CMC · California Mechanical Code
What are the mechanical combustion air supply rates and interlock requirements?
If all combustion air is supplied by a mechanical fan, size the fan at 0.35 CFM per 1,000 Btu/h of total appliance input, provide extra makeup air for any exhaust, and interlock each appliance so it cannot fire if the air supply is off (CMC **§ 701.9**, **§ 701.9.1**, **§ 701.9.2**, **§ 701.9.3**).
Last reviewed: July 6, 2026
What the code requires — 2-4 sentences
Where all combustion air is provided by a mechanical air supply system, the CMC requires the outdoor supply to be sized at not less than 0.35 cubic feet per minute (CFM) per 1,000 Btu/h of total appliance input (metric: 0.034 m3/min per kW) and to be delivered to the space containing the appliances — § 701.9.
If there are exhaust fans, the mechanical supply must provide additional makeup air to replace exhaust — § 701.9.1.
Each appliance served must be interlocked so the main burner cannot operate unless the mechanical combustion-air supply is running — § 701.9.2.
When the building’s ventilation system supplies combustion air, it must deliver the specified combustion-air rate in addition to any ventilation requirements — § 701.9.3.
The single most critical rule: if you use a mechanical fan to supply all combustion air, size it at least 0.35 CFM per 1,000 Btu/h and make sure each burner cannot fire unless that fan (or proved air) is operating. § 701.9 / § 701.9.2.
Requirements in detail
1) Supply rate (how to size the combustion-air fan)
- Required minimum supply: 0.35 CFM per 1,000 Btu/h of the total input of all appliances located within the space. § 701.9.
- Metric conversion published in the same section: 0.034 m3/min per kW. § 701.9.
2) Makeup air when exhaust devices are present
- If the space has exhaust fans (kitchen hoods, dryers, other exhaust), the supply must include additional air to replace the exhausted air so appliances do not become starved for combustion air. § 701.9.1.
3) Interlock and proof-of-air requirements
- Each appliance served must be interlocked to the mechanical air supply so the main burner cannot operate when the mechanical combustion-air supply is not in operation. This is an absolute requirement for safety and is stated in § 701.9.2.
- The interlock must reliably prove the air supply is running (examples in other CMC locations show “motor proves open” or “fan-running” proof devices are used — see Related Provisions). If the code text you have does not show a required proof-device detail for your appliance size, follow the appliance manufacturer and AHJ guidance. § 701.9.2.
4) Using the building ventilation system
- If you rely on the building’s mechanical ventilation system to supply combustion air, the system must provide the specified combustion-air rate in addition to required ventilation air (do not double-count ventilation air for both purposes). § 701.9.3.
Quick decision table (values you will use on plans / calculations)
| Decision variable | Value / threshold | When to use it | Code reference |
|---|---|---|---|
| Mechanical combustion-air rate | 0.35 CFM / 1,000 Btu/h | All appliances in space when all combustion air is from a mechanical supply | § 701.9 |
| Metric equivalent | 0.034 m3/min per kW | Use for SI calculations | § 701.9 |
| Exhaust fan presence | Additional makeup air required | When any exhaust fan removes air from the appliance space | § 701.9.1 |
| Appliance interlock | Main burner prevented when supply off | Every appliance served by the mechanical combustion-air system | § 701.9.2 |
| Building ventilation supplying combustion air | Provide specified combustion-air rate in addition to ventilation air | When the building ventilation system is used for combustion air | § 701.9.3 |
Exceptions & special cases
- The chapter’s general applicability excludes certain appliances (for example, direct-vent appliances and specific dryer provisions). These appliance-specific exceptions and applicability rules are found in the earlier applicability section of Chapter 7 (see § 701.1). If an appliance manufacturer’s instructions differ, follow the appliance instructions and the AHJ.
- For engineered installations or unusual systems where the standard methods cannot apply, the code allows engineering design methods (see § 701.8). If you choose an engineered approach, document the design calculations and prove adequacy to the AHJ.
- If combustion air is ducted through louvers, dampers or the building system, additional code provisions (e.g., louvers, damper proof-of-open requirements) may apply — see Related Provisions below. If the retrieval you have does not include the full damper/louver interlock text needed for your exact device, consult the AHJ and the full CMC text.
Common mistakes
- Assuming the ventilation system’s normal OA flow automatically satisfies combustion-air requirements. The code requires the specified combustion-air rate in addition to ventilation air when the building system is used for combustion air (§ 701.9.3).
- Forgetting to add makeup air to compensate for exhaust fans: exhaust removes air and the mechanical combustion-air system must replace that air (§ 701.9.1).
- Failing to interlock each appliance to the combustion-air fan (or otherwise prove air is provided) so burners can start with no combustion airflow — the code forbids main burner operation unless the mechanical supply is running (§ 701.9.2).
- Under-sizing the fan by using the per-appliance rate instead of the total input basis: the code applies 0.35 CFM per 1,000 Btu/h for all appliances located within the space — sum the inputs first (§ 701.9).
Worked example — concrete scenario with numbers
Scenario: a mechanical room contains a gas boiler (input 150,000 Btu/h) and a gas water heater (input 50,000 Btu/h). A separately installed exhaust fan exhausts 200 CFM for general ventilation.
