CMC · California Mechanical Code
Are dampers allowed in combustion air openings?
In plain English: the California Mechanical Code (CMC **§ 701.12**) forbids manual dampers in combustion‑air openings. You may use motorized louvers only with prior approval and only if they are electrically interlocked so the appliance cannot receive fuel unless those louvers are fully open; also avoid routing combustion air through locations that would require fire dampers .
Last reviewed: July 6, 2026
What the code requires — 2–4 sentences
The short answer: No — manual dampers are prohibited in combustion air openings. The California Mechanical Code explicitly states that combustion air ducts or plenums shall not be installed so as to require openings in or penetrations through construction where fire dampers are required, and that manually operated dampers shall not be installed in combustion air openings. With prior approval, power‑actuated movable louvers are permitted only if they are electrically interlocked so fuel cannot be supplied unless the louvers are fully open (§ 701.12) .
The single most important rule: manual dampers are not allowed in combustion‑air openings — only approved motorized louvers are permitted and then only when interlocked to prevent ignition unless fully open (see § 701.12) .
Requirements in detail
Prohibitions and routing
- Combustion air ducts or plenums must not be routed in a way that would force combustion‑air openings to be placed where fire dampers are required; i.e., avoid penetrating fire‑resistance assemblies in a way that would require fire damper protection (§ 701.12) .
- Manually operated dampers are expressly forbidden in combustion air openings (§ 701.12) .
When powered louvers are allowed
- Power‑actuated movable louvers may be used only with prior approval and must be electrically interlocked with the main burner fuel‑supply valve so fuel delivery is prevented unless the louvers are in the fully open position (§ 701.12) .
- For larger burner installations and specialized burners, supervised startup / proof‑of‑air interlocks and proof‑of‑open checks may also be required by other provisions or by the Authority Having Jurisdiction (see Appendix requirements for high‑input burners) .
Decision‑relevant summary table
| Decision item | Allowed? | Key requirement / value | Code Reference |
|---|---|---|---|
| Manual damper in combustion air opening | No | Manual dampers are prohibited (do not install) | § 701.12 |
| Power‑actuated movable louver | Yes, only with prior approval | Must be electrically interlocked to prevent fuel delivery unless louvers are fully open | § 701.12 |
| Routing combustion air through penetrations requiring fire dampers | No | Combustion air ducts/plenums shall not be installed so openings/penetrations would require fire dampers | § 701.12 |
| Proof‑of‑open for large burners / dampered openings | Depends | Installations with dampered combustion‑air openings commonly require proof‑of‑open before burner ignition (see supervisory startup / interlock rules) | Appendix C and AHJ requirements |
| Minimum vertical clearance for exterior intake lowest side | N/A to dampers but decision‑relevant for intake location | 12 inches (305 mm) above adjacent finished ground level | (combustion air intake location rules) |
How to comply (practical checklist)
- Do not supply combustion air through a grille or duct with a manual damper.
- If you plan to use a motorized louver, obtain AHJ approval and provide an electrical interlock that proves the louver is fully open before permitting fuel flow.
- Avoid routing combustion air ducts through fire‑resistance‑rated assemblies where a fire damper would otherwise be required.
- Provide documented proof (sensors/controls) that the powered louvers are in the full‑open position prior to burner ignition when required by appliance controls or AHJ.
Exceptions & special cases
- The Code allows power‑actuated movable louvers only with prior approval and with the interlock described; there is no general exception permitting manual dampers in any combustion‑air opening (§ 701.12) .
- Some appliance installations (high‑input burners, special fuel operations) have additional startup, interlock, or proving requirements in other sections/appendices; those may require supervised startup or proof‑of‑air/damper open before ignition — check the appliance‑specific controls and Appendix C language and consult the AHJ for applicability .
- The prohibition on manual dampers is about the combustion‑air opening itself; unrelated required dampers (e.g., backdraft dampers for ventilation fans) are governed elsewhere (see energy/ventilation sections) and do not override § 701.12 .
Common mistakes
- Installing a manual damper on an outside louver or intake grille because “it reduces drafts” — this is prohibited by § 701.12 .
- Assuming a screen or fixed louver equals a damper and is therefore acceptable; screens/louvers reduce free area and must be accounted for in sizing, but a manually operable damper is not permitted (see sizing and free‑area rules elsewhere) .
