Appendix J — COMBINATION OF INDOOR AND OUTDOOR COMBUSTION AND VENTILATION OPENING DESIGN
2025 California Plumbing Code (Title 24, Part 5) · 2025 edition · updated 2026-07-27 · California
Italicized text is a California amendment to the model code, as printed in the official publication.
(Matrix Adoption Tables are non-regulatory, intended only as an aid to the code user. See Chapter 1 for state agency authority and building applications.)
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| Adopting Agency | BSC | BSC- CG |
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This state agency does not adopt sections identified with the following symbol: The Office of the State Fire Marshal’s adoption of this chapter or individual sections is applicable to structures regulated by other state agencies pursuant to Section 1.11.0.
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APPENDIX J
COMBINATION OF INDOOR AND OUTDOOR COMBUSTION AND
VENTILATION OPENING DESIGN
The provisions contained in this appendix are not mandatory unless specifically adopted by a state agency, or referenced in the adopting ordinance.
J 101.0 General.
J 101.1 Applicability. This appendix provides general guidelines for the sizing of combination indoor and outdoor combustion and ventilation air openings. J 101.2 Example of Combination Indoor and Outdoor Combustion Air Opening Design. Determine the required combination of indoor and outdoor combustion air opening sizes for the following appliance installation example.
Example Installation: A fan-assisted furnace and a drafthood-equipped water heater with the following inputs are located in a 15 foot by 30 foot (4572 mm by 9144 mm) basement with an 8 foot (2438 mm) ceiling. No additional indoor spaces can be used to help meet the appliance combustion air needs.
Fan-Assisted Furnace Input: 100 000 British thermal units per hour (Btu/h) (29 kW)
Draft Hood-Equipped Water Heater Input: 40 000 Btu/h (11.7 kW)
Solution:
(1) Determine the total available room volume. Appliance room volume:
15 feet by 30 feet (4572 mm by 9144 mm) with an 8 foot (2438 mm) ceiling = 3600 cubic feet (101.94 m [3] )
(2) Determine the total required volume. The standard method to determine combustion air is used to calculate the required volume. The combined input for the appliances located in the basement is calculated as follows:
100 000 Btu/h (29 kW) + 40 000 Btu/h (11.7 kW) = 140 000 Btu/h (41 kW)
The standard method requires that the required volume be determined based on 50 cubic feet per 1000 Btu/h (4.83 m [3] /kW). Using Table J 101.2, the required volume for a 140 000 Btu/h (41 kW) combined input is 7000 cubic feet (198.22 m [3] ).
Conclusion: The indoor volume is insufficient to supply combustion air since the total of 3600 cubic feet (101.94 m [3] ) does not meet the required volume of 7000 cubic feet (198.22 m [3] ). Therefore, additional combustion air shall be provided from the outdoors.
(3) Determine ratio of the available volume to the required
volume:
| 3600 cubic feet | = 0.51 |
|---|---|
| 7000 cubic feet | 7000 cubic feet |
(4) Determine the reduction factor to be used to reduce the full outdoor air opening size to the minimum required based on the ratio of indoor spaces:
1.00 – 0.51 (from Step 3) = 0.49
(5) Determine the single outdoor combustion air opening size as though all combustion air is to come from outdoors. In this example, the combustion air opening directly communicates with the outdoors:
140 000 Btu/h = 47 square inches (0.03 m [2] ) 3000 British thermal units per square inch (Btu/in [2] )
(6) Determine the minimum outdoor combustion air opening area:
Outdoor opening area = 0.49 (from Step 4) x 47 square inches (0.03 m [2] ) = 23 square inches (0.01 m [2] )
Section 506.5.3(3) requires the minimum dimension of the air opening should not be less than 3 inches (76 mm).
