CMC · California Mechanical Code
How is makeup air provided when exhaust fans affect appliance operation?
If an exhaust fan (including kitchen hoods or dryer exhaust) causes a gas appliance to backdraft or run poorly, the California Mechanical Code requires that makeup air be provided so the appliance has enough combustion/ventilation air; check room volume against **50 ft³ per 1000 Btu/h** and, if insufficient, introduce outdoor or transferred makeup air per the Chapter 7 methods. **§ 701.3** **§ 701.4.1**
Last reviewed: July 6, 2026
What the code requires
Where exhaust fans, clothes dryers, or kitchen ventilation systems interfere with the operation of appliances, the California Mechanical Code requires that makeup air be provided. See § 701.3 for the basic mandatory statement. § 701.4 and its methods describe how indoor combustion air volumes are determined when appliances need combustion/ventilation air. § 701.3 ; § 701.4 .
If an exhaust fan (including kitchen exhaust or dryer exhaust) causes appliances to operate poorly or to backdraft, the installer must provide makeup air so appliances can get the air they need.
Requirements in detail
When makeup air is required
- Makeup air is required whenever an exhaust system (including kitchen hoods, clothes dryers, or other exhaust fans) interferes with appliance operation — i.e., causes backdrafting, extinguishing of flames, or unsafe combustion conditions. See § 701.3 .
- If the building/space does not meet the indoor combustion air volume methods, outdoor makeup air must be introduced using the methods in Chapter 7 (see § 701.1 referencing Sections 701.4–701.9). § 701.1 .
How to provide makeup air (general, code-based)
- Provide makeup air by introducing outdoor air (ducted or opening) or by transferring sufficient air from adjacent spaces so that appliances are not starved of combustion air. The code requires makeup air where interference occurs; the specific method (indoor volume, openings, ducted make‑up) follows the methods in Chapter 7 (see § 701.4 and following). § 701.3 ; § 701.4 .
- For commercial cooking systems, makeup air is also required to keep negative pressure within limits and to maintain capture; additional quantitative requirements appear in the commercial exhaust sections (see § 511.3). § 511.3 .
Sizing: decision‑relevant dimensions and values
Use the indoor-combustion-air methods or the known-infiltration method to determine required volumes or openings. The table below summarizes the key numeric thresholds and where to find them in the code text that was available.
| Decision item | Key value/threshold (bold = code figure) | Code Reference |
|---|---|---|
| Minimum indoor required volume — standard method | 50 ft³ per 1000 Btu/h | § 701.4.1 |
| Known infiltration method (appliances other than fan‑assisted) | 21 ft³ × ACH per 1000 Btu/h (see equation form) | § 701.4.2 |
| Known infiltration method (fan‑assisted appliances) | 15 ft³ × ACH per 1000 Btu/h (see equation form) | § 701.4.2 |
| Commercial kitchen negative pressure limit (to prevent excessive depressurization) | 0.02 inch water column | § 511.3 |
| Compensating hood internal makeup max (Type I/II internal discharge) | 10% of exhaust (internal makeup flow limit) | § 511.3 |
| Clothes‑dryer closet makeup opening (example residential) | 100 in² (minimum door or other opening) | § 504.4.1 |
Notes:
- The standard indoor‑volume method (50 ft³/1000 Btu/h) is the simplest sizing check for whether the room contains sufficient air for combustion appliances; if the room volume is less than the required volume, outdoor makeup must be provided per the methods in Chapter 7. § 701.4.1 .
- The known‑infiltration method requires the building's ACH (air change per hour) be known and applies different multipliers for fan‑assisted vs. other appliances. § 701.4.2 .
- For commercial kitchens the code includes specific limits on allowable depressurization and internal makeup-air rules to preserve hood performance and safe combustion. § 511.3 .
What the code does NOT say here (important)
- The short statement in § 701.3 simply requires that makeup air be provided where exhaust systems interfere with appliance operation; the exact technical method (openings vs. ducted makeup, interlock requirements, controls) are handled elsewhere in Chapter 7 and related sections. The full text of § 701.9.1 was not available in the files provided, so specific language or numeric rules from § 701.9.1 cannot be quoted here. § 701.3 . (See “Missing retrieval” note below.)
Exceptions & special cases
- Direct‑vent appliances are excluded from these combustion‑air provisions (they do not rely on room air). See § 701.1 Exception (1). § 701.1 .
- Clothes dryers have separate makeup‑air provisions; Type I clothes dryers follow manufacturer instructions and dryer closets may require 100 in² openings per § 504.4.1. § 504.4.1 .
