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Building Code Reference

Got a CMC 701.6 correction?

Combustion air openings

For builders in California, Greater Seattle, the Las Vegas area, the Denver area and Kansas City · CrossBeam Permits

What this code requires, how to fix it, and how cities and counties review it

What this code requires

CMC 701.6 is the California Mechanical Code section that governs how combustion air openings must be sized for fuel-burning appliances installed in confined spaces. When a furnace, boiler, or water heater sits in a sealed closet or small mechanical room, the space has to have openings or ducts that supply enough air for safe combustion. Most residential and light-commercial permits that include a new or relocated gas appliance in a utility closet get flagged here when the drawings do not show those openings or do not size them correctly.

How to fix it

  1. Calculate whether your space is confined

    Add up the total rated Btu/hr input for every fuel-burning appliance in the room. Divide the room interior volume (length x width x ceiling height, in cubic feet) by that total input in thousands of Btu/hr. If the result is less than 50 cubic feet per 1,000 Btu/hr, the space is confined and CMC 701.6 applies. For example: a 96 cu ft closet with a 60,000 Btu/hr furnace plus a 40,000 Btu/hr water heater gives 96 / 100 = 0.96 cu ft per 1,000 Btu/hr. That is well below 50, so you need combustion air openings. Note the calculation on the plans.

  2. Choose a combustion air method and note it on the drawings

    CMC Chapter 7 gives you two main paths. The indoor air method (CMC 701.5) brings air in from adjacent interior spaces through two openings: one within 12 inches of the top of the enclosure, one within 12 inches of the bottom. The outdoor air method (CMC 701.6) brings air from outside through openings or ducts. Pick one and label it on the mechanical sheet before sizing. If the closet is interior with no exterior wall, indoor air is usually simpler. If the closet is on an exterior wall, direct outdoor air can give you a smaller duct.

  3. Size indoor-air openings at 1 sq in per 1,000 Btu/hr, 100 sq in minimum

    For the indoor air method, each opening (high and low) must provide 1 square inch of free area per 1,000 Btu/hr of total appliance input, with a minimum of 100 square inches per opening. A 100,000 Btu/hr combined load needs 100 square inches free area per opening. Note: free area accounts for louver blade blockage. A standard fixed louver is typically 25 to 60 percent free area. If you are using a louvered grille rated at 50 percent free area, specify a grille that is 200 square inches total face area to deliver 100 square inches free. Label the free area and face area separately on the detail.

  4. Size outdoor-air openings per CMC 701.6.1 or 701.6.2

    For the outdoor air method, opening size depends on total Btu/hr input and how the air arrives. With two openings (CMC 701.6.1), each needs 1 sq in per 4,000 Btu/hr directly or through vertical ducts, or 1 sq in per 2,000 Btu/hr through horizontal ducts. One opening near the top (CMC 701.6.2) needs 1 sq in per 3,000 Btu/hr and at least the total vent connector area. At 100,000 Btu/hr, two vertical-duct openings need 25 sq in each. Show the duct route, size, and termination on the plans.

  5. Add a combustion air detail to the mechanical sheet

    A note that says combustion air per code is almost always insufficient and will get you another correction. Instead, add a boxed detail on the mechanical sheet showing the closet dimensions, the appliances and their input ratings, the opening or duct locations, sizes, and free-area calculations. Include the method name (indoor or outdoor), the openings heights above finished floor, and the free-area values. One clear detail eliminates the ambiguity that causes re-reviews.

  6. Verify the adjacent space is adequate for indoor-air method

    If you are using indoor air, the adjacent space that supplies the air must itself not be confined. If both the utility closet and the room next to it are small spaces, the plan-checker may flag that neither can supply adequate air. The adjacent space needs to have enough volume or its own connection to outside air. Document the adjacent room dimensions and confirm the combined volume meets the 50 cu ft per 1,000 Btu/hr threshold, or show that the adjacent space is openly connected to additional volume.

  7. Check for direct-vent sealed combustion exceptions

    If the furnace or water heater is a direct-vent (sealed combustion) appliance that pulls all its combustion air from outside through a dedicated sealed pipe, CMC 701.1 exempts direct-vent appliances from the Chapter 7 combustion air rules because the combustion air never comes from the room. If every appliance in the space is direct-vent, you may not need room openings at all. Confirm this by noting on the plans that each appliance is direct-vent with a sealed combustion air intake, and include the manufacturer model confirming direct-vent operation. Plan-checkers will want to see the notation clearly so they do not have to guess.

