HVAC Combustion Air and Makeup Air — CMC 2022 LA Guide (2026)
California Mechanical Code 2022 §§701 through 709 — the combustion air and makeup air provisions — determine whether your gas water heater, furnace, or range hood will pass LADBS inspection. The rules are quantitative: 1 square inch of net free area per 4,000 BTU input for high-ventilation openings, commercial range hoods above 400 CFM trigger a mechanical makeup-air (MUA) unit, and garage return-air connections are entirely prohibited under §602.3. CEC 2025 Title 24 Part 6 §150.0(o) layers on top with its own kitchen-ventilation efficacy requirements. This guide lays out the calculations, the common inspection failures, and the 2026 cost impact on LA kitchen and mechanical-room designs.
CMC §701 — the combustion-air definition
CMC §701 defines combustion air as air supplied to fuel-burning appliances for combustion. The code distinguishes between appliances within the thermal envelope (indoor), in unconditioned spaces (garage, attic, crawl space), and outdoor-installed.
Fuel-burning appliances subject to combustion-air requirements: gas furnaces, gas water heaters, gas fireplaces (vented), gas cooktops (usually exempt under §701 but still subject to CEC ventilation), oil-fired appliances, and wood-burning fireplaces with mechanical draft.
Direct-vent (sealed-combustion) appliances pull combustion air directly from outdoors through a dedicated intake. These are exempt from §701 combustion-air sizing because they do not depend on indoor air for combustion. The 2026 high-efficiency water-heater market is dominated by direct-vent models (Rinnai, Rheem tankless, Navien) that sidestep the §701 sizing entirely.
Power-vent appliances (fan-forced exhaust but atmospheric combustion-air intake) still require §701-compliant combustion air. The fan only handles exhaust, not intake.
§701.6 sizing — the 1 sq in per 4,000 BTU rule
CMC §701.6.2.1 (All Air from Outdoors, Two-Opening Method) requires two openings: one within 12 inches of the top of the enclosure and one within 12 inches of the bottom. Each opening must have a net free area of not less than 1 square inch per 4,000 BTU per hour of total input rating.
Example: a 40,000 BTU tank water heater plus a 60,000 BTU gas furnace totaling 100,000 BTU needs 25 square inches per opening — a 5-by-5 grille or equivalent louver net free area.
CMC §701.6.2.2 (All Air from Outdoors, One-Opening Method) allows a single opening within 12 inches of the top of the enclosure. The single opening must have a net free area of 1 square inch per 3,000 BTU (larger than the two-opening method) and must not be less than the sum of the appliance connector cross-sections.
Louver net free area is typically 60 to 75 percent of gross opening area for metal louvers and 20 to 25 percent for wood louvers. The net-free-area correction is what inspectors verify — a 5-by-5 gross louver with 60 percent NFA provides only 15 square inches of net free area, failing the 25 square inch requirement.
CEC 2025 Title 24 §150.0(o) — kitchen ventilation overlay
CEC 2025 Title 24 Part 6 §150.0(o) requires every kitchen with gas cooking to have a range hood that meets specific CFM by kitchen floor area. The 2026 numbers: kitchens over 1,500 sqft need 180 CFM (gas) or 110 CFM (electric induction) hoods; kitchens 1,000 to 1,500 sqft need 250/110; kitchens 750 to 1,000 sqft need 280/130; kitchens under 750 sqft need 280/160.
All range hoods must duct to the exterior. Recirculating hoods do not meet code for gas cooking. The duct-termination location must not reintroduce the exhaust air into building openings — minimum 3 feet horizontal from windows and 10 feet from property lines per CMC §504.
The range-hood CFM triggers the makeup-air requirement: CMC §505.8.1 requires mechanical makeup air for exhaust systems moving more than 400 CFM. A 600 CFM residential hood in a new kitchen requires a 600 CFM MUA fan that supplies outdoor air at balanced pressure.
Residential kitchens with hoods under 400 CFM avoid the MUA requirement, which is why most 2026 residential kitchen designs land at 395 CFM hoods — slightly below the trigger — unless the homeowner specifically wants a higher-CFM commercial-style hood.
Commercial kitchen hood MUA — the §508 full trigger
CMC §508 governs commercial hood makeup air. Any commercial kitchen hood over 400 CFM requires mechanical makeup air at 80 to 100 percent of the hood's exhaust CFM.
The MUA system must be interlocked with the hood exhaust — the MUA fan starts when the hood fan starts and stops when the hood fan stops. The MUA must be tempered (heated in winter, cooled in summer when air conditioning is present) to avoid drafts at the cook line.
2026 MUA unit pricing for a 1,500 CFM commercial kitchen hood: $14,000 to $22,000 for the MUA unit, $8,000 to $14,000 for ductwork and roof-curb installation, $2,400 to $4,800 for the electrical and controls. Total MUA install often doubles the hood project cost.