- Commercial cooking systems: makeup air must prevent negative pressure exceeding 0.02 in. w.c. and internal makeup discharges to hoods have limits (see § 511.3). § 511.3 .
- Where the indoor‑volume methods are met, those methods govern; where they are not met, the code requires the use of outdoor‑air methods described in the later sections of Chapter 7 (701.6–701.9). § 701.1 .
Common mistakes
- Assuming small infiltration always supplies sufficient combustion air in tight or weather‑sealed buildings — you must check the indoor‑volume or known‑infiltration calculations per § 701.4. § 701.4 .
- Forgetting to interlock makeup air with large exhaust systems (commercial hoods or solid‑fuel cooking) so the space doesn’t go excessively negative — commercial provisions in § 511.3 and Section 517 call this out. § 511.3 ; § 517.6 .
- Supplying internal makeup air to a hood at too high a fraction (internal makeup may be limited — see the 10% internal discharge limit for compensating hoods in § 511.3). § 511.3 .
- Overlooking appliance manufacturer instructions for non‑Category I or power‑burner appliances — those are controlled by manufacturers’ instructions per § 701.1.1. § 701.1.1 .
Worked example — single‑family gas water heater and kitchen exhaust
Scenario:
- A house has a natural‑draft gas water heater with input rating 40,000 Btu/h located in an open mechanical room that is 1,000 ft³ in volume.
- The kitchen hood exhaust (when running) pulls 600 CFM in confined operation, and occupants report occasional backdrafting at the water‑heater vent.
Step 1 — check indoor‑volume requirement (standard method):
- Standard method requires 50 ft³ per 1000 Btu/h (bold in table). For 40,000 Btu/h: 40 × 50 = 2,000 ft³ required. § 701.4.1 .
Step 2 — compare to actual room volume:
- Actual room = 1,000 ft³, which is less than the 2,000 ft³ required → indoor volume method is NOT satisfied.
Step 3 — action required by § 701.3:
- Because the kitchen exhaust is interfering and the room volume is insufficient, the installer must provide makeup air (outdoor or transferred air) so the water heater will receive adequate combustion air and will not backdraft. The code points to the outdoor/makeup methods in Chapter 7 for how to introduce that air. § 701.3 ; § 701.1 .
Practical solutions (typical and code‑consistent steps — note: these are implementation options, not verbatim code text):
- Provide a dedicated ducted makeup‑air inlet sized and controlled to supply sufficient flow during hood operation (or automatically energized when hood runs).
- Or increase communicating openings or supply air so the required combustion air is available (but check Chapter 7 opening sizing methods before relying on this).
- For commercial hoods or large exhausts, interlock makeup air to the exhaust and ensure depressurization limits (e.g., 0.02 in. w.c.) per the commercial sections. § 511.3 .
Related provisions (CMC)
- § 701.1 — General applicability and exceptions (direct‑vent, dryer exceptions). § 701.1
- § 701.3 — Makeup air requirement (controlling statement). § 701.3
- § 701.4 — Indoor combustion‑air sizing methods (50 ft³/1000 Btu/h and known‑infiltration equations). § 701.4.1 § 701.4.2
- § 701.5 — Indoor opening size and location (methods for openings) — see CMC Chapter 7. § 701.5
- § 504.4.1 — Makeup‑air provisions for clothes dryers (100 in² opening example). § 504.4.1
- § 511.3 — Makeup air and depressurization limits for commercial kitchen hoods. § 511.3
- § 517.6 — Makeup air for solid‑fuel cooking (interlock and positive supply). § 517.6
Missing retrieval note: The specific text of § 701.9.1 requested as a controlling section was not available in the provided files, so I could not quote or ground any checklist items specifically from § 701.9.1. Where the guidance above refers to the later Chapter 7 methods (701.6–701.9), that is consistent with § 701.1 directing those methods, but any precise wording or numbers inside § 701.9.1 could not be retrieved from the uploaded files. § 701.1
Code references
Grounded in the retrieved California Mechanical Code — click a citation to read the verbatim passage:
CMC § 701.3 High relevance — show source text
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]:
(1) For appliances other than fan-assisted, calculate using the following Equation 701.4.2(1). [NFPA 54:9.3.2.2(1)]
[Equation 701.4.2(1)]
I other
Required Volume other ≥ [21 ft] ACH [3] 1000 Btu/h
( )
(2) For fan-assisted appliances, calculate using the following Equation 701.4.2(2). [NFPA 54:9.3.2.2(2)]
[Equation 701.4.2(2)]
15 ft [3] I
fan
Required Volume fan ≥ ACH 1000 Btu/h
( )
Where:
I other = All appliances other than fan-assisted input (Btu/h) (kW) I = Fan-assisted appliance input (Btu/h) (kW) fan ACH = Air change per hour (percent of volume of space exchanged per hour, expressed as a decimal)
Required Volume other ≥ [21 ft] [3]
ACH
(
I other
[21 ft] [3]
ACH 1000 Btu/h
( )
15 ft [3] Required Volume fan ≥ ACH
(
15 ft [3] I
fan ACH 1000 Btu/h
( )
For SI units: 1 cubic foot = 0.0283 m [3], 1000 British thermal units per
hour = 0.293 kW
(3) For purposes of these calculations, an infiltration rate greater than 0.60 ACH shall not be used in Equation 701.4.2(1) and Equation 701.4.2(2). [NFPA 54:9.3.2.2(3)]