- Sum appliance input: 150,000 + 50,000 = 200,000 Btu/h.
- Mechanical combustion-air minimum: 0.35 CFM / 1,000 Btu/h × (200,000 / 1,000) = 0.35 × 200 = 70 CFM. (§ 701.9)
- Replace exhaust: The exhaust fan removes 200 CFM; code requires additional air to replace exhausted air (§ 701.9.1). That means the mechanical combustion-air system must be capable of delivering at least 70 CFM (combustion requirement) + 200 CFM (makeup for exhaust) = 270 CFM total to the space.
- Interlock: Each appliance’s main burner must be prevented from operating unless the combustion-air fan (or proven air supply) is functioning. Install controls so loss of the 270 CFM supply or fan shutdown trips a lockout preventing burner firing — § 701.9.2.
Notes: If the building ventilation system were being used to supply combustion air, you could not count its normal ventilation flow toward both ventilation and combustion purposes — you must provide the specified 70 CFM in addition to required ventilation flows (§ 701.9.3).
Related provisions
- § 701.1 — Applicability and appliance exceptions (e.g., direct-vent appliances).
- § 701.3 — Makeup air general requirement when exhaust devices interfere with appliance operation.
- § 701.4 — Indoor combustion-air methods (volume-based methods for indoor air).
- § 701.8 — Engineered installations (when engineering methods are required/used).
- § 701.10 / § 701.10.2 — Louvers, grilles, and motorized louver interlocks/proving open requirements (relevant when combustion air passes through motorized louvers).
- Chapter appendices (Appendix F, G, etc.) — example calculations and additional guidance for sizing combustion-air openings and outdoor air rates.
Code references
Grounded in the retrieved California Mechanical Code — click a citation to read the verbatim passage:
CMC § 9.3.4 High relevance — show source text
[NFPA 54:9.3.4(3)] 701.8 Engineered Installations. Engineered combustion air installations shall provide an adequate supply of combustion, ventilation, and dilution air determined using engineering methods. [NFPA 54:9.3.5] 701.9 Mechanical Combustion Air Supply. Where all combustion air is provided by a mechanical air supply system, the combustion air shall be supplied from outdoors at the minimum rate of not less than 0.35 cubic feet per minute per 1000 Btu/h [0.034 (m [3] /min)/kW] for all appliances located within the space. [NFPA 54:9.3.6]
701.9.1 Exhaust Fans. Where exhaust fans are installed, additional air shall be provided to replace the exhausted air. [NFPA 54:9.3.6.1]
701.9.2 Interlock. Each of the appliances served shall be interlocked to the mechanical air supply system to prevent main burner operation where the mechanical air supply system is not in operation. [NFPA 54:9.3.6.2] 701.9.3 Specified Combustion Air. Where combustion air is provided by the building’s mechanical ventilation system, the system shall provide the specified combustion air rate in addition to the required ventilation air. [NFPA 54:9.3.6.3] 701.10 Louvers, Grilles, and Screens. The required size of openings for combustion, ventilation, and dilution air shall be based on the net free area of each opening. Where the free area through a design of louver, grille, or screen is known, it shall be used in calculating the size opening required to provide the free area specified. Where the louver and grille design and free area are not known, it shall be assumed that wood louvers have 25 percent free area and metal louvers and grilles have 75 percent free area. Nonmotorized louvers and grilles shall be fixed in the open position. [NFPA 54:9.3.7.1]
701.10.1 Minimum Screen Mesh Size. Screens shall not be smaller than [1] ⁄ 4 of an inch (6.4 mm) mesh. [NFPA 54:9.3.7.2]
701.10.2 Motorized Louvers. Motorized louvers shall be interlocked with the appliance so they are proven in the full open position prior to main burner ignition and during main burner operation. Means shall be provided to prevent the main burner from igniting should the louver fail to open during burner startup and to shut down the main burner if the louvers close during burner operation.
[NFPA 54:9.3.7.3]
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COMBUSTION AIR
701.11 Combustion Air Ducts. Combustion air ducts shall comply with the following [NFPA 54:9.3.8]:
(1) Ducts shall be constructed of galvanized steel or a material having equivalent corrosion resistance, strength, and rigidity.
CMC § 701.6.1 High relevance — show source text
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COMBUSTION AIR
FIGURE 701.6.1(3) ALL COMBUSTION AIR FROM OUTDOORS
THROUGH HORIZONTAL DUCTS
[NFPA 54: FIGURE A.9.3.3.1(2)]
FIGURE 701.6.2
ALL COMBUSTION AIR FROM OUTDOORS THROUGH
SINGLE COMBUSTION AIR OPENING
[NFPA 54: FIGURE A.9.3.3.2]
701.7.1 Indoor Openings. Where used, openings connecting the interior spaces shall comply with Section 701.5. [NFPA 54:9.3.4(1)]
701.7.2 Outdoor Opening(s) Location. Outdoor opening(s) shall be located in accordance with Section 701.6. [NFPA 54:9.3.4(2)]
701.7.3 Outdoor Opening(s) Size. The outdoor opening(s) size shall be calculated in accordance with the following:
(1) The ratio of the interior spaces shall be the available volume of all communicating spaces divided by the required volume.