- Routing combustion air through a shaft or duct that penetrates fire barriers and then adding a fire damper — the Code disallows arranging combustion air ducts so they would require fire‑dampers (§ 701.12) .
- Using a motorized louver without an interlock that prevents fuel flow when the louver is closed or partially open — that defeats the safety purpose and violates § 701.12 .
Worked example — concrete scenario
Scenario: A contractor is installing a gas furnace (80,000 Btu/h) in a small mechanical room and wants to provide outdoor combustion air via a wall louver. The installer proposes mounting a manual butterfly damper behind the louver to “balance” flows.
Application of the rule:
- The Code forbids manually operated dampers in combustion air openings — the proposed manual butterfly damper is not allowed (§ 701.12) .
- Options compliant with the Code:
- Remove the manual damper and provide a fixed louver or screened opening sized for the required free area (size per combustion‑air sizing rules found elsewhere).
- Or (with prior approval) install a power‑actuated louver that is electrically interlocked so the furnace’s main fuel valve cannot open unless the louver controller proves the blades are fully open — provide wiring/controls and documentation to the AHJ demonstrating the interlock function (§ 701.12) .
- For larger or special burners, the AHJ may require that the system demonstrate proof‑of‑open during supervised startup or have proof‑of‑air interlocks incorporated into the burner control scheme (see Appendix / appliance control rules) .
Result: The installer must either eliminate the manual damper or replace it with an approved, interlocked motorized louver and receive AHJ approval and acceptance of the interlock scheme.
Related provisions
- § 701.12 — Dampers Prohibited (primary controlling section)
- § 702.0 — Extra Device or Attachment (heading for related device limitations)
- § 702.1 — General (no device/attachment shall impair combustion of gas)
- Chapter 7 — Combustion Air (general context and sizing rules; see the Chapter 7 discussion)
- Appendix C / C109 (supervised startup / proof‑of‑air interlocks for certain burners) — informative but may apply to larger burners or as required by AHJ
Code references
Grounded in the retrieved California Mechanical Code — click a citation to read the verbatim passage:
CMC § 9.3.8.7 High relevance — show source text
Exception: Direct vent appliances designed for installation in a solid fuel-burning fireplace where installed in accordance with the manufacturer’s installation instructions. [NFPA 54:9.3.8.7]
(8) Combustion air intake openings located on the exterior of the building shall have the lowest side of the combustion air intake openings located at least 12 inches (305 mm) vertically from the adjoining finished ground level.
[NFPA 54:9.3.8.8]
701.12 Dampers Prohibited. Combustion air ducts or plenums shall not be installed so as to require openings in or penetrations through construction where fire dampers are required. Manually operated dampers shall not be installed in combustion air openings. With prior approval, power-actuated movable louvers admitting combustion air shall be permitted to be used and, where installed, shall be electrically interlocked with the main burner fuel-supply valve so as to prevent fuel delivery unless the louvers are in the fully open position.
702.0 Extra Device or Attachment.
702.1 General. No device or attachment shall be installed on any appliance that could in any way impair the combustion of gas. [NFPA 54:9.1.15]
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CALIFORNIA MECHANICAL CODE – MATRIX ADOPTION TABLE
CHAPTER 8 – CHIMNEYS AND VENTS
(Matrix Adoption Tables are non-regulatory, intended only as an aid to the code user. See Chapter 1 for state agency authority and building applications.)
Adopting Agency BSC BSC-
CGSFM 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-
CGSFM 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 Adopt Entire Chapter as
amended (amended sections
listed below)Adopt only those sections
that are listed belowChapter/Section 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 § 9.3.8.1 High relevance — show source text
Exception: Within dwellings units, unobstructed stud and joist spaces shall not be prohibited from conveying combustion air, provided that not more than one fireblock is removed. [NFPA 54:9.3.8.1]
(2) Ducts shall terminate in an unobstructed space, allowing free movement of combustion air to the appliances.
