; that table gives the per-diameter maximum horizontal lengths.
If my connector is longer than the table maximum, do I need to enlarge the diameter?
Not necessarily — you must reduce the connector capacity by 10% for each additional multiple of the Table 803.2.1 maximum length (see § 803.2.2); if the reduced capacity is insufficient, you must increase diameter or use engineering methods.
When must I use engineering calculations instead of the tables?
Use engineering methods when the vent height is below 6 ft or above the heights listed in the vent tables, or when your configuration isn’t represented in the tables — see § 803.2.26.
Are extra elbows already included in table capacities?
Tables include allowance for two 90° elbows; additional elbows require capacity reductions (5% or 10% depending on elbow angle). See § 803.2.6.
Can I mix single-wall and Type B pipe sizes in a run?
Yes, but only if all sizes and types used are permitted by the tables for the entire run; otherwise size to the larger required dimension. See § 803.2.22.
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