(2) The outdoor size reduction factor shall be 1 minus the ratio of interior spaces.
(3) The minimum size of outdoor opening(s) shall be the full size of outdoor opening(s) calculated in accordance with Section 701.6, multiplied by the reduction factor. The minimum dimension of air openings shall not be less than 3 inches (76 mm).
[NFPA 54:9.3.4(3)] 701.8 Engineered Installations. Engineered combustion air installations shall provide an adequate supply of combustion, ventilation, and dilution air determined using engineering methods. [NFPA 54:9.3.5] 701.9 Mechanical Combustion Air Supply. Where all combustion air is provided by a mechanical air supply system, the combustion air shall be supplied from outdoors at the minimum rate of not less than 0.35 cubic feet per minute per 1000 Btu/h [0.034 (m [3] /min)/kW] for all appliances located within the space. [NFPA 54:9.3.6]
701.9.1 Exhaust Fans. Where exhaust fans are installed, additional air shall be provided to replace the exhausted air. [NFPA 54:9.3.6.1]
701.9.2 Interlock. Each of the appliances served shall be interlocked to the mechanical air supply system to prevent main burner operation where the mechanical air supply system is not in operation. [NFPA 54:9.3.6.2] 701.9.3 Specified Combustion Air. Where combustion air is provided by the building’s mechanical ventilation system, the system shall provide the specified combustion air rate in addition to the required ventilation air. [NFPA 54:9.3.6.3] 701.10 Louvers, Grilles, and Screens. The required size of openings for combustion, ventilation, and dilution air shall be based on the net free area of each opening.
CMC § 3663.4 High relevance — show source text
(5) More than 12 500 000 Btu/h (3663.4 kW) per burner inputs. These burners shall comply with the requirements of the appropriate standards listed in Chapter 18 and the following:
(a) One approved manual shutoff valve lever handle.
(b) One approved fuel-oil filter, installed on the supply piping.
(c) Two safety shutoff valves in series, one with proof of closure, with a combined flame failure response and valve closing time not to exceed 2 seconds with strainer directly before the valves.
(d) Programmed electronic flame safeguard including proven low-fire start, manual reset lockout, 100 percent shutoff (both pilot and main burner), and a separately supervised and proven pilot.
Flame-sensing systems utilizing a UV scanner shall prove pilot and interrupt ignition spark prior to main burner valves being energized.
(e) Two controls, one operating and one high limit, activated by temperature or pressure.
(f) Burners relying on mechanical means to provide air for combustion shall have actual proof-of-air interlock device.
(g) Power burners must include proven prepurge of not less than 60 seconds at high-fire damper settings. This prepurge shall occur before every burner cycle, regardless of reason.
(h) Installations with dampered combustion air openings shall prove damper open position before trial for burner ignition.
(i) Vent dampers and flue dampers shall be interlocked to prevent burner ignition unless safely open.
(j) One high oil or liquid fuel-pressure interlock; reset from flame safeguard or manually.
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APPENDIX C
(k) A manual firing cock.
(l) Where hot water or steam, two low water cut-offs.
(m) An atomizing medium proving switch.
(n) A low oil temperature switch for oil or liquid fuel requiring preheating.
(o) A high oil temperature interlock for oil or liquid fuel requiring pre-heating.
(p) A separate firing rate control valve.
(q) The burner oil pump shall automatically not operate or rotate while the alternate fuel is firing.
(r) A pressure-relief valve shall be provided between safety shutoff valves and between pump and safety valves where an integral valve is used with a pump.
(s) A separate relief device is required on each transfer
pump.
(t) One low oil or liquid fuel-pressure interlock, reset from flame safeguard or manually.
(u) Burners with automatic controls, prepurge, proofof-closure, modulation, or postpurge shall not use relays external to the flame safeguard to accomplish these functions.
(6) Shutoff Valve.
(a) Oil or liquid fuel burner installations shall include a non-electric shutoff valve that is held open by a fusible link designed to close at 165°F (74°C), installed near the burner in the same room as the burner. This shall prevent the flow of oil or liquid fuel to the burner through the supply pipe. A check valve is required in the return line if the tank is higher than the burner.
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CALIFORNIA MECHANICAL CODE – MATRIX ADOPTION TABLE
CMC § 506.4.2 High relevance — show source text
The opening shall directly communicate with the outdoors or shall communicate through a vertical or horizontal duct to the outdoors or spaces that freely communicate with the outdoors (see Figure 506.4.2) and shall have a minimum free area of the following: (1) One square inch per 3000 Btu/h (0.0007 m [2] /kW) of the total input rating of all appliances located in the enclosure.
