CMC · California Mechanical Code

How do pressure differences and makeup air affect combustion appliances?

Q: What exactly does “prevent any difference in pressure” mean for my water heater?

It means a draft hood or barometric regulator must be located inside the same room or enclosure as the appliance’s combustion-air source so the hood/regulator is exposed to the same pressure as the combustion air. That prevents backdrafting or spillage ( § 701.2 ) .

Q: Are direct-vent appliances covered by these rules?

No. Direct-vent appliances are excluded from the chapter’s combustion-air requirements because they take combustion air directly from outdoors (see § 701.1 exception) .

Q: Can I rely on building leakage (infiltration) for combustion air?

You can if you use the known-infiltration method in § 701.4.2 , but you must use the correct equations and not assume an infiltration rate greater than 0.60 ACH in calculations ( § 701.4.2(3) ) .

If exhaust fans or kitchen hoods depressurize a space they can make gas appliances backdraft or run poorly. The CMC says draft hoods or barometric regulators must be in the same room as the combustion-air supply (**§ 701.2**) and that makeup air must be provided whenever exhaust systems interfere with appliance operation (**§ 701.3**) .

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Last reviewed: July 6, 2026

What the code requires — 2-4 sentences

The California Mechanical Code requires that where a draft hood or barometric draft regulator is used, it be installed in the same room or enclosure as the appliance it serves so there is no pressure difference between the hood/regulator and the combustion air supply (§ 701.2) . The code also requires that makeup air be provided when exhaust fans, clothes dryers, or kitchen ventilation systems interfere with appliance operation (§ 701.3) .

The single most important rule: if a draft hood or barometric regulator is used, it must share the same air space as the combustion-air source so negative or positive pressure does not upset the appliance’s draft (§ 701.2) .

Requirements in detail

Pressure differences — location of draft hoods and regulators

The code requires that a draft hood or barometric draft regulator be installed in the same room or enclosure as the appliance it serves so as to prevent any difference in pressure between the hood/regulator and the combustion air supply (§ 701.2) .
Why: pressure differences can cause spillage of flue products, poor combustion, or backdrafting of hazardous gases.

Makeup air — when it’s required and how it’s supplied

Makeup air must be provided whenever exhaust fans, clothes dryers, or kitchen ventilation systems interfere with proper appliance operation (§ 701.3) .
If all combustion air is supplied mechanically, the minimum outdoor mechanical supply is 0.35 cfm per 1,000 Btu/h for all appliances within the space; mechanical systems must supply additional air to replace exhausts and shall be interlocked to prevent burner operation if the supply is off (§ 701.9, §§ 701.9.1–701.9.3) .
When indoor combustion air is used, the code provides volume-based sizing (see 50 ft³ per 1,000 Btu/h) and an alternate method based on known infiltration rates (§ 701.4) .

Decision-relevant dimensions / values

Decision dimension	Typical value / threshold	When it matters	Code Reference
Location of draft hood / barometric regulator	Same room/enclosure as appliance	Always required when a draft hood or regulator is used to avoid pressure difference	§ 701.2
Trigger for makeup air	Exhaust fans, clothes dryers, kitchen ventilation interfering with appliance	Anytime these exhausts can affect appliance draft or combustion	§ 701.3
Mechanical combustion-air minimum	0.35 cfm per 1,000 Btu/h (0.034 m³/min per kW)	When all combustion air is provided mechanically	§ 701.9
Indoor required volume (standard method)	50 ft³ per 1,000 Btu/h (4.83 m³/kW)	When using indoor-combustion-air sizing	§ 701.4.1
Known infiltration adjustment	Use specified equations; do not use > 0.60 ACH in calculations	When infiltration rate is known and used for sizing	§ 701.4.2

Exceptions & special cases

Direct-vent appliances are exempt from the combustion-air chapter requirements (they draw combustion air directly from outdoors) — see applicability and exceptions in § 701.1 .
Type I clothes dryers are treated specially; makeup-air provisions for Type I clothes dryers are addressed elsewhere (Chapter 5, e.g., § 504.4.1) and are exempt from the general Chapter 7 makeup-air rule listed in § 701.1 (Exception 2) .
Appliances that are not natural draft, that are not Category I, or that have power burners must follow the appliance manufacturer’s instructions for combustion air rather than the Chapter 7 default methods (§ 701.1.1) .
Where mechanical combustion-air systems serve appliances, the system must be interlocked to prevent burner operation if the mechanical supply fails (§ 701.9.2) .