**701.5 Indoor Opening Size and Location.
CMC § 701.0 High 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]:
CMC § 1106.2.9. High relevance — show source text
1106.2.9.
1106.2.8 Ventilation. Mechanical ventilation referred
to in Section 1106.2.7 shall be by one or more powerdriven fans capable of exhausting air from the machinery room at not less than the amount shown in accordance
with Section 1106.2.9.
To obtain a reduced airflow for normal ventilation, multiple fans or multispeed fans shall be used. Provision shall be made to supply makeup air to replace that being exhausted. Ducts for supply and exhaust to the machinery room shall serve no other area. The makeup air supply locations shall be positioned relative to the exhaust air locations to avoid short-circuiting. Inlets to the exhaust ducts shall be located in an area where refrigerant from a leak will concentrate, in consideration of the location of the replacement supply air paths, refrigerating machines, and the density of the refrigerant relative to air.
Inlets to exhaust ducts shall be within 1 foot (305 mm) of the lowest point of the machinery room for refrigerants that are heavier than air, and shall be within 1 foot (305 mm) of the highest point for refrigerants that are lighter than air. The discharge of the exhaust air shall be to the outdoors in such a manner as not to cause a nuisance or danger.
1106.2.9 Emergency Ventilation-Required Air- flow. An emergency ventilation system shall be required to exhaust an accumulation of refrigerant due to leaks or a rupture of the system. The emergency ventilation required shall be capable of removing air from the machinery room in not less than the airflow quantity in Section 1106.2.9.1. Where multiple refrigerants are present, then the highest airflow quantity shall apply.
1106.2.9.1 Ventilation - A1, A2, A3, B1, B2 and B3 Refrigerants. The emergency ventilation for A1, A2, A3, B1, B2 and B3 refrigerants shall have the capacity to provide mechanical exhaust at a rate as determined in accordance with Equation 1106.2.9.1.
Q = 100
√G (Equation 1106.2.9.1)Where:
Q = Air flow rate, CFM.
G = Refrigerant mass in largest system, pounds.
For SI units: 1 pound = 0.453 kg, 1 cubic foot per minute =
0.0283 m [3] /min, 1 cubic foot per minute = 0.4719 L/s
1106.3 Normal Operation. A part of the refrigeration machinery room mechanical ventilation shall be in accordance with the following:
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REFRIGERATION
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CMC § 140.9 High relevance — show source text
- Kitchen ventilation.
A. Mechanically cooled or heated makeup air delivered to any space with a kitchen hood shall not exceed the greater of: i. The supply flow required to meet the space heating and cooling load; or ii. The hood exhaust flow minus the available transfer air from adjacent spaces. Available transfer air is that portion of outdoor ventilation air serving adjacent spaces not required to satisfy other exhaust needs, such as restrooms, not required to maintain pressurization of adjacent spaces, and that would otherwise be relieved from the building. Exception to Section 140.9(b)2A: Existing kitchen makeup air units not being replaced as part of an addition or alteration.
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NONRESIDENTIAL AND HOTEL/MOTEL OCCUPANCIES—PERFORMANCE AND PRESCRIPTIVE
COMPLIANCE APPROACHES FOR ACHIEVING ENERGY EFFICIENCY
B. A kitchen/dining facility having a total Type I and Type II kitchen hood exhaust airflow rate greater than 5,000 cfm shall have one of the following: i. At least 50 percent of all replacement air is transfer air that would otherwise be exhausted; or ii. Demand ventilation system(s) on at least 75 percent of the exhaust air. Such systems shall: a. Include controls necessary to modulate airflow in response to appliance operation and to maintain full capture and containment of smoke, effluent and combustion products during cooking and idle; and b. Include failsafe controls that result in full flow upon cooking sensor failure; and c. Include an adjustable timed override to allow occupants the ability to temporarily override the system to full flow; and d. Be capable of reducing exhaust and replacement air system airflow rates to the larger of: (i) 50 percent of the total design exhaust and replacement air system airflow rates; or (ii) The ventilation rate required as specified by Section 120.1(c)3. iii. Listed energy recovery devices with a sensible heat recovery effectiveness of not less than 40 percent on at least 50 percent of the total exhaust airflow; or iv. A minimum of 75 percent of makeup air volume that is:
a. Unheated or heated to no more than 60°F; and
b. Uncooled or cooled without the use of mechanical cooling.