(2) Not less than the sum of the areas of all vent connectors in the space. [NFPA 54:9.3.3.2]
506.5 Combination Indoor and Outdoor Combustion Air. The use of a combination of indoor and outdoor com bustion air shall be in accordance with Section 506.5.1 through Section 506.5.3. [NFPA 54:9.3.4] (See Appendix J for example calculations) 506.5.1 Indoor Openings. Where used, openings connecting the interior spaces shall comply with Section 506.3. [NFPA 54:9.3.4(1)] 506.5.2 Outdoor Opening(s) Location. Outdoor opening(s) shall be located in accordance with Section 506.4. [NFPA 54:9.3.4(2)] 506.5.3 Outdoor Opening(s) Size. The outdoor opening(s) size shall be calculated in accordance with the following:
(1) The ratio of the interior spaces shall be the available volume of all communicating spaces divided by the required volume.
(2) The outdoor size reduction factor shall be 1 minus the ratio of interior spaces.
(3) The minimum size of outdoor opening(s) shall be the full size of outdoor opening(s) calculated in accordance with Section 506.4, multiplied by the reduction factor. The minimum dimension of air
openings shall not be less than 3 inches (76 mm).
[NFPA 54:9.3.4(3)]
506.6 Engineered Installations. Engineered combustion air installations shall provide an adequate supply of combustion, ventilation, and dilution air determined using engineering methods. [NFPA 54:9.3.5]
506.7 Mechanical Combustion Air Supply. Where all combustion air is provided by a mechanical air supply system, the combustion air shall be supplied from outdoors at the minimum rate of 0.35 cubic feet per minute per 1000 Btu/h [0.034 (m [3] /min)/kW] for all appliances located within the space. [NFPA 54:9.3.6]
506.7.1 Exhaust Fans. Where exhaust fans are installed, additional air shall be provided to replace the exhausted air. [NFPA 54:9.3.6.1]
506.7.2 Interlock. Each of the appliances served shall be interlocked to the mechanical air supply system to prevent main burner operation where the mechanical air supply system is not in operation. [NFPA 54:9.3.6.2]
506.7.3 Specified Combustion Air. Where combustion air is provided by the building’s mechanical ventilation system, the system shall provide the specified combustion air rate in addition to the required ventilation air. [NFPA 54:9.3.6.3]
CMC § 117.2 High relevance — show source text
(g) Installations with dampered combustion air openings shall prove damper open position before trial for burner ignition.
(h) Vent dampers and flue dampers shall be properly interlocked to prevent burner ignition unless safely
open.
(2) Four hundred thousand and one to 999 999 Btu/h (117.2 kW to 292.9 kW) per burner.
(a) One approved manual shutoff valve lever handle.
(b) One approved fuel-oil filter, installed on the supply piping.
(c) Two safety shutoff valves in series, maximum five seconds closing time.
(d) One electronic flame safeguard pilot control providing a separately supervised and proven pilot, 100 percent shutoff manual reset. Flame failure response time shall not exceed the control manufacturer’s
instructions.
Direct-spark ignition shall be allowed where approved by the Authority Having Jurisdiction and where used on number 2 or lighter oil.
(e) Two controls, one operating and one high limit, activated by temperature or pressure, as appropriate.
(f) Burners relying on mechanical means to provide air for combustion shall have actual proof-of-air interlock device.
(g) Power burners shall include proven prepurge of not less than 60 seconds at high-fire damper settings. This prepurge shall occur before every burner cycle, regardless of reason.
(h) Installations with dampered combustion air openings shall prove damper open position before trial for burner ignition.
(i) Vent dampers and flue dampers shall be interlocked to prevent burner ignition unless safely open.
(j) One high oil or liquid fuel-pressure interlock, reset from flame safeguard or manually.
(k) Where hot water or steam, one low water cutoff.
(l) An atomizing medium proving switch.
(m) A low oil temperature switch for oil or liquid fuel requiring preheating.
(n) A high oil temperature interlock for oil or liquid fuel requiring preheating.
(o) The burner oil pump shall automatically not operate or rotate while the alternate fuel is firing.
(p) A pressure-relief valve shall be provided between safety shutoff valves and between pump and safety valves where an integral valve is used with a pump.
(q) A separate relief device is required on each transfer
pump.
(3) One million to 2 499 999 Btu/h (293 kW to 732 kW) input per burner.
(a) One approved manual shutoff valve lever handle.
(b) One approved fuel-oil filter, installed on the supply piping.
(c) Two safety shutoff valves in series with a combined flame failure response and valve closing time not to exceed 5 seconds with strainer directly before the valves.
(d) Programmed electronic flame safeguard including proven low-fire start, manual reset lockout, 100 percent shutoff (both pilot and main burner), and a separately supervised and proven pilot.
Flame-sensing systems utilizing a UV scanner shall prove pilot and interrupt ignition spark prior to main burner valves being energized.
(e) Two controls, one operating and one high limit, activated by temperature or pressure.
(f) Burners relying on mechanical means to provide air for combustion shall have actual proof-of-air interlock device.
(g) Power burners shall include proven prepurge of not less than 60 seconds at high-fire damper settings. This prepurge shall occur before every burner cycle, regardless of reason.
(h) Installations with dampered combustion air openings shall prove damper open position before trial for burner ignition.
(i) Vent dampers and flue dampers shall be interlocked to prevent burner ignition unless safely open.