[NFPA 54:9.3.8.2]
(3) Ducts shall serve a single space. [NFPA 54:9.3.8.3]
(4) Ducts shall not serve both upper and lower combustion air openings where both such openings are used. The separation between ducts serving upper and lower combustion air openings shall be maintained to the source of combustion air. [NFPA 54:9.3.8.4]
(5) Ducts shall not be screened where terminating in an attic space. [NFPA 54:9.3.8.5]
(6) Horizontal upper combustion air ducts shall not slope downward toward the source of combustion air. [NFPA 54:9.3.8.6]
(7) The remaining space surrounding a chimney liner, gas vent, special gas vent, or plastic piping installed within a masonry, metal, or factory built chimney shall not be used to supply combustion air.
Exception: Direct vent appliances designed for installation in a solid fuel-burning fireplace where installed in accordance with the manufacturer’s installation instructions. [NFPA 54:9.3.8.7]
(8) Combustion air intake openings located on the exterior of the building shall have the lowest side of the combustion air intake openings located at least 12 inches (305 mm) vertically from the adjoining finished ground level.
[NFPA 54:9.3.8.8]
701.12 Dampers Prohibited. Combustion air ducts or plenums shall not be installed so as to require openings in or penetrations through construction where fire dampers are required. Manually operated dampers shall not be installed in combustion air openings. With prior approval, power-actuated movable louvers admitting combustion air shall be permitted to be used and, where installed, shall be electrically interlocked with the main burner fuel-supply valve so as to prevent fuel delivery unless the louvers are in the fully open position.
702.0 Extra Device or Attachment.
702.1 General. No device or attachment shall be installed on any appliance that could in any way impair the combustion of gas. [NFPA 54:9.1.15]
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CALIFORNIA MECHANICAL CODE – MATRIX ADOPTION TABLE
CHAPTER 8 – CHIMNEYS AND VENTS
(Matrix Adoption Tables are non-regulatory, intended only as an aid to the code user. See Chapter 1 for state agency authority and building applications.)
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).
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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 § 3663.4 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 § 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 § 3663.4 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 § 1.4.2. High relevance — show source text
Backdraft dampers. All fan systems, regardless of volumetric capacity, that exchange air between the building conditioned space and the outside of the building shall be provided with backdraft or automatic dampers to prevent unintended air leakage through the fan system when the fan system is not operating.
Gravity ventilation dampers. All gravity ventilating systems that serve conditioned space shall be provided with either automatic or readily accessible, manually operated dampers in all openings to the outside except combustion inlet and outlet air openings and elevator shaft vents.
Protection of insulation. Insulation shall be protected from damage, including that due to sunlight, moisture, equipment maintenance and wind, but not limited to the following: Insulation exposed to weather shall be suitable for outdoor service, e.g., protected by aluminum, sheet metal, painted canvas or plastic cover. Cellular foam insulation shall be protected as above or painted with a coating that is water retardant and provides shielding from solar radiation that can cause degradation of the material.
Porous inner core flex duct. Flexible ducts having porous inner cores shall have a non-porous layer or air barrier between the inner core and the outer vapor barrier.
Duct system sealing and leakage testing. When space-conditioning systems utilize forced air duct systems to supply conditioned air to an occupiable space, the ducts shall be sealed as confirmed through field verification and diagnostic testing in accordance with all applicable procedures specified in Reference Residential Appendix RA3.1, and conforming to one of the following Subsections A, B or C as applicable. Air handler airflow for calculation of duct leakage rate compliance targets shall be determined according to methods specified in Reference Residential Appendix RA3.1.4.2. A. For single-family dwellings and townhouses with the air-handling unit installed and the ducts connected directly to the air handler, the total leakage of the duct system shall not exceed 5 percent of the air handler airflow as determined utilizing the procedures in Reference Residential Appendix Section RA3.1.4.3.1. B. For single-family dwellings and townhouses at the rough-in stage of construction prior to installation of the dwelling’s interior finishing: i. Air-handling unit installed. If the air-handling unit is installed and the ducts are connected directly to the air handler, the total leakage of the duct system shall not exceed 5 percent of the air handler airflow as determined utilizing the procedures in Reference Residential Appendix Sections RA3.1.4.3.2, RA3.1.4.3.2.1 and RA3.1.4.3.3. ii. Air-handling unit not yet installed. If the air-handling unit is not yet installed, the total leakage of the duct system shall not exceed 4 percent of the air handler airflow as determined utilizing the procedures in Reference Residential Appendix Sections RA3.1.4.3.2, RA3.1.4.3.2.2 and RA3.1.4.3.3.