Common mistakes

Installing a draft hood or barometric regulator outside the appliance room (e.g., in a corridor or plenum) — this violates § 701.2 and can create harmful pressure differentials .
Assuming building leakage (infiltration) will always provide makeup air; the designer must verify infiltration rate and use the proper method in § 701.4.2, and must not assume > 0.60 ACH for calculations (§ 701.4.2(3)) .
Forgetting to provide makeup air for large kitchen hoods or powerful exhaust fans. If the exhaust “interferes” with appliance operation, makeup air is required per § 701.3 .
Not interlocking mechanical makeup-air systems with the appliances they serve — required where mechanical supply provides combustion air (§ 701.9.2) .

Worked example — concrete scenario with numbers

Scenario: A mechanical room contains two natural-draft appliances with combined input of 50,000 Btu/h. A kitchen exhaust hood on the same floor can exhaust 300 cfm when running.

Step 1 — Draft hood location:

If either appliance uses a draft hood or barometric draft regulator, the hood/regulator must be in the same room as the appliances’ combustion-air source (do not locate the hood outside the room) — § 701.2 .

Step 2 — Is makeup air required?

Because the kitchen hood can exhaust 300 cfm and could interfere with appliance operation, makeup air is required per § 701.3 .

Step 3 — If you provide mechanical combustion air for the appliances:

Minimum mechanical combustion-air rate = 0.35 cfm per 1,000 Btu/h (from § 701.9) .
For 50,000 Btu/h: 0.35 × 50 = 17.5 cfm (minimum) of outdoor combustion air required to serve the appliances.
But the kitchen hood exhausts 300 cfm. To avoid depressurizing the appliance room and interfering with combustion, you must provide makeup air to replace the exhausted air. Practically, that means the mechanical system must supply at least the larger of:
- the combustion-air minimum (17.5 cfm), and
- additional makeup air to replace the 300 cfm exhausted by the hood (so at least 300 cfm total makeup if the hood draws through the same space).
Also consider that mechanical makeup-air systems serving combustion air must be interlocked so burners cannot fire if the supply is not operating (§ 701.9.2) .

Step 4 — If using indoor-volume method instead:

Standard indoor volume requirement = 50 ft³ per 1,000 Btu/h (§ 701.4.1) .
For 50,000 Btu/h: 50 × 50 = 2,500 ft³ minimum room volume would be required to rely on indoor air alone. If the room is smaller, outdoor makeup or mechanical supply will be needed.

Practical takeaway: Even if the code minimum combustion-air rate is small (17.5 cfm), makeup air to offset an exhausting hood (300 cfm) governs the practical design — provide enough makeup air to keep the appliance room near neutral pressure and interlock mechanical supplies as required.

Related provisions

§ 701.1 — Applicability and exceptions (direct-vent exemption; Type I clothes dryer note)
§ 701.4 — Indoor combustion-air sizing methods (50 ft³/1,000 Btu/h and known-infiltration method)
§ 701.9 — Mechanical combustion-air supply (minimum 0.35 cfm/1,000 Btu/h, interlocks, exhaust replacement)
§ 701.10–701.11 — Louvers/grilles and combustion-air duct construction (free-area assumptions, materials)

Code references

Grounded in the retrieved California Mechanical Code — click a citation to read the verbatim passage:

CMC § 701.0 High relevance — show source text
2025 CALIFORNIA MECHANICAL CODE 145

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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 § 608.9 High relevance — show source text
608.9 Floor Registers . . . . . . . . . . . . . . . 143