CMC § 506.4 High relevance — show source text
(3) The minimum size of outdoor opening(s) shall be the full size of outdoor opening(s) calculated in accordance with Section 506.4, multiplied by the reduction factor. The minimum dimension of air
openings shall not be less than 3 inches (76 mm).
[NFPA 54:9.3.4(3)]
506.6 Engineered Installations. Engineered combustion air installations shall provide an adequate supply of combustion, ventilation, and dilution air determined using engineering methods. [NFPA 54:9.3.5]
506.7 Mechanical Combustion Air Supply. Where all combustion air is provided by a mechanical air supply system, the combustion air shall be supplied from outdoors at the minimum rate of 0.35 cubic feet per minute per 1000 Btu/h [0.034 (m [3] /min)/kW] for all appliances located within the space. [NFPA 54:9.3.6]
506.7.1 Exhaust Fans. Where exhaust fans are installed, additional air shall be provided to replace the exhausted air. [NFPA 54:9.3.6.1]
506.7.2 Interlock. Each of the appliances served shall be interlocked to the mechanical air supply system to prevent main burner operation where the mechanical air supply system is not in operation. [NFPA 54:9.3.6.2]
506.7.3 Specified Combustion Air. Where combustion air is provided by the building’s mechanical ventilation system, the system shall provide the specified combustion air rate in addition to the required ventilation air. [NFPA 54:9.3.6.3]
506.8 Louvers, Grilles, and Screens. The required size of openings for combustion, ventilation, and dilution air shall be based on the net free area of each opening. Where the free area through a design of louver, grille, or screen is known, it shall be used in calculating the size opening required to provide the free area specified. Where the louver and grille design and free area are not known, it shall be assumed that wood louvers have 25 percent free area, and metal louvers and grilles have 75 percent free area. Nonmotorized louvers and grilles shall be fixed in the open position. [NFPA 54:9.3.7.1]
506.8.1 Minimum Screen Mesh Size. Screens shall not be smaller than [1] ⁄ 4 of an inch (6.4 mm) mesh. [NFPA 54:9.3.7.2]
506.8.2 Motorized Louvers. Motorized louvers shall be interlocked with the appliance so they are proven in the full open position prior to main burner ignition and during main burner operation. Means shall be provided to prevent the main burner from igniting should the louver fail to open during burner startup and to shut down the main burner if the louvers close during burner operation.
[NFPA 54:9.3.7.3]
506.9 Combustion Air Ducts. Combustion air ducts shall comply with the following [NFPA 54:9.3.8]:
(1) Ducts shall be constructed of galvanized steel or a material having equivalent corrosion resistance, strength, and rigidity.
CMC § 3663.4 High relevance — show source text
(d) Programmed electronic flame safeguard including proven low-fire start, manual reset lockout, 100 percent shutoff (both pilot and main burner), and a separately supervised and proven pilot.
Flame-sensing systems utilizing a UV scanner shall prove pilot and interrupt ignition spark prior to main burner valves being energized.
(e) Two controls, one operating and one high limit, activated by temperature or pressure.
(f) Burners relying on mechanical means to provide air for combustion shall have actual proof-of-air interlock device.
(g) Power burners shall include proven prepurge of not less than 60 seconds at high-fire damper settings. This prepurge shall occur before every burner cycle, regardless of reason.
(h) Installations with dampered combustion air openings shall prove damper open position before trial for burner ignition.
(i) Vent dampers and flue dampers shall be interlocked to prevent burner ignition unless safely open.
(j) One high oil or liquid fuel-pressure interlock, reset from flame safeguard or manually.
(k) Where hot water or steam, two low water cutoffs.
(l) An atomizing medium proving switch.
(m) A low oil temperature switch for oil or liquid fuel requiring preheating.
(n) A high oil temperature interlock for oil or liquid fuel requiring preheating.
(o) A separate firing rate control valve.
(p) The burner oil pump shall automatically not operate or rotate while the alternate fuel is firing.
(q) A pressure-relief valve shall be provided between safety shutoff valves and between pump and safety valves where an integral valve is used with a pump.
(r) A separate relief device is required on each transfer
pump.
(s) One low oil or liquid fuel-pressure interlock reset from flame safeguard or manually.
(t) Burners with automatic controls, prepurge, proofof-closure, modulation, or postpurge shall not use relays external to the flame safeguard to accomplish these functions.
(5) More than 12 500 000 Btu/h (3663.4 kW) per burner inputs. These burners shall comply with the requirements of the appropriate standards listed in Chapter 18 and the following:
(a) One approved manual shutoff valve lever handle.
(b) One approved fuel-oil filter, installed on the supply piping.
(c) Two safety shutoff valves in series, one with proof of closure, with a combined flame failure response and valve closing time not to exceed 2 seconds with strainer directly before the valves.
(d) Programmed electronic flame safeguard including proven low-fire start, manual reset lockout, 100 percent shutoff (both pilot and main burner), and a separately supervised and proven pilot.
Flame-sensing systems utilizing a UV scanner shall prove pilot and interrupt ignition spark prior to main burner valves being energized.
(e) Two controls, one operating and one high limit, activated by temperature or pressure.
(f) Burners relying on mechanical means to provide air for combustion shall have actual proof-of-air interlock device.