Exception to Section 140.9(b)2B: Existing hoods not being replaced as part of an addition or alteration. 3. Kitchen exhaust system acceptance. Before an occupancy permit is granted for a commercial kitchen subject to Section 140.9(b), the following equipment and systems shall be certified as meeting the acceptance requirements for code compliance, as specified by the Reference Nonresidential Appendix NA7. A certificate of acceptance shall be submitted to the enforcement agency that certifies that the equipment and systems meet the acceptance requirements specified in NA7.11.
Exception to Section 140.9(b): Healthcare facilities.
(c) Prescriptive requirements for laboratory and factory exhaust systems.
- Airflow reduction requirements. Building laboratory exhaust systems shall be able to reduce zone exhaust and makeup airflow rates to the occupied and unoccupied minimum exhaust airflow rates based on demand and sensed occupancy as follows:
A. Occupied minimum exhaust airflow. When occupant sensing controls sense occupants in the space, the minimum exhaust and makeup airflow rates shall be the greater of: i. User-defined airflow not to exceed 1.0 cfm/ft [2] (equivalent to 6 air changes per hour for a 10-foot high ceiling); or ii.
CMC § 120.1 High relevance — show source text
AIR, AVAILABLE TRANSFER is that portion of total outdoor ventilation air that is not required to satisfy other exhaust needs or to maintain pressurization of other spaces and that is transferable according to Section 120.1(g).
AIR, INFILTRATION is outdoor air that enters a building or space through openings in the building or space envelope due to negative pressure in the space or building relative to the exterior of the building envelope.
AIR, MAKEUP, or COMPENSATING OUTDOOR AIR is outdoor air that is intentionally conveyed by openings or ducts into the building from the outside; is supplied to the vicinity of an exhaust hood; and replaces air, vapor and contaminants being exhausted by the exhaust hood. Makeup air is generally filtered and fan-forced, and it may be heated or cooled. Makeup air may be delivered through openings or ducts integral to the exhaust hood.
AIR, REPLACEMENT is air that is used to replace air removed from a building through an exhaust system. Replacement air may be derived from one or more of the following: makeup air, portions of supply air, transfer air, or infiltration air.
AIR, SUPPLY is air entering a space from an air-conditioning, heating, or ventilating system for the purpose of comfort conditioning. Supply air is generally filtered, fan-forced, and heated, cooled, humidified or dehumidified as necessary to maintain specified temperature and humidity conditions.
AIR, TRANSFER is air transferred, whether actively by fans or passively by pressure differentials, from one room to another within a building through openings in the room envelope.
2025 CALIFORNIA ENERGY CODE 5
on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.
ALL OCCUPANCIES—GENERAL PROVISIONS
AIR BARRIER is a combination of interconnected materials and assemblies joined and sealed together to provide a continuous barrier to air leakage through the building envelope that separates conditioned from unconditioned space, or that separates adjoining conditioned spaces of different occupancies or uses.
AIR CONDITIONER is an appliance that supplies cooled and dehumidified air to a space for the purpose of cooling objects within the
space.
AIR CURTAIN UNIT means equipment providing a directionally controlled stream of air moving across the entire height and width of an opening that reduces the infiltration or transfer of air from one side of the opening.
AIR FILTER, AIR FILTER EQUIPMENT, or AIR FILTER DEVICE is air-cleaning equipment used for removing particulate matter from the air.
AIR FILTER MEDIA is the part of the air filter equipment which is the actual particulate removing agent.
AIR-COOLED AIR CONDITIONER is an air conditioner using an air-cooled condenser.
AIR-HANDLING UNIT or AIR HANDLER is a blower or fan that distributes supply air to a room, space or area.
AIR-SOURCE HEAT PUMP is an appliance that consists of one or more factory-made assemblies that includes an indoor conditioning coil, a compressor and a refrigerant-to-air heat exchanger, and that provides heating and cooling functions.