Air filtration.
A. System types specified in Subsections i, ii, and iii shall be provided with air filters in accordance with Sections 150.0(m)12B, 150.0(m)12C, and 150.0(m)12D. System types specified in Subsection i shall also comply with Section 150.0(m)12E.
i. Mechanical space-conditioning systems that supply air to an occupiable space through ductwork exceeding 10 feet (3 m) in length. ii. Mechanical supply-only ventilation systems and makeup air systems that provide outside air to an occupiable
space.
CMC § 160.3 High relevance — show source text
ii. For duct wrap, installed thickness shall be assumed to be 75 percent (25 percent compression) of nominal thickness.
iii. For factory-made flexible air ducts, the installed thickness shall be determined by dividing the difference between the actual outside diameter and nominal inside diameter by two. F. Duct labeling. Insulated flexible duct products installed to meet this requirement shall include labels, in maximum intervals of 3 feet, showing the thermal performance R- value for the duct insulation itself (excluding air films, vapor retarder or other duct components), based on the tests in Section 160.3(b)5D and the installed thickness determined by Section 160.3(b)5Eiii. G. Backdraft dampers. All fan systems, regardless of volumetric capacity, that exchange air between the building conditioned space and the outside of the building shall be provided with backdraft or automatic dampers to prevent unintended air leakage through the fan system when the fan system is not operating. H. Gravity ventilation dampers. All gravity ventilating systems that serve conditioned space shall be provided with either automatic or readily accessible, manually operated dampers in all openings to the outside except combustion inlet and outlet air openings and elevator shaft vents. I. Protection of insulation. Insulation shall be protected from damage, including that due to sunlight, moisture, equipment maintenance and wind but not limited to the following: Insulation exposed to weather shall be suitable for outdoor service (e.g., protected by aluminum, sheet metal, painted canvas or plastic cover). Cellular foam insulation shall be protected as above or painted with a coating that is water retardant and provides shielding from solar radiation that can cause degradation of the material. J. Porous inner core flex duct. Flexible ducts having porous inner cores shall have a nonporous layer or air barrier between the inner core and the outer vapor barrier. K. Duct system sealing and leakage testing. When space-conditioning systems utilize forced air duct systems to supply conditioned air to an individual dwelling unit, the ducts shall be sealed, as confirmed through field verification and diagnostic testing, in accordance with all applicable procedures specified in Reference Residential Appendix RA3.1. Air handler airflow for calculation of duct leakage rate compliance targets shall be determined according to methods specified in Reference Residential Appendix RA3.1.4.2. For multifamily dwellings with the air-handling unit installed and the ducts connected directly to the air handler, regardless of duct system location: i. The total leakage of the duct system shall not exceed 12 percent of the air handler airflow as determined utilizing the procedures in Reference Residential Appendix Section RA3.1.4.3.1; or ii. The duct system leakage to outside shall not exceed 6 percent of the air handler airflow as determined utilizing the procedures in Reference Residential Appendix Section RA3.1.4.3.4.
- Exception 1 to Section 160.3(b)5K: The field verification and ECC-Provider data registry requirements of Reference Residential Appendix RA2 and RA3 are not required for multifamily dwelling units in buildings four
216 2025 CALIFORNIA ENERGY CODE
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MULTIFAMILY BUILDINGS—MANDATORY REQUIREMENTS
habitable stories and greater. The installer shall certify that diagnostic testing was performed in accordance with the applicable procedures. Exception 2 to Section 160.3(b)5K: Multifamily dwelling units in buildings four habitable stories and greater in Climate Zones 1, 3, 5 and 7. L.