608.10 Exterior Wall and Interior Stud

Partitions. . . . . . . . . . . . . . . . . . . . 143

608.11 Wall Register . . . . . . . . . . . . . . . . 143

608.12 Distance from Combustible . . . . . 143

608.13 Vapor Barrier . . . . . . . . . . . . . . . . 143

608.14 Prohibited . . . . . . . . . . . . . . . . . . . 143

609.0 Automatic Shutoffs . . . . . . . . . . . . 143

609.1 Air-Moving Systems and Smoke Detectors . . . . . . . . . . . . . . . . . . . 143

609.2 Air-Moving Systems and Smoke Detectors in Group I-2 Occupancies . . . . . . . . . . . . . . . . 144

CHAPTER 7 COMBUSTION AIR . . . . . . . . . . . 145

701.0 General. . . . . . . . . . . . . . . . . . . . . 147

701.1 Applicability. . . . . . . . . . . . . . . . . . 147

701.2 Pressure Difference . . . . . . . . . . . 147

701.3 Makeup Air . . . . . . . . . . . . . . . . . . 147

701.4 Indoor Combustion Air . . . . . . . . . 147

701.5 Indoor Opening Size and Location . . . . . . . . . . . . . . . . . . . . 147

701.6 Outdoor Combustion Air . . . . . . . . 147

701.7 Combination Indoor and Outdoor

Combustion Air . . . . . . . . . . . . . . . 148

701.8 Engineered Installations . . . . . . . . 149

701.9 Mechanical Combustion Air

Supply. . . . . . . . . . . . . . . . . . . . . . 149

701.10 Louvers, Grilles, and Screens . . . 149

701.11 Combustion Air Ducts. . . . . . . . . . 150

701.12 Dampers Prohibited . . . . . . . . . . . 150

702.0 Extra Device or Attachment . . . . . 150

702.1 General. . . . . . . . . . . . . . . . . . . . . 150

CHAPTER 8 CHIMNEYS AND VENTS. . . . . . . 151

801.0 General. . . . . . . . . . . . . . . . . . . . . 153

801.1 Applicability. . . . . . . . . . . . . . . . . . 153

801.2 Venting of Gas Appliances . . . . . . 153

801.3 Appliances Fueled by Other Fuels. 153

802.0 Venting of Appliances. . . . . . . . . . 153

xxxiv
CMC § 506.1.2 Medium 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 § 701.3 Medium 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 § 2025 Medium relevance — show source text
Chapter 7 Combustion Air. Chapter 7 regulates combustion air requirements for ventilation and dilution of flue gases for appliances installed in buildings. Fuel-gas appliances not regulated by chapter include direct vent appliances and Type I clothes dryers. Makeup air requirements for Type I clothes dryers are located in Chapter 5. Chapter 7 provides acceptable methods for supplying satisfactory combustion air to ensure proper combustion. Combustion air can be supplied by using indoor combustion air or by introducing the air from the outdoors.

Combustion is the rapid oxidation of fuel to release energy. The oxygen required to release the energy from the fuel normally comes from the air. Incomplete combustion of fuel occurs when inadequate oxygen is provided to the appliance. Combustion is needed to provide ventilation cooling for the casing and internal controls. When a lack of oxygen occurs, some of the carbon is not oxidized, and carbon monoxide forms.

Chapter 8 Chimneys and Vents. Chapter 8 regulates the installation, design, and construction of venting systems for fuel-burning appliances. The provisions addressed within this chapter follow procedures an installer would use to design or evaluate a venting system. Many requirements apply to the design and construction of venting systems, chimneys, installation of gas vents, and the sizing of venting system for a Category I appliance. Sizing venting systems require rigorous engineering calculations. However, the venting sizing requirements and sizing tables in this chapter already perform the calculations for the benefit of the end user.

Combustion appliances produce products of incomplete combustion, including potentially harmful carbon monoxide (CO). It is desirable to vent these products to the outdoors. Although the gas is clean-burning fuel, the products of combustion must not be allowed to collect within a building.

Chapter 9 Installation of Specific Appliances. Chapter 9 regulates the minimum requirements for the design, construction and installation of specific appliances. The provisions address the minimum requirements for gas-fired appliances, oil-fired appliances, wood-fired appliances, and electric-type appliances. In addition to the requirements of this chapter, appliances are also required to comply with the general requirements of Chapter 3.

Chapter 10 Boilers and Pressure Vessels. Chapter 10 regulates the construction, installation, operation, repair, and alteration of boilers and pressure vessels. The safety provisions within this chapter address controls and limit devices for automatic boilers, methods of determining expansion tank capacities, discharge piping, relief valves, shutoff valves, gas-pressure and combustion regulators, and inspections and tests. Potable water heaters are free from the requirements of Chapter 10 as they are within the scope of the Uniform Plumbing Code (UPC).

Pressure vessels store large amounts of energy and must comply with ASME Boiler and Pressure Vessel Code (BPVC) Section VIII.1. The stored energy must be contained to prevent disastrous failures. Boilers must comply with ASME BPVC Section I, ASME BPVC Section IV, or NFPA 85. Installing a safety relief valve and expansion tank prevents pressures in the tank from exceeding the design threshold.