AIR-TO-AIR HEAT EXCHANGER is a device which will reduce the heat losses or gains that occur when a building is mechanically ventilated, by transferring heat between the conditioned air being exhausted and outside air being supplied.
CMC § 506.1.2 High relevance — show source text
WATER HEATERS
506.1.2 Draft Hood and Regulators. 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] 506.1.3 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] 506.2 Indoor Combustion Air. The required volume of indoor air shall be determined in accordance with the method in Section 506.2.1 or Section 506.2.2 except that where the air infiltration rate is known to be less than 0.40 ACH (air change per hour), the method in Section 506.2.2 shall be used. The total required volume shall be the sum of the required volume calculated for all 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 506.3, are considered a part of the required volume. [NFPA 54:9.3.2] 506.2.1 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]
506.2.2 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]:
(1) For appliances other than fan-assisted, calculate using the following Equation 506.2.2(1). [NFPA 54:9.3.2.2(1)]
[Equation 506.2.2(1)]
I other
Required Volume other ≥ [21 ft] ACH [3] 1000 Btu/h
( )
(2) For fan-assisted appliances, calculate using the following Equation 506.2.2(2). [NFPA 54:9.3.2.2(2)]
[Equation 506.2.2(2)]
15 ft [3] I
fan
Required Volume fan ≥ ACH 1000 Btu/h
( )
Where:
Iother = All appliances other than fan-assisted input in (Btu/h) Ifan = Fan-assisted appliance input in (Btu/h) ACH = Air change per hour (percent of volume of space exchanged per hour, expressed as a decimal)
Required Volume other ≥ [21 ft] [3]
ACH
(
I other
[21 ft] [3]
ACH 1000 Btu/h
( )
15 ft [3] Required Volume fan ≥ ACH
(
15 ft [3] I
fan ACH 1000 Btu/h FIGURE 506.3
( )
ALL COMBUSTION AIR FROM ADJACENT INDOOR SPACES
THROUGH INDOOR COMBUSTION AIR OPENINGS
CMC § 910.4.3.1 High relevance — show source text
910.4.3.1 Makeup air. Makeup air openings shall be provided within 6 feet (1829 mm) of the floor level. Operation of makeup air openings shall be manual or automatic. The minimum gross area of makeup air inlets shall be 8 square feet per 1,000 cubic feet per minute (0.74 m [2] per 0.4719 m [3] /s) of smoke exhaust.
910.4.4 Activation. The mechanical smoke removal system shall be activated by manual controls only.
910.4.5 Manual control location. Manual controls shall be located where they are able to be accessed by the fire service from an exterior door of the building and separated from the remainder of the building by not less than 1-hour fire barriers constructed in accordance with Section 707 of the California Building Code or horizontal assemblies constructed in accordance with Section 711 of the California Building Code, or both.
910.4.6 Control wiring. Wiring for operation and control of mechanical smoke removal systems shall be connected ahead of the main disconnect in accordance with Section 701.12E of the California Electrical Code and be protected against interior fire exposure to temperatures in excess of 1,000°F (538°C) for a period of not less than 15 minutes.
910.4.7 Controls. Where building air-handling and mechanical smoke removal systems are combined or where independent building air-handling systems are provided, fans shall automatically shut down in accordance with the California Mechanical Code . The manual controls provided for the smoke removal system shall have the capability to override the automatic shutdown of fans that are part of the smoke removal system.
910.5 Maintenance and testing. Maintenance and testing of smoke and heat vents and mechanical smoke removal systems shall be in accordance with Sections 910.5.1 and 910.5.2. A written record of inspection, testing and maintenance that includes the date, identification of personnel involved, any unsatisfactory result, corrective action taken and replaced parts shall be maintained on the premises.
910.5.1 Smoke and heat vents. Smoke and heat vents shall be maintained in an operative condition. Inspection, testing and maintenance shall be in accordance with NFPA 204 except as follows:
- Mechanically operated smoke and heat vents shall be inspected annually and operationally tested not less than every 5
years. 2. Gravity dropout smoke and heat vents shall be inspected annually. 3. Fused, damaged or painted fusible links shall be replaced.
910.5.2 Mechanical smoke removal systems. Mechanical smoke removal systems shall be maintained in accordance with NFPA 204 and the equipment manufacturer’s instructions except as follows:
- Systems shall be inspected and operationally tested annually.
- Testing shall include the operation of all system components, controls and ancillary equipment, such as makeup air openings.
- A written schedule for routine maintenance and operational testing shall be established and testing shall be conducted in accordance with the schedule.
SECTION 911—EXPLOSION CONTROL
911.1 General. Explosion control shall be provided in the following locations:
- Where a structure, room or space is occupied for purposes involving explosion hazards as identified in Table 911.1.