(g) Power burners must include proven prepurge of not less than 60 seconds at high-fire damper settings. This prepurge shall occur before every burner cycle, regardless of reason.
(h) Installations with dampered combustion air openings shall prove damper open position before trial for burner ignition.
(i) Vent dampers and flue dampers shall be interlocked to prevent burner ignition unless safely open.
(j) One high oil or liquid fuel-pressure interlock; reset from flame safeguard or manually.
2025 CALIFORNIA MECHANICAL CODE 401
CMC § 108.1 High relevance — show source text
The tag stating the date of the test and the name of the tester shall be
attached to the appliance at the main valve.
(b) Oil and liquid fuel burners of 1 000 000 Btu/h (293 kW) input or more require a supervised startup in accordance with Section C 108.1(10)(a).
(c) Installation of oxygen trim systems, modulating dampers, or other draft control or combustion devices require a supervised startup in accordance with Section C 108.1(10)(a).
2025 CALIFORNIA MECHANICAL CODE 399
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APPENDIX C
(d) Direct-fired heaters shall require a supervised startup in accordance with Section C 108.1(10)(a).
(11)The complete control diagram of the installation and operating instructions shall be supplied and posted by the installer of the appliance.
C 109.0 Special Requirements Based on Btu/h Input.
C 109.1 General.
(1) Zero to 400 000 Btu/h (0 kW to 117 kW) per burner.
(a) One approved manual shutoff valve lever handle.
(b) One approved fuel oil filter, installed on the supply piping.
(c) Approved automatic safety shutoff valve to provide 100 percent shutoff of all oil.
(d) A flame safeguard control capable of providing 100 percent shutoff in the event of flame failure. Flame failure response timing shall not exceed the control manufacturer’s instructions.
(e) Two controls, one operating and one high limit, activated by temperature or pressure, as appropriate.
(f) Burners relying on mechanical means to provide air for combustion shall have actual proof-of-air interlock device.
(g) Installations with dampered combustion air openings shall prove damper open position before trial for burner ignition.
(h) Vent dampers and flue dampers shall be properly interlocked to prevent burner ignition unless safely
open.
(2) Four hundred thousand and one to 999 999 Btu/h (117.2 kW to 292.9 kW) per burner.
(a) One approved manual shutoff valve lever handle.
(b) One approved fuel-oil filter, installed on the supply piping.
(c) Two safety shutoff valves in series, maximum five seconds closing time.
(d) One electronic flame safeguard pilot control providing a separately supervised and proven pilot, 100 percent shutoff manual reset. Flame failure response time shall not exceed the control manufacturer’s
instructions.
Direct-spark ignition shall be allowed where approved by the Authority Having Jurisdiction and where used on number 2 or lighter oil.
(e) Two controls, one operating and one high limit, activated by temperature or pressure, as appropriate.
(f) Burners relying on mechanical means to provide air for combustion shall have actual proof-of-air interlock device.
(g) Power burners shall include proven prepurge of not less than 60 seconds at high-fire damper settings. This prepurge shall occur before every burner cycle, regardless of reason.
(h) Installations with dampered combustion air openings shall prove damper open position before trial for burner ignition.
(i) Vent dampers and flue dampers shall be interlocked to prevent burner ignition unless safely open.
(j) One high oil or liquid fuel-pressure interlock, reset from flame safeguard or manually.
(k) Where hot water or steam, one low water cutoff.
(l) An atomizing medium proving switch.
CMC § 3663.3 High relevance — show source text
(n) A high oil temperature interlock for oil or liquid fuel requiring preheating.
(o) The burner oil pump shall automatically not operate or rotate while the alternate fuel is firing.
(p) A pressure-relief valve shall be provided between safety shutoff valves and between pump and safety valves where an integral valve is used with a pump.
(q) A separate relief device is required on each transfer
pump.
(3) One million to 2 499 999 Btu/h (293 kW to 732 kW) input per burner.
(a) One approved manual shutoff valve lever handle.
(b) One approved fuel-oil filter, installed on the supply piping.
(c) Two safety shutoff valves in series with a combined flame failure response and valve closing time not to exceed 5 seconds with strainer directly before the valves.
(d) Programmed electronic flame safeguard including proven low-fire start, manual reset lockout, 100 percent shutoff (both pilot and main burner), and a separately supervised and proven pilot.
Flame-sensing systems utilizing a UV scanner shall prove pilot and interrupt ignition spark prior to main burner valves being energized.
(e) Two controls, one operating and one high limit, activated by temperature or pressure.
(f) Burners relying on mechanical means to provide air for combustion shall have actual proof-of-air interlock device.
(g) Power burners shall include proven prepurge of not less than 60 seconds at high-fire damper settings. This prepurge shall occur before every burner cycle, regardless of reason.
(h) Installations with dampered combustion air openings shall prove damper open position before trial for burner ignition.
(i) Vent dampers and flue dampers shall be interlocked to prevent burner ignition unless safely open.
(j) One high oil or liquid fuel-pressure interlock, reset from flame safeguard or manually.
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APPENDIX C
(k) Where hot water or steam, two low water cutoffs.