Chapter 11 Refrigeration. Chapter 11 regulates the design, installation, and construction requirements of refrigeration systems and the installation and construction of cooling towers. Refrigeration is a method used for achieving heat transfer to cool spaces. Refrigerants are the most common medium used to transfer the heat energy from the low-temperature level to the high-temperature level. Table

2025 CALIFORNIA MECHANICAL CODE xv

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FORMAT OF THE UNIFORM MECHANICAL CODE
CMC § 904.1 Medium relevance — show source text
904.1 Application. Central heating furnaces and boilers shall be listed in accordance with the following:

(1) Central heating furnaces and boilers having input ratings up to and including 400 000 Btu/hr (117 kW) shall be listed in accordance with the following as applicable:

(a) Furnaces listed in accordance with CSA/ANSI Z21.47/CSA 2.3.

»

»

902.13 Process Air. In addition to air needed for combus tion in commercial or industrial processes, process air shall be provided as required for cooling of appliances, equipment, or material; for controlling dew point, heating, drying, oxidation, dilution, safety exhaust, odor control, and air for compressors; and for comfort and proper working conditions for personnel. [NFPA 54:9.1.7]

902.14 Gas Appliance Pressure Regulators. Where the gas supply pressure is higher than that at which the appliance is designed to operate or varies beyond the design pressure limits of the appliance, a gas appliance pressure regulator listed in accordance with CSA/ANSI Z21.18/CSA 6.3 shall

be installed. [NFPA 54:9.1.17]

902.15 Bleed Lines for Diaphragm-Type Valves. Bleed lines shall comply with the following requirements:

(1) Diaphragm-type valves shall be equipped to convey bleed gas to the outdoors or into the combustion chamber adjacent to a continuous pilot.

(2) In the case of bleed lines leading outdoors, means shall be employed to prevent water from entering this piping and also to prevent blockage of vents by insects and foreign

matter.

(3) Bleed lines shall not terminate in the appliance flue or exhaust system.

(4) In the case of bleed lines entering the combustion chamber, the bleed line shall be located so the bleed gas is readily ignited by the pilot and the heat liberated thereby does not adversely affect the normal operation of the safety shutoff system. The terminus of the bleed line shall be securely held in a fixed position relative to the pilot. For manufactured gas, the need for a flame arrester in the bleed line piping shall be determined.

(5) A bleed line(s) from a diaphragm-type valve and a vent line(s) from an appliance pressure regulator shall not be connected to a common manifold terminating in a combustion chamber. Bleed lines shall not terminate in positive-pressure-type combustion chambers. [NFPA 54:9.1.18]

903.0 Air-Conditioning Appliances.

903.1 Electric Air Conditioners. Electric air conditioning systems designed for permanent installation shall comply with UL 1995 or UL 60335-2-40.

903.2 Gas-Fired Air Conditioners and Heat Pumps. Gas-fired air conditioners shall comply with Section 903.2.1 through Section 903.2.7.

903.2.1 Application. Gas-fired air conditioners and heat pumps shall be listed in accordance with ANSI Z21.40.1/CGA 2.91 or ANSI Z21.40.2/CGA 2.92.

[NFPA 54:10.2.1]

903.2.2 Independent Gas Piping. Gas piping serving heating appliances shall be permitted to also serve cooling appliances where heating and cooling appliances cannot be operated simultaneously. [NFPA 54:10.2.2]

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CMC § 0.120 Medium 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 § 9.3.4 Medium 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 § 2025 Medium relevance — show source text
Systems that carry nonabrasive exhaust, such as smoke, moderate abrasives such as sawdust, and high abrasives such as manganese or acid vapors use product-conveying ducts. Part II provides the minimum fire safety requirements related to the design, installation, inspection, and maintenance of grease-type operations, such as cooking, for both fuel-gas and solid fuel. Cooking produces a significant amount of smoke, fumes, vapors, heat, and other pollutants. Therefore, acceptable kitchen ventilation is necessary to prevent fires from the build-up of grease and to provide the occupants protection from smoke, unpleasant odors, pollutants, and dangerous gases. There are two types of exhaust hoods (Type I and Type II) used in commercial kitchen applications.