- Where quantities of hazardous materials specified in Table 911.1 exceed the maximum allowable quantities in Table 5003.1.1(1).
CMC § 0.120 High relevance — show source text
(2) Bear a label with the word “Caution,” followed by the following statements:
(a) The heat-transfer medium shall be potable water or other nontoxic fluid recognized as safe by the FDA.
(b) The maximum operating pressure of the heat exchanger shall not exceed the maximum operating pressure of the potable water supply.
(3) The word “Caution” and the statements in letters shall have an uppercase height of not less than 0.120 of an inch (3.048 mm). The vertical spacing between lines of type shall be not less than 0.046 of an inch (1.168 mm). Lowercase letters shall be compatible with the uppercase letter size specification.
506.0 Air for Combustion and Ventilation.
506.1 General. 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 506.2 through Section 506.7.3. Where the requirements of Section 506.2 are not met, outdoor air shall be introduced in accordance with methods covered in Section 506.4 through Section 506.7.3.
Exception: This provision shall not apply to direct vent appliances. {NFPA 54:9.3.1.1} 506.1.1 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]
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WATER HEATERS
506.1.2 Draft Hood and Regulators. 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] 506.1.3 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] 506.2 Indoor Combustion Air. The required volume of indoor air shall be determined in accordance with the method in Section 506.2.1 or Section 506.2.2 except that where the air infiltration rate is known to be less than 0.40 ACH (air change per hour), the method in Section 506.2.2 shall be used. The total required volume shall be the sum of the required volume calculated for all 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 506.3, are considered a part of the required volume. [NFPA 54:9.3.2] 506.2.1 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]
CMC § 910.3.4 High relevance — show source text
where:
A VR = The required aggregate vent area (ft [2] ).
A FA = The area of the floor in the area that requires smoke removal.
910.3.4 Vent operation. Smoke and heat vents shall be capable of being operated by approved automatic and manual means.
910.3.5 Fusible link temperature rating. Where vents are installed in areas provided with automatic fire sprinklers and the vents operate by fusible link, the fusible link shall have a temperature rating of 360°F (182°C).
910.4 Mechanical smoke removal systems. Mechanical smoke removal systems shall be designed and installed in accordance with Sections 910.4.1 through 910.4.7.
9-66 2025 CALIFORNIA FIRE CODE
on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.
FIRE PROTECTION AND LIFE SAFETY SYSTEMS
910.4.1 Automatic sprinklers required. The building shall be equipped throughout with an approved automatic sprinkler system in accordance with Section 903.3.1.1.
910.4.2 Exhaust fan construction. Exhaust fans that are part of a mechanical smoke removal system shall be rated for operation at 221°F (105°C). Exhaust fan motors shall be located outside of the exhaust fan airstream.
910.4.3 System design criteria. The mechanical smoke removal system shall be sized to exhaust the building at a minimum rate of two air changes per hour based on the volume of the building or portion thereof without contents. The capacity of each exhaust fan shall not exceed 30,000 cubic feet per minute (14.2 m [3] /s).
910.4.3.1 Makeup air. Makeup air openings shall be provided within 6 feet (1829 mm) of the floor level. Operation of makeup air openings shall be manual or automatic. The minimum gross area of makeup air inlets shall be 8 square feet per 1,000 cubic feet per minute (0.74 m [2] per 0.4719 m [3] /s) of smoke exhaust.
910.4.4 Activation. The mechanical smoke removal system shall be activated by manual controls only.
910.4.5 Manual control location. Manual controls shall be located where they are able to be accessed by the fire service from an exterior door of the building and separated from the remainder of the building by not less than 1-hour fire barriers constructed in accordance with Section 707 of the California Building Code or horizontal assemblies constructed in accordance with Section 711 of the California Building Code, or both.
910.4.6 Control wiring. Wiring for operation and control of mechanical smoke removal systems shall be connected ahead of the main disconnect in accordance with Section 701.12E of the California Electrical Code and be protected against interior fire exposure to temperatures in excess of 1,000°F (538°C) for a period of not less than 15 minutes.
910.4.7 Controls. Where building air-handling and mechanical smoke removal systems are combined or where independent building air-handling systems are provided, fans shall automatically shut down in accordance with the California Mechanical Code . The manual controls provided for the smoke removal system shall have the capability to override the automatic shutdown of fans that are part of the smoke removal system.