[NFPA 54:I.1]
140 000 Btu/h = 47 square inches (0.03 m [2] ) 3000 British thermal units per square inch (Btu/in [2] )
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APPENDIX J
TABLE J 101.2 STANDARD METHOD: REQUIRED VOLUME, ALL APPLIANCES*
[NFPA 54: TABLE A.9.3.2.1]
| APPLIANCE INPUT (Btu/h) |
REQUIRED VOLUME (cubic feet) |
|---|---|
| 5000 | 250 |
| 10 000 | 500 |
| 15 000 | 750 |
| 20 000 | 1000 |
| 25 000 | 1250 |
| 30 000 | 1500 |
| 35 000 | 1750 |
| 40 000 | 2000 |
| 45 000 | 2250 |
| 50 000 | 2500 |
| 55 000 | 2750 |
| 60 000 | 3000 |
| 65 000 | 3250 |
| 70 000 | 3500 |
| 75 000 | 3750 |
| 80 000 | 4000 |
| 85 000 | 4250 |
| 90 000 | 4500 |
| 95 000 | 4750 |
| 100 000 | 5000 |
| 105 000 | 5250 |
| 110 000 | 5500 |
| 115 000 | 5750 |
| 120 000 | 6000 |
| 125 000 | 6250 |
| 130 000 | 6500 |
| 135 000 | 6750 |
| 140 000 | 7000 |
| 145 000 | 7250 |
| 150 000 | 7500 |
| 160 000 | 8000 |
| 170 000 | 8500 |
| 180 000 | 9000 |
| 190 000 | 9500 |
| 200 000 | 10 000 |
| 210 000 | 10 500 |
| 220 000 | 11 000 |
| 230 000 | 11 500 |
| 240 000 | 12 000 |
| 250 000 | 12 500 |
| 260 000 | 13 000 |
| 270 000 | 13 500 |
| 280 000 | 14 000 |
| 290 000 | 14 500 |
| 300 000 |
15 000 |
For SI units: 1000 British thermal units per hour = 0.293 kW, 1 cubic foot = 0.0283 m [3]
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CALIFORNIA PLUMBING CODE – MATRIX ADOPTION TABLE
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Ask AI about this code▸ Contents — 2025 California Plumbing Code (Title 24, Part 5)
- Chapter 1 — GENERAL CODE PROVISIONS
- Chapter 2 — DEFINITIONS
- Chapter 3 — GENERAL REGULATIONS
- Chapter 4 — PLUMBING FIXTURES AND FIXTURE FITTINGS
- Chapter 5 — WATER HEATERS
- Chapter 6 — WATER SUPPLY AND DISTRIBUTION
- Chapter 7 — SANITARY DRAINAGE
- Chapter 8 — INDIRECT WASTES
- Chapter 9 — VENTS
- Chapter 10 — TRAPS AND INTERCEPTORS
- Chapter 11 — STORM DRAINAGE
- Chapter 12 — FUEL GAS PIPING
- Chapter 13 — HEALTH CARE FACILITIES AND MEDICAL GAS AND MEDICA…
- Chapter 14 — FIRESTOP PROTECTION
- Chapter 15 — ALTERNATE WATER SOURCES FOR NONPOTABLE APPLICATIONS
- Chapter 16 — NONPOTABLE RAINWATER CATCHMENT SYSTEMS
- Chapter 17 — REFERENCED STANDARDS
- Appendix A — RECOMMENDED RULES FOR SIZING THE WATER SUPPLY SYS…
- Appendix B — EXPLANATORY NOTES ON COMBINATION WASTE AND VENT S…
- Appendix C — ALTERNATE PLUMBING SYSTEMS
- Appendix D — SIZING STORM WATER DRAINAGE SYSTEMS
- Appendix E — MANUFACTURED/MOBILE HOME PARKS AND RECREATIONAL V…
- Appendix F — FIREFIGHTER BREATHING AIR REPLENISHMENT SYSTEMS
- Appendix G — SIZING OF VENTING SYSTEMS
- Appendix H — PRIVATE SEWAGE DISPOSAL SYSTEMS
- Appendix I — INSTALLATION STANDARDS
- Appendix J — COMBINATION OF INDOOR AND OUTDOOR COMBUSTION AND …
- Appendix K — POTABLE RAINWATER CATCHMENT SYSTEMS
- Appendix L — SUSTAINABLE PRACTICES
- Appendix M — PEAK WATER DEMAND CALCULATOR
- Appendix N — IMPACT OF WATER TEMPERATURE ON THE POTENTIAL FOR …
- Appendix O — NON-SEWERED SANITATION SYSTEMS
- Appendix P — PROFESSIONAL QUALIFICATIONS
- Appendix Q — INDOOR HORTICULTURAL FACILITIES
- Appendix R — TINY HOUSES
- Appendix S — ONSITE STORMWATER TREATMENT SYSTEMS