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FORMAT OF THE UNIFORM MECHANICAL CODE

Type I hoods are intended to be installed above equipment or appliances that generate grease or smoke. Type II hoods are intended to be installed above equipment or appliances that generate steam, vapor, heat, or odors and do not generate grease or smoke.

Chapter 6 Duct Systems. Chapter 6 regulates requirements for ducts and plenums that are portions of a heating, cooling, ventilation, or exhaust system. This chapter contains material and installation requirements for metal, gypsum, factory-made, flexible, and plastic ducts. It also contains provisions for duct insulation, dampers, fire protection, and automatic shutoff for the building’s air distribution system. It is worth noting that exhaust ducts are regulated by Chapter 5, and combustion air ducts are regulated by Chapter 7.

Chapter 7 Combustion Air. Chapter 7 regulates combustion air requirements for ventilation and dilution of flue gases for appliances installed in buildings. Fuel-gas appliances not regulated by chapter include direct vent appliances and Type I clothes dryers. Makeup air requirements for Type I clothes dryers are located in Chapter 5. Chapter 7 provides acceptable methods for supplying satisfactory combustion air to ensure proper combustion. Combustion air can be supplied by using indoor combustion air or by introducing the air from the outdoors.

Combustion is the rapid oxidation of fuel to release energy. The oxygen required to release the energy from the fuel normally comes from the air. Incomplete combustion of fuel occurs when inadequate oxygen is provided to the appliance. Combustion is needed to provide ventilation cooling for the casing and internal controls. When a lack of oxygen occurs, some of the carbon is not oxidized, and carbon monoxide forms.

Chapter 8 Chimneys and Vents. Chapter 8 regulates the installation, design, and construction of venting systems for fuel-burning appliances. The provisions addressed within this chapter follow procedures an installer would use to design or evaluate a venting system. Many requirements apply to the design and construction of venting systems, chimneys, installation of gas vents, and the sizing of venting system for a Category I appliance. Sizing venting systems require rigorous engineering calculations. However, the venting sizing requirements and sizing tables in this chapter already perform the calculations for the benefit of the end user.

Combustion appliances produce products of incomplete combustion, including potentially harmful carbon monoxide (CO). It is desirable to vent these products to the outdoors. Although the gas is clean-burning fuel, the products of combustion must not be allowed to collect within a building.
CMC § 9.1.18 Medium relevance — show source text
(4) In the case of bleed lines entering the combustion chamber, the bleed line shall be located so the bleed gas is readily ignited by the pilot and the heat liberated thereby does not adversely affect the normal operation of the safety shutoff system. The terminus of the bleed line shall be securely held in a fixed position relative to the pilot. For manufactured gas, the need for a flame arrester in the bleed line piping shall be determined.

(5) A bleed line(s) from a diaphragm-type valve and a vent line(s) from an appliance pressure regulator shall not be connected to a common manifold terminating in a combustion chamber. Bleed lines shall not terminate in positive-pressure-type combustion chambers. [NFPA 54:9.1.18]

903.0 Air-Conditioning Appliances.

903.1 Electric Air Conditioners. Electric air conditioning systems designed for permanent installation shall comply with UL 1995 or UL 60335-2-40.

903.2 Gas-Fired Air Conditioners and Heat Pumps. Gas-fired air conditioners shall comply with Section 903.2.1 through Section 903.2.7.

903.2.1 Application. Gas-fired air conditioners and heat pumps shall be listed in accordance with ANSI Z21.40.1/CGA 2.91 or ANSI Z21.40.2/CGA 2.92.

[NFPA 54:10.2.1]

903.2.2 Independent Gas Piping. Gas piping serving heating appliances shall be permitted to also serve cooling appliances where heating and cooling appliances cannot be operated simultaneously. [NFPA 54:10.2.2]

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INSTALLATION OF SPECIFIC APPLIANCES

904.3.3 Listed and Unlisted Units. Central heating furnaces and low-pressure boilers shall be permitted to be installed with reduced clearances to combustible material, provided that the combustible material or appliance is protected as described in Table 303.10.1 and Figure 303.10.1(1) through Figure 303.10.1(3), and such reduction is allowed by the manufacturer’s installation instructions. [NFPA 54:10.3.3.3]

904.3.4 Front Clearance. Front clearance shall be sufficient for servicing the burner and the furnace or boiler.

[NFPA 54:10.3.3.4]