Residential 'commercial-style' hoods (Wolf Pro, Viking Pro, Zephyr) over 600 CFM trigger MUA when installed in a residential setting per LADBS interpretation of CMC §508. The 2026 MUA requirement for a 1,200 CFM Wolf residential hood is $8,000 to $14,000 installed.
Garage return-air prohibition — §602.3
CMC §602.3 prohibits return-air openings in garages. A forced-air furnace installed in a garage cannot draw return air from the garage space, and a return duct from living space cannot pass through the garage without being sealed per CBC §406.3.4.
The rationale: carbon monoxide from vehicles and gasoline vapor from stored containers must not be distributed through the HVAC system into living spaces. The return-air prohibition is absolute — no exceptions for 'well-ventilated garages.'
Common violation: existing homes where a 1970s-era forced-air furnace in the garage was designed with garage-return and has been operating out-of-compliance for decades. Remodels that touch the mechanical system trigger compliance with current code, requiring relocation of the return-air intake or relocation of the furnace entirely.
Furnace relocation options: move the furnace to an exterior closet with outdoor combustion air, move to an interior mechanical closet with dedicated return-air from the living space, or replace with a split-system heat pump that has no combustion component. Relocation costs $4,800 to $12,000 depending on ducting complexity.
Attic combustion-air — §701.6.3
CMC §701.6.3 allows combustion air to be supplied from a ventilated attic. The attic must have unrestricted communication with outdoors (standard vented attic with sufficient soffit and ridge vents per CBC §1203).
Combustion-air ducts from the attic to the appliance enclosure must be sized per §701.6.3.1: 1 square inch of net free area per 4,000 BTU input, with a minimum of 100 square inches for any single duct.
Unvented attics (as increasingly specified for high-performance homes using spray foam at the roof deck) CANNOT provide combustion air under §701.6.3. Homes with unvented attics require outdoor-sourced combustion air or direct-vent appliances.
Attic-sourced combustion-air installations require access for inspection — a 22-by-30 inch attic access opening within 20 feet of the appliance per §304.1.
Crawl-space combustion-air and alternative paths
CMC §701.6.2.3 allows combustion air from a crawl space provided the crawl space has unrestricted communication with outdoors and meets the §701.6 sizing. Crawl-space openings to the outside must total 1 square foot per 25 linear feet of foundation wall per CBC §1203.3.
Many LA homes built before 1995 have insufficient crawl-space ventilation for combustion-air compliance. Retrofits add automatic foundation vents (adjustable louvers that open at high temperatures for cooling and close at low temperatures).
Combination indoor-outdoor air methods per §701.6.2.4 are allowed but rarely specified in LA because the calculation is complex and inspectors prefer the straightforward outdoor-only or attic-only methods.
Mechanical combustion-air supply (a fan-powered combustion-air intake) per §701.6.2.5 is occasionally specified for mechanical rooms that cannot provide natural-flow combustion air. The fan must be interlocked with the appliance ignition — the fan starts before the appliance lights and remains on for the duration of operation plus a post-purge period.
2026 permit and inspection picture
LADBS mechanical permits required for: any new or replacement furnace, any new or replacement water heater (note: like-for-like water-heater swaps can qualify for Express permit), any new hood installation or hood replacement changing CFM, any ductwork modification.
2026 mechanical permit fees: $180 to $380 for a water-heater-only permit, $280 to $620 for a furnace and ductwork permit, $540 to $1,200 for a full kitchen-hood project including MUA.
Inspection sequence: rough-in inspection (duct, gas piping, combustion-air openings visible), final inspection (appliances connected, operational test with CO and draft measurement for atmospheric appliances).
Common inspection failures: insufficient combustion-air opening size, missing CO alarms per CBC §420 within bedrooms where gas appliances are nearby, improper hood duct termination, and MUA fan not interlocked with hood fan.
For homeowners integrating HVAC and combustion-air planning into a full kitchen remodel scope, see the kitchen remodeling service page at https://askbaily.com/kitchen-remodeling-los-angeles.
Still have questions?
Ask Baily — pre-seeded for this topic.
Loading chat…
Who is Baily?
Baily is named after Francis Baily — an English stockbroker who retired at 51, became an astronomer, and in 1836 described something on the edge of a solar eclipse that nobody had properly articulated before: a string of bright beads of sunlight breaking through the valleys along the moon’s rim.
He wasn’t the first to see them. Edmond Halley saw them in 1715 and barely noticed. Baily’s contribution was clarity — describing exactly what was happening, in plain language, so vividly that the whole field of astronomy paid attention. The phenomenon is still called Baily’s beads.
That’s what we wanted our AI to do. Every inbound call and text has signal in it — a homeowner’s real question, a timeline, a budget, a hesitation that means “yes but.” Baily listens to every one, 24/7, and finds the beads of light.
Baily was a businessman before he was a scientist. That’s our vibe too.