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Why you got flagged

The most common trigger is a utility closet with a furnace or water heater drawn on the plans with no combustion air notation at all. A plan-checker reading the mechanical sheet sees the appliance, notes the closet dimensions, and flags it the moment the room area cannot supply air naturally on its own. Confined-space rules kick in when the volume of the room is less than 50 cubic feet per 1,000 Btu/hr of the total installed appliance input. If your closet is 3 feet by 4 feet by 8 feet, that is 96 cubic feet. A 60,000 Btu/hr furnace needs 3,000 cubic feet of volume for the unconfined-air method to apply. That closet is confined, and you need to show the openings.

A second common scenario involves ADU conversions where the mechanical room was sized for the original house layout but a second appliance gets added. The total Btu/hr input goes up, the volume-per-Btu ratio drops below the threshold, and the space that was borderline now clearly requires combustion air openings. The drawings often miss this because the designer sized the room for the old appliance and did not recalculate after the scope change.

The third scenario is a plan that does show openings but specifies them incorrectly. CMC 701.6 allows two primary methods: the indoor air method, which uses two openings (one high, one low) each sized at 1 square inch free area per 1,000 Btu/hr of total input with a 100-square-inch minimum, and the outdoor air method, which ducts air directly from outside with different sizing criteria depending on duct length and configuration. A common drawing mistake is showing a single louvered door and calling it combustion air, which does not satisfy either method as written.

The official code

CMC Chapter 7 — Combustion Air, California Mechanical Code (state adoption reference)

Read the source →

Common questions

Does CMC 701.6 apply to direct-vent sealed combustion appliances?
No. Direct-vent appliances draw all their combustion air through a sealed pipe connected directly to the outside. Because no room air is used for combustion, CMC 701.6 room-opening requirements do not apply to those units. To use this exception, every appliance in the confined space must be direct-vent. If you have a direct-vent furnace paired with an atmospheric-vent water heater in the same closet, you still need combustion air openings for the water heater, and those openings have to be sized for the total input of all appliances that draw room air.
What is the minimum opening size for indoor combustion air under CMC 701.6?
Each opening (one high, one low) must provide at least 100 square inches of free area, regardless of appliance input. That is the floor. If your total appliance input exceeds 100,000 Btu/hr, you need 1 square inch of free area per 1,000 Btu/hr, so a 150,000 Btu/hr load needs 150 square inches per opening. Free area is not the same as face area. A standard louvered grille is roughly 25 to 60 percent free, depending on blade angle. Always specify both the required free area and the actual grille face area on the detail so the plan-checker can verify the math without guessing.
Can a louvered closet door count as a combustion air opening?
Sometimes, but it depends on placement and sizing. CMC 701.6 requires one opening within 12 inches of the ceiling and one within 12 inches of the floor. A standard door louver sits at mid-door height and typically fails the low-opening placement requirement. If the door has two separate louver sections, one at the bottom and one at the top, and each provides the required free area, it can work. Many plan-checkers want to see both openings called out separately on the drawings, not just louvered door. A door louver that covers only the lower half of the door might satisfy the low opening but not the high opening.
Does CMC 701.6 apply to ADU utility closets?
Yes. ADUs are subject to the same CMC requirements as primary dwellings. ADU utility closets are frequently small by design, so they almost always meet the confined space threshold. If the ADU shares a wall with the main house and you are using the indoor air method, the adjacent space has to be in the same building, which gets complicated with attached ADUs that have fire separation walls. In those cases, the outdoor air method is often cleaner and avoids questions about whether air can actually flow through the fire-rated assembly.
My closet has an existing louvered grille but the plan-checker still flagged CMC 701.6 — why?
A few reasons come up repeatedly. First, the existing grille may not be sized for the current appliance load, especially if a new appliance was added. Second, the plans may show the grille without noting the free-area calculation, and the plan-checker cannot verify compliance without it. Third, the grille placement may be wrong for the method, since both a high and a low opening are required for the indoor air method. The fix is to add a detail that documents the existing grille free area, confirms placement (height above finished floor), and shows the math matching the current appliance input.
How do I calculate if my mechanical room is a confined space under CMC 701.6?
Multiply the room interior length by width by ceiling height to get volume in cubic feet. Add up the rated Btu/hr input of every fuel-burning appliance in the room. Divide volume by total input in thousands of Btu/hr. If the result is less than 50, the space is confined and you need combustion air openings. Example: a closet that is 4 ft x 3 ft x 8 ft = 96 cubic feet, with an 80,000 Btu/hr furnace, gives 96 / 80 = 1.2. That is far below 50, so it is confined. Put the calculation on the plans with the room dimensions and appliance inputs clearly labeled.

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