(l) An atomizing medium proving switch.
(m) A low oil temperature switch for oil or liquid fuel requiring preheating.
(n) A high oil temperature interlock for oil or liquid fuel requiring preheating.
(o) The burner oil pump shall automatically not operate or rotate while the alternate fuel is firing.
(p) A pressure-relief valve shall be provided between safety shutoff valves and between pump and safety valves where an integral valve is used with a pump.
(q) A separate relief device is required on each transfer
pump.
(r) One low oil or liquid fuel-pressure interlock, reset from flame safeguard or manually.
(s) Burners with automatic controls, prepurge, proofof-closure, modulation, or postpurge shall not use relays external to the flame safeguard to accomplish these functions.
(4) Two million five hundred thousand to 12 499 999 Btu/h (733 kW to 3663.3 kW) per burner.
(a) One approved manual shutoff valve lever handle.
(b) One approved fuel-oil filter, installed on the supply piping.
(c) Two safety shutoff valves in series, with a combined flame failure response and valve closing time not to exceed 5 seconds with strainer directly before the valves.
CMC § 3663.3 High relevance — show source text
(j) One high oil or liquid fuel-pressure interlock, reset from flame safeguard or manually.
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APPENDIX C
(k) Where hot water or steam, two low water cutoffs.
(l) An atomizing medium proving switch.
(m) A low oil temperature switch for oil or liquid fuel requiring preheating.
(n) A high oil temperature interlock for oil or liquid fuel requiring preheating.
(o) The burner oil pump shall automatically not operate or rotate while the alternate fuel is firing.
(p) A pressure-relief valve shall be provided between safety shutoff valves and between pump and safety valves where an integral valve is used with a pump.
(q) A separate relief device is required on each transfer
pump.
(r) One low oil or liquid fuel-pressure interlock, reset from flame safeguard or manually.
(s) Burners with automatic controls, prepurge, proofof-closure, modulation, or postpurge shall not use relays external to the flame safeguard to accomplish these functions.
(4) Two million five hundred thousand to 12 499 999 Btu/h (733 kW to 3663.3 kW) per burner.
(a) One approved manual shutoff valve lever handle.
(b) One approved fuel-oil filter, installed on the supply piping.
(c) Two safety shutoff valves in series, with a combined flame failure response and valve closing time not to exceed 5 seconds with strainer directly before the valves.
(d) Programmed electronic flame safeguard including proven low-fire start, manual reset lockout, 100 percent shutoff (both pilot and main burner), and a separately supervised and proven pilot.
Flame-sensing systems utilizing a UV scanner shall prove pilot and interrupt ignition spark prior to main burner valves being energized.
(e) Two controls, one operating and one high limit, activated by temperature or pressure.
(f) Burners relying on mechanical means to provide air for combustion shall have actual proof-of-air interlock device.
(g) Power burners shall include proven prepurge of not less than 60 seconds at high-fire damper settings. This prepurge shall occur before every burner cycle, regardless of reason.
(h) Installations with dampered combustion air openings shall prove damper open position before trial for burner ignition.
(i) Vent dampers and flue dampers shall be interlocked to prevent burner ignition unless safely open.
(j) One high oil or liquid fuel-pressure interlock, reset from flame safeguard or manually.
(k) Where hot water or steam, two low water cutoffs.
(l) An atomizing medium proving switch.
(m) A low oil temperature switch for oil or liquid fuel requiring preheating.
(n) A high oil temperature interlock for oil or liquid fuel requiring preheating.
(o) A separate firing rate control valve.
(p) The burner oil pump shall automatically not operate or rotate while the alternate fuel is firing.
(q) A pressure-relief valve shall be provided between safety shutoff valves and between pump and safety valves where an integral valve is used with a pump.
(r) A separate relief device is required on each transfer
pump.
(s) One low oil or liquid fuel-pressure interlock reset from flame safeguard or manually.
CMC § 8.13.6 Medium relevance — show source text
[ASHRAE 15:8.13.6]
»
(1) Operated, where occupied, to supply not less than 0.5 CFM/ft [2] [2.54 L/(s•m [2] )] of machinery room area or 20 cubic feet per minute (9.44 L/s) per person.
(2) Operable, where occupied at a volume required to not exceed the higher of a temperature rise of 18°F (10°C) above inlet air temperature or a maximum temperature of 122°F (50°C).
1106.4 Natural Ventilation. When a refrigerating system is located outdoors more than 20 feet (6096 mm) from building openings and is enclosed by a penthouse, lean-to, or other open structure, natural or mechanical ventilation shall be provided. The requirements for such natural ventilation shall be in accordance with the following:
(1) The free-aperture cross section for the ventilation of a machinery room shall be not less than as determined in accordance with Equation 1106.4.
F = √ G (Equation 1106.4)
Where:
F = The free opening area, ft [2] (m [2] ).
G = The mass of refrigerant in the largest system, any part of which is located in the machinery room, pounds.
For SI units: 1 square foot = 0.0929 m [2], 1 pound = 0.453 kg
(2) Locations of the gravity ventilation openings shall be based on the relative density of the refrigerant to air.