CMC § 90.1 High relevance — show source text
[ASHRAE 90.1: TABLE 6.4.3.4.3]
Col1 OUTDOOR AIR INTAKE (CFM/ft2) Col3 EXHAUST/RELIEF (CFM/ft2) Col5 CLIMATE ZONE NONMOTORIZED1 MOTORIZED NONMOTORIZED3 MOTORIZED 0, 1, 2 0, 1, 2 0, 1, 2 0, 1, 2 0, 1, 2 Any height 20 4 20 4 3 3 3 3 3 Any height 20 10 20 10 4, 5B, 5C 4, 5B, 5C 4, 5B, 5C 4, 5B, 5C 4, 5B, 5C Fewer than three stories 204 10 20 10 Three or more stories 204 10 204 10 5A, 6, 7, 8 5A, 6, 7, 8 5A, 6, 7, 8 5A, 6, 7, 8 5A, 6, 7, 8 Fewer than three stories 204 4 20 4 Three or more stories 204 4 204 4 For SI units: 1 square foot = 0.0929 m [2], 1 cubic foot per minute = 0.00047 m [3] /s, 1 cubic foot per minute = 0.4719 L/s, 1 cubic foot per minute per square foot = 5.08 [(L/s)/m [2] ], 1 inch water gauge = 0.249 kPa
Notes:
1 When tested in accordance with AMCA 500D.
2 Dampers smaller than 12 inches (305 mm) in height, width, or diameter need not be tested but shall be of the same design and construction as the smallest tested damper meeting the listed leakage rate requirement.
3 Nonmotorized dampers smaller than 24 inches (610 mm) in height, width, or diameter shall be permitted to have a leakage rate of 40 CFM/ft 2 [0.203 (m 3 /s)/m 2 ].
4 Where permitted by Section E 503.4.6.4.1, exception 2.
2025 CALIFORNIA MECHANICAL CODE 425
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APPENDIX E
(2) Systems serving zones where humidity levels are required to be maintained with precision of not more than ±5 percent relative humidity to comply with applicable codes or accreditation standards or as approved by the Authority Having Jurisdiction.
[ASHRAE 90.1:6.4.3.6.2]
CMC § 717.3.3.5 High relevance — show source text
717.3.3.5 Corridor damper actuation. Corridor damper actuation shall be in accordance with Sections 717.3.3.1 and 717.3.3.2.
717.4 Access and identification. Access and identification of fire and smoke dampers shall comply with Sections 717.4.1 through 717.4.2.
717.4.1 Access. Fire and smoke dampers shall be provided with an approved means of access that is large enough to permit inspection and maintenance of the damper and its operating parts. Dampers equipped with fusible links, internal operators, or both shall be provided with an access door that is not less than 12 inches (305 mm) square or provided with a removable duct section.
717.4.1.1 Access openings. The access shall not affect the integrity of fire-resistance-rated assemblies. The access openings shall not reduce the fire-resistance rating of the assembly. Access doors in ducts shall be tight fitting and suitable for the required duct construction.
717.4.1.2 Restricted access. Where space constraints or physical barriers restrict access to a damper for periodic inspection and testing, the damper shall be a single- or multi-blade type damper and shall comply with the remote inspection requirements of NFPA 80 or NFPA 105.
717.4.2 Identification. Access points shall be permanently identified on the exterior by a label having letters not less than [1] / 2 inch (12.7 mm) in height reading: “FIRE/SMOKE DAMPER,” “SMOKE DAMPER” or “FIRE DAMPER.”
717.5 Where required. Fire dampers, smoke dampers, combination fire/smoke dampers, ceiling radiation dampers and corridor dampers shall be provided at the locations prescribed in Sections 717.5.1 through 717.5.7 and 717.6. Where an assembly is required to have both fire dampers and smoke dampers, combination fire/smoke dampers or a fire damper and a smoke damper shall be provided.
717.5.1 Fire walls. Ducts and air transfer openings permitted in fire walls in accordance with Section 706.11 shall be protected with listed fire dampers installed in accordance with their listing.
717.5.1.1 Horizontal exits. A listed smoke damper designed to resist the passage of smoke shall be provided at each point a duct or air transfer opening penetrates a fire wall that serves as a horizontal exit.
717.5.2 Fire barriers. In other than Group A, E, H, I, L and R occupancies, high-rise buildings, and other applications listed in Section 1.11 regulated by the Office of the State Fire Marshal. Ducts and air transfer openings of fire barriers shall be protected with listed fire dampers installed in accordance with their listing. Ducts and air transfer openings shall not penetrate enclosures for interior exit stairways and ramps and exit passageways, except as permitted by Sections 1023.5 and 1024.6, respectively.