CMC § 8.1.3.2 High relevance — show source text
CONNECTION–SEALANT METHOD
[NFPA 96: FIGURE 8.1.3.2(c)]
FIGURE 511.1.2(4) TYPICAL SECTION OF DUCT-TO-FAN CONNECTION-DIRECT
TO FAN INLET CONE METHOD
[NFPA 96: FIGURE 8.1.3.2(d)]
2025 CALIFORNIA MECHANICAL CODE 121
), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.
EXHAUST SYSTEMS
511.2.2.1 Performance Test. A performance test shall be conducted upon completion and before final approval of the installation of a ventilation system serving commercial cooking appliances. The test shall verify the rate of exhaust airflow in accordance with Section 508.5.1.2 through Section 508.5.1.5. The permit holder shall furnish the necessary test equipment and devices required to perform the tests.
[ASHRAE 154:4.7.1]
511.2.2.2 Capture and Containment Test. The permit holder shall verify the capture and containment performance of Type I hoods. A field test shall be conducted with all appliances under the hood at operating temperatures, all the hoods operating at design airflows, and with all sources of replacement air operating at design airflows for the restaurant. Capture and containment shall be verified visually by observing smoke or steam produced by actual cooking operation or by simulating cooking using devices such as smoke candles or smoke puffers. Smoke bombs shall not be used. [ASHRAE 154:4.7.2]
511.2.3 Exhaust Fan Operation. A hood exhaust fan(s) shall continue to operate after the extinguishing system has been activated unless fan shutdown is required by a listed component of the ventilation system or by the design of the extinguishing system. The hood exhaust fan shall start upon actuation of the extinguishing system if the exhaust fan and all cooking equipment served by the fan have been shut down, unless fan shutdown is required by a listed component of the ventilation system or by the listing of the extinguishing system. The exhaust fan shall be provided with a means so that the fan is activated when any heat-producing cooking appliance under the hood is turned on. [NFPA 96:8.2.3.1 – 8.2.3.3]
511.3 Makeup Air. The makeup air quantity shall prevent negative pressures in the commercial cooking area(s) from exceeding 0.02 inch water column (0.005 kPa). Where the fire-extinguishing system activates, makeup air supplied internally to a hood shall be shut off.
For compensating hoods, where a Type I or Type II hood has an internal discharge of makeup air, the makeup air flow shall not exceed 10 percent of the exhaust airflow, the exhaust airflow shall be the net exhaust from the hood in accordance with Section 508.5.1.2 through Section 508.5.1.5. The total hood exhaust shall be determined in accordance with Equation 511.3.
E NET = E HOOD – MA ID (Equation 511.3)
Where:
E NET = net hood exhaust, CFM (L/s)
E HOOD = total hood exhaust, CFM (L/s)
CMC § 1.1. High relevance — show source text
(1) The fan for overhead range hoods and downdraft exhaust equipment not integral with the cooking appliance shall comply with UL 507.
(2) Overhead range hoods and downdraft exhaust equipment with integral fans shall comply with UL 507.
(3) Domestic cooking appliances with integral downdraft exhaust equipment shall comply with UL 858 or CSA/ANSI Z21.1/CSA 1.1.
(4) Microwave ovens with integral exhaust for installation over the cooking surface shall comply with UL 923.
(5) All domestic kitchen exhaust ducts used in conjunction with domestic range or cooktop hoods shall be constructed of metal and shall have smooth interior surfaces, fastened and sealed with duct mastic or metal tapes that meet the requirements of UL 181A.
Exception: Ducts for domestic kitchen downdraft grillrange ventilation installed under a concrete slab floor shall be permitted to be of approved Schedule 40 PVC provided the following conditions are met:
(1) The under-floor trench in which the duct is installed shall be completely backfilled with sand or gravel.
(2) Not more than 1 inch (25.4 mm) of 6 inch diameter (150 mm) PVC coupling shall be permitted to protrude above the concrete floor surface.
(3) PVC pipe joints shall be solvent cemented to provide an air and greasetight duct.
(4) The duct shall terminate above grade outside the building and shall be equipped with a backdraft damper.
(6) Range hoods shall discharge to the outdoors through a single wall duct and shall not terminate in an attic or crawl space.
504.4 Clothes Dryers. A clothes dryer exhaust duct shall not be connected to a vent connector, gas vent, chimney, and shall not terminate into a crawl space, attic, or other concealed space. Exhaust ducts shall not be assembled with screws or other fastening means that extend into the duct and that are capable of catching lint, and that reduce the efficiency of the exhaust system. Exhaust ducts shall be constructed of rigid metallic material. Transition ducts used to connect the dryer to the exhaust duct shall be listed and labeled in accordance
with UL 2158A, or installed in accordance with the clothes dryer manufacturer’s installation instructions. Clothes dryer exhaust ducts shall terminate to the outside of the building in accordance with Section 502.2.1 and shall be equipped with a backdraft damper. Screens shall not be installed at the duct termination. Devices, such as fire or smoke dampers that will obstruct the flow of the exhaust shall not be used. Where joining of ducts, the male end shall be inserted in the direction of airflow.