[ASHRAE 15:8.14]
1106.5 Combustion Air. No open flames that use combustion air from the machinery room shall be installed where refrigerant is used. Combustion equipment shall not be installed in the same machinery room with refrigerant-containing equipment except under one of the following conditions:
(1) Combustion air shall be ducted from outside the machinery room and sealed in such a manner as to prevent refrigerant leakage from entering the combustion chamber.
(2) A refrigerant detector, that is in accordance with Section 1106.2.5, shall be installed to automatically shut down the combustion process in the event of refrigerant leak age.
Exception: Machinery rooms where carbon dioxide (R-744) or water (R-718) is the refrigerant.
1106.6 Ventilation Intake. Makeup air intakes to replace the exhaust air shall be provided to the refrigeration machinery room directly from outside the building. Intakes shall be located as required by other sections of the code and fitted with backdraft dampers or other approved flow-control means to prevent reverse flow. Distribution of makeup air shall be arranged to provide thorough mixing within the refrigeration machinery room to prevent short circuiting of the makeup air directly to the exhaust.
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REFRIGERATION
1106.11.6.1 Mechanical Ventilation. The machinery room shall have a mechanical ventilation system in accordance with Section 1106.11.11. The mechanical ventilation system shall:
(1) Run continuously, and failure of the mechanical ventilation system actuates an alarm, or
CMC § 701.0 Medium relevance — show source text
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CHAPTER 7
COMBUSTION AIR
701.0 General.
701.1 Applicability. Air for combustion, ventilation, and dilution of flue gases for appliances installed in buildings shall be obtained by application of one of the methods covered in Section 701.4 through Section 701.9.3. Where the requirements of Section 701.4 are not met, outdoor air shall be introduced in accordance with methods covered in Section
701.6 through Section 701.9.3.
Exceptions:
(1) This provision shall not apply to direct vent appliances.
(2) Type 1 clothes dryers that are provided with makeup air in accordance with Section 504.4.1. [NFPA 54:9.3.1.1]
701.1.1 Other Types of Appliances. Appliances of other than natural draft design, appliances not designated as Category I vented appliances, and appliances equipped with power burners shall be provided with combustion, ventilation, and dilution air in accordance with the appliance manufacturer’s instructions. [NFPA 54:9.3.1.2]
701.2 Pressure Difference. Where used, a draft hood or a barometric draft regulator shall be installed in the same room or enclosure as the appliance served so as to prevent any difference in pressure between the hood or regulator and the combustion air supply. [NFPA 54:9.3.1.4]
701.3 Makeup Air. Where exhaust fans, clothes dryers, and kitchen ventilation systems interfere with the operation of appliances, makeup air shall be provided. [NFPA 54:9.3.1.5]
701.4 Indoor Combustion Air. The required volume of indoor air shall be determined in accordance with the method
in Section 701.4.1 or Section 701.4.2 except that where the air infiltration rate is known to be less than 0.40 ACH (air change per hour), the method in Section 701.4.2 shall be used. The total required volume shall be the sum of the required volume calculated for appliances located within the space. Rooms communicating directly with the space in which the appliances are installed through openings not furnished with doors, and through combustion air openings sized and located in accordance with Section 701.5, are considered a part of the required volume. [NFPA 54:9.3.2]
701.4.1 Standard Method. The minimum required volume shall be 50 cubic feet per 1000 British thermal units per hour (Btu/h) (4.83 m [3] /kW). [NFPA 54:9.3.2.1]
701.4.2 Known Air Infiltration Rate Method. Where
the air infiltration rate of a structure is known, the minimum required volume shall be determined as follows
[NFPA 54:9.3.2.2]:
Frequently asked questions
What if my appliances are in different rooms but share one mechanical combustion-air fan?
You still size the fan based on the total input of all appliances the fan serves and provide ducts/makeup air to each room as required. Each appliance served must be interlocked so the burner cannot operate without the mechanical supply running — § 701.9 and § 701.9.2.
Can normal building ventilation counts toward combustion-air supply?
Only if you explicitly arrange for the building mechanical ventilation to provide the combustion-air rate, and then it must deliver the specified combustion-air rate in addition to required ventilation air — you cannot double-count the same flow for both purposes. See § 701.9.3.
How do I prove the combustion-air fan is “running” for interlock purposes?
The code requires an interlock to prevent burner operation when the mechanical supply is not operating (§ 701.9.2). Common proving methods accepted by AHJs include fan-run contacts, pressure or flow switches in the duct, or louver open-position switches for motorized dampers — also review related louver/damper provisions. If the code excerpts you have do not show the exact proof-device detail for your application, follow the appliance manufacturer and AHJ requirements.
If I have a small exhaust hood that pulls 50 CFM, do I always add that 50 CFM to the combustion-air fan capacity?
Yes — any exhaust that removes air from the space must be replaced so appliances are not starved of combustion air. Provide the combustion-air minimum plus the makeup air equal to the exhaust flow per § 701.9.1.
Do these mechanical requirements apply to direct-vent appliances?
No — direct-vent appliances are excluded from the general provisions; check § 701.1 for applicability and the appliance manufacturer’s instructions.
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