Exceptions: Fire dampers are not required at penetrations of fire barriers where any of the following apply:
- Penetrations are tested in accordance with ASTM E119 or UL 263 as part of the fire-resistance-rated assembly.
- Ducts are used as part of an approved smoke control system in accordance with Section 909 and where the use of a fire damper would interfere with the operation of a smoke control system.
CMC § 101.0 High relevance — show source text
2025 CALIFORNIA PLUMBING CODE 487
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488 2025 CALIFORNIA PLUMBING CODE
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APPENDIX J
COMBINATION OF INDOOR AND OUTDOOR COMBUSTION AND
VENTILATION OPENING DESIGN
The provisions contained in this appendix are not mandatory unless specifically adopted by a state agency, or referenced in the adopting ordinance.
J 101.0 General.
J 101.1 Applicability. This appendix provides general guidelines for the sizing of combination indoor and outdoor combustion and ventilation air openings. J 101.2 Example of Combination Indoor and Outdoor Combustion Air Opening Design. Determine the required combination of indoor and outdoor combustion air opening sizes for the following appliance installation example.
Example Installation: A fan-assisted furnace and a drafthood-equipped water heater with the following inputs are located in a 15 foot by 30 foot (4572 mm by 9144 mm) basement with an 8 foot (2438 mm) ceiling. No additional indoor spaces can be used to help meet the appliance combustion air needs.
Fan-Assisted Furnace Input: 100 000 British thermal units per hour (Btu/h) (29 kW)
Draft Hood-Equipped Water Heater Input: 40 000 Btu/h (11.7 kW)
Solution:
(1) Determine the total available room volume. Appliance room volume:
15 feet by 30 feet (4572 mm by 9144 mm) with an 8 foot (2438 mm) ceiling = 3600 cubic feet (101.94 m [3] )
(2) Determine the total required volume. The standard method to determine combustion air is used to calculate the required volume. The combined input for the appliances located in the basement is calculated as follows:
100 000 Btu/h (29 kW) + 40 000 Btu/h (11.7 kW) = 140 000 Btu/h (41 kW)
The standard method requires that the required volume be determined based on 50 cubic feet per 1000 Btu/h (4.83 m [3] /kW). Using Table J 101.2, the required volume for a 140 000 Btu/h (41 kW) combined input is 7000 cubic feet (198.22 m [3] ).
Conclusion: The indoor volume is insufficient to supply combustion air since the total of 3600 cubic feet (101.94 m [3] ) does not meet the required volume of 7000 cubic feet (198.22 m [3] ). Therefore, additional combustion air shall be provided from the outdoors.
(3) Determine ratio of the available volume to the required
volume: Col2 3600 cubic feet = 0.51 7000 cubic feet 7000 cubic feet (4) Determine the reduction factor to be used to reduce the full outdoor air opening size to the minimum required based on the ratio of indoor spaces:
1.00 – 0.51 (from Step 3) = 0.49
(5) Determine the single outdoor combustion air opening size as though all combustion air is to come from outdoors. In this example, the combustion air opening directly communicates with the outdoors:
Frequently asked questions
Can I install a manual damper upstream of the combustion air opening (in the duct)?
No. The Code prohibits manually operated dampers in combustion air openings and bars configurations that would effectively use manual dampers to control combustion air that serves appliances (§ 701.12) .
Are motorized louvers automatically acceptable?
No. Motorized (power‑actuated) louvers are allowed only with prior approval and must be electrically interlocked so the main burner fuel valve cannot open unless the louvers are fully open (§ 701.12) .
What does “electrically interlocked” mean in practice?
It means the louver position must be proven (via a switch or position sensor) and tied into the burner/fuel‑valve control so fuel flow or ignition is prevented unless the louver is in the fully open position, per § 701.12 .
My duct passes through a fire‑rated floor — can I put a combustion‑air duct there if I use fire dampers?
No. The Code says combustion air ducts or plenums shall not be installed so as to require openings or penetrations where fire dampers are required — avoid routing combustion air through such assemblies (§ 701.12) .
Who decides when a power‑actuated louver is acceptable?
The Authority Having Jurisdiction (AHJ) must give prior approval for power‑actuated louvers and will review the proposed interlock and controls to ensure safety and code compliance (§ 701.12) .
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