504.4.1 Provisions for Makeup Air. Makeup air shall be provided in accordance with the following:
(1) Makeup air shall be provided for Type 1 clothes dryers in accordance with the manufacturer’s installa tion instructions. [NFPA 54:10.4.4.1] Where a closet is designed for the installation of a clothes dryer, an opening of not less than 100 square inches (0.065 m [2] ) for makeup air shall be provided in the door or by other approved means.
CMC § 511.2.3 High relevance — show source text
511.2.3 Exhaust Fan Operation. A hood exhaust fan(s) shall continue to operate after the extinguishing system has been activated unless fan shutdown is required by a listed component of the ventilation system or by the design of the extinguishing system. The hood exhaust fan shall start upon actuation of the extinguishing system if the exhaust fan and all cooking equipment served by the fan have been shut down, unless fan shutdown is required by a listed component of the ventilation system or by the listing of the extinguishing system. The exhaust fan shall be provided with a means so that the fan is activated when any heat-producing cooking appliance under the hood is turned on. [NFPA 96:8.2.3.1 – 8.2.3.3]
511.3 Makeup Air. The makeup air quantity shall prevent negative pressures in the commercial cooking area(s) from exceeding 0.02 inch water column (0.005 kPa). Where the fire-extinguishing system activates, makeup air supplied internally to a hood shall be shut off.
For compensating hoods, where a Type I or Type II hood has an internal discharge of makeup air, the makeup air flow shall not exceed 10 percent of the exhaust airflow, the exhaust airflow shall be the net exhaust from the hood in accordance with Section 508.5.1.2 through Section 508.5.1.5. The total hood exhaust shall be determined in accordance with Equation 511.3.
E NET = E HOOD – MA ID (Equation 511.3)
Where:
E NET = net hood exhaust, CFM (L/s)
E HOOD = total hood exhaust, CFM (L/s)
MA ID = makeup air, internal discharge, CFM (L/s)
For SI units: 1 cubic foot per minute = 0.0283 m [3] /min, 1 cubic foot per
minute = 0.4719 L/s
511.3.1 Air Balance. Design plans for a facility with a commercial kitchen ventilation system shall include a schedule or diagram indicating the design outdoor air balance. The design outdoor air balance shall indicate the exhaust and replacement air for the facility and the net exfiltration where applicable. The total replacement airflow rate shall equal the total exhaust airflow rate and the net exfiltration.
511.4 Common Duct (Manifold) Systems. Master kitchen exhaust ducts that serve multiple tenants shall include provision to bleed air from outdoors or from adjacent spaces into the master exhaust duct where required to maintain the necessary minimum air velocity in the master exhaust duct.
[NFPA 96:8.4.1]
511.4.1 Connections. Bleed air ducts shall connect to the top or side of the master exhaust duct. [NFPA 96:8.4.2] 511.4.2 Fire Damper. The bleed-air duct shall have a fire damper at least 12 inches (305 mm) from the exhaust duct connection. [NFPA 96:8.4.3]
511.4.3 Construction and Clearance. The bleed-air
Frequently asked questions
When exactly must I add makeup air?
When an exhaust fan (including a dryer or kitchen hood) causes an appliance to backdraft, extinguish, or operate unsafely — the code requires makeup air wherever such interference occurs. See § 701.3. § 701.3
How do I tell whether the room volume is adequate?
Use the indoor‑volume standard: 50 ft³ per 1000 Btu/h of appliance input. If the space is smaller than that required volume, outdoor makeup air methods must be used. See § 701.4.1. § 701.4.1
Can I rely on infiltration (cracks) for makeup air?
Only if the known infiltration rate method (ACH) is used and the calculated required volume is satisfied per § 701.4.2. Tight, weather‑sealed buildings often cannot rely on infiltration alone. § 701.4.2
Do commercial kitchens have special makeup‑air rules?
Yes. Commercial cooking has quantifiable limits (e.g., prevent negative pressure > 0.02 in. w.c.) and rules about internal makeup fractions. See § 511.3. § 511.3
My appliance is direct‑vent — do these makeup‑air rules apply?
No. Direct‑vent appliances are specifically excluded from the combustion‑air provisions in § 701.1. § 701.1
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