904.3.5 Adjacent to Plaster or Noncombustible Materials. Where the furnace plenum is adjacent to plaster on metal lath or noncombustible material attached to combustible material, the clearance shall be measured to the surface of the plaster or other noncombustible finish where the clearance specified is 2 inches (51 mm) or less. [NFPA 54:10.3.3.5]

904.3.6 Interference. The clearances to these appliances shall not interfere with combustion air, draft hood clearance and relief, and accessibility for servicing.
CMC § 1106.2.5 Medium relevance — show source text
(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

(2) Be activated by one or more refrigerant detectors, conforming to requirements of Section 1106.11.8. [ASHRAE 15:8.13.6.1] 1106.11.6.2 Detection System. Detection of refrigerant concentration that exceeds 25 percent of the LFL or the upper detection limit of the refrigerant detector, whichever is lower, shall automatically de-energize the following equipment in the machin ery room:

(1) Refrigerant compressors

(2) Refrigerant pumps

(3) Normally closed automatic refrigerant valves

(4) Other unclassified electrical sources of ignition with apparent power rating greater than 1 kVA, where the apparent power is the product of the circuit voltage and current rating. [ASHRAE 15:8.13.6.2] 1106.11.7 Mechanical Equipment Control. Remote control of the mechanical equipment in the refrigerating machinery room shall be provided immediately outside the machinery room door solely for the purpose of shutting down the equipment in an emergency. Ventilation fans shall be on a separate electrical circuit and have a control switch located immediately outside the machinery room door. [ASHRAE 15:8.13.7] 1106.11.8 Refrigerant Detectors. Each refrigerating machinery room in accordance with Section 1106.11 shall contain one or more refrigerant detectors in accordance with Section 1106.11.9. The detector(s) sensing element shall be located in areas where refrigerant from a leak will concentrate, with one or more set points that activate responses in accordance with Section 1106.11.10 for alarms and Section 1106.11.11 for mechanical ventilation. Multiport-type devices shall be prohibited. {ASHRAE 15:8.13.8} 1106.11.9 Refrigerant Detectors Requirements. Refrigerant detectors required by Section 1106.11 shall meet all of the following conditions:

CMC § 1.11.0. Medium relevance — show source text

Adopting Agency	BSC	BSC- CG	SFM	HCD	Col6	Col7	DSA	Col9	Col10	OSHPD	Col12	Col13	Col14	Col15	Col16	Col17	BSCC	DPH	AGR	DWR	CEC	CA	SL	SLC
Adopting Agency	BSC	BSC- CG	SFM	1	2	1-AC	AC	SS	SS/CC	1	1R	2	3	4	5	6	6	6	6	6	6	6	6	6
Adopt Entire Chapter	X		X	X	X			X		X	X	X	X	X	X	X		X
Adopt Entire Chapter as amended (amended sections listed below)
Adopt only those sections that are listed below
Chapter/Section

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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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]

Frequently asked questions

What exactly does “prevent any difference in pressure” mean for my water heater?

It means a draft hood or barometric regulator must be located inside the same room or enclosure as the appliance’s combustion-air source so the hood/regulator is exposed to the same pressure as the combustion air. That prevents backdrafting or spillage (§ 701.2) .

If I have a powerful kitchen hood, how much makeup air must I provide?

You must provide makeup air whenever the exhaust interferes with appliance operation (§ 701.3) . Practically, supply enough makeup air to prevent depressurization of the appliance room — often equal to the hood exhaust flow — and ensure any mechanical combustion-air supply meets the minimum 0.35 cfm per 1,000 Btu/h when used (§ 701.9) .

Are direct-vent appliances covered by these rules?

No. Direct-vent appliances are excluded from the chapter’s combustion-air requirements because they take combustion air directly from outdoors (see § 701.1 exception) .

Can I rely on building leakage (infiltration) for combustion air?

You can if you use the known-infiltration method in § 701.4.2, but you must use the correct equations and not assume an infiltration rate greater than 0.60 ACH in calculations (§ 701.4.2(3)) .

If my mechanical makeup-air system serves both ventilation and combustion, what must I do?

The system must provide the specified combustion-air rate in addition to required ventilation and must be interlocked so appliances cannot operate if the mechanical combustion-air supply fails (§ 701.9.2–701.9.3) .

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