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Building Code Reference

Got a CMC 701.4 correction?

Combustion air gas appliances

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 701.4 requires gas-fired appliances installed in enclosed spaces — closets, utility alcoves, or mechanical rooms — to have a verified supply of combustion air. The code covers furnaces, water heaters, and boilers rated for natural gas or propane. If the enclosed room is too small to supply air through infiltration alone, you must add dedicated air openings or ducts that pull from outdoors or from adjacent conditioned space. Plan-checkers flag this section most often on furnaces and water heaters tucked into tight utility closets where no combustion air calculations appear on the mechanical sheet.

How to fix it

  1. Identify total appliance BTU input in the enclosed space

    Add up the input ratings (not output) for every gas-fired appliance in the enclosed room. Input ratings are stamped on the appliance data plate — typically listed as BTU/hr. A standard 80,000 BTU/hr furnace and a 40,000 BTU/hr water heater give a combined input of 120,000 BTU/hr. Write this number at the top of your calculation sheet; every downstream sizing decision depends on it.

  2. Choose an air supply method and calculate room volume or opening size

    CMC 701.4 offers two main approaches. The all-air-from-indoors method requires the room volume to be at least 50 cubic feet per 1,000 BTU/hr of total appliance input. At 120,000 BTU/hr that means 6,000 cubic feet — far more than most closets. The openings method is more practical for small spaces: you add two openings, one within 12 inches of the ceiling and one within 12 inches of the floor, each sized at 1 square inch of free area per 1,000 BTU/hr of input when drawing from adjacent conditioned space (minimum 100 square inches each), or 1 square inch per 4,000 BTU/hr when pulling directly from outdoors or through vertical ducts (1 per 2,000 BTU/hr through horizontal ducts). Show the method selected and the calculation on the mechanical sheet.

  3. Size and specify the combustion air openings or ducts

    If you’re using the openings method, specify both the high and low opening locations on the plan and calculate the minimum net free area for each. Note the grille or louver model and its free-area factor (louver free area is usually 50–75% of the rough opening). Label each opening clearly: ‘High combustion air opening — min 120 sq in net free area’ and ‘Low combustion air opening — min 120 sq in net free area.’ If drawing air from outdoors, show the duct route, duct diameter, and the termination point on the exterior elevation.

  4. Verify the door assembly doesn’t create an airtight seal

    A solid door with a weather-stripped threshold can make the room air-tight even if you have code-compliant openings elsewhere. The door to the mechanical space should not have door sweeps or threshold seals that block air movement. If the architect has specified an insulated, gasketed door on the mechanical room, either add a transfer grille in the door panel, switch to a louvered door, or route the combustion air duct entirely from outside so the door seal doesn’t matter. Note the door treatment on the door schedule.

  5. Show the combustion air note on the correct sheet

    Add a combustion air summary table to the mechanical plan sheet (typically M-series). The table should list: (1) appliances and their input BTU/hr, (2) total combined input, (3) method chosen, (4) calculated minimum opening area or room volume, (5) provided opening area or room volume. Label the openings on the floor plan with a note like ‘CA-1’ and ‘CA-2’ keyed to the table. Plan-checkers want to find all of this in one place — if it’s scattered across multiple sheets with no cross-reference, expect another correction.

  6. Cross-check with Chapter 8 venting requirements

    Combustion air and venting work together. If the appliances are category I natural-draft (most older furnaces and atmospheric water heaters), they vent through a common flue and need the air supply sized for all appliances firing simultaneously. Confirm the flue serving the appliances in the mechanical room is shown on the mechanical plan with diameter, rise, and termination height. A correction on 701.4 sometimes reveals a separate CMC Chapter 8 venting deficiency hiding in the same plan set.

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Why you got flagged

Combustion air corrections on enclosed appliances come up in a handful of specific situations.

The most common is a missing calculation. A contractor specifies a 80,000 BTU furnace and a 40-gallon water heater in a 3-by-4 utility closet, and the plan set has no note, table, or detail showing the room can supply adequate air. The plan-checker sees the BTU load, does the math in their head, and flags it. Under the all-air-from-indoors method, the combined appliance input in BTUs per hour governs the minimum room volume. For that furnace-and-water-heater combination, the required volume typically exceeds what a small closet provides, which means dedicated openings are mandatory.

The second scenario is undersized openings. The contractor does show combustion air openings on the plan, but the grille or louver dimensions don’t match the BTU load. CMC 701.4 ties opening area directly to appliance input rating — both the lower (floor-level) opening and the upper opening must meet minimum square-inch requirements based on the total input BTUs. A single 4-by-12 return-air grille that looks generous on a garage wall may still fall short if the appliance load is high.

A third scenario shows up on retrofits and ADUs where an original open utility room gets partially enclosed by a new wall or door. The appliances were code-compliant in the open configuration, but once the door goes on, the same space becomes an enclosed room subject to 701.4. The correction letter arrives because nobody rechecked the combustion air conditions after the layout changed.

The fourth trigger is a sealed or weatherstripped door to the mechanical space. Even if the room volume appears adequate on paper, a door that seals tightly against the threshold starves the appliances during operation. Plan-checkers catch this when the door schedule shows an airtight assembly on a room with gas appliances and no dedicated air path.

The official code

California Mechanical Code — Chapter 7 Combustion Air

Read the source →

Common questions

Does CMC 701.4 apply to a water heater in a garage?
Not automatically. A water heater in an open garage with unobstructed air circulation typically gets its combustion air through normal infiltration and doesn’t need dedicated openings. The section kicks in when the appliance is in an enclosed space — meaning a room with walls, ceiling, and a door that restricts free air movement. If you’re installing a water heater in a garage utility closet with a door, that closet is an enclosed space and 701.4 applies. An open alcove without a door usually doesn’t trigger the calculation requirement, but check your local jurisdiction’s interpretation before assuming.
What's the minimum room size for a 100,000 BTU furnace under CMC 701.4?
Using the all-air-from-indoors method, the minimum room volume is 50 cubic feet per 1,000 BTU/hr of appliance input. A 100,000 BTU/hr furnace needs 5,000 cubic feet — about a 20-by-20 room with 12-foot ceilings. That’s why the all-air-from-indoors method is almost never practical for a dedicated utility closet. The openings method is the standard fix for small mechanical rooms: two openings (high and low), each providing at least 1 square inch of net free area per 1,000 BTU/hr from adjacent conditioned space, minimum 100 square inches each.
Can I use a single large opening instead of two openings?
No. CMC 701.4 specifically requires two openings when you’re supplying air from adjacent conditioned space: one within 12 inches of the ceiling and one within 12 inches of the floor. The high-low configuration creates a natural convective air path — cooler dense air enters at the bottom, warm air with combustion byproducts circulates up and out at the top during appliance operation. A single opening, even oversized, doesn’t satisfy the two-opening requirement. The only single-opening option is when you’re drawing from outdoors through a horizontal duct that runs to within 12 inches of the floor.
My plans show combustion air openings but the plan-checker still flagged 701.4 — why?
A few things trip this up even when openings are shown. First, the opening free area might be too small for the BTU load. If you drew a 6-by-10 grille but the net free area is only 40 square inches and you need 80, it fails. Second, the opening location may not comply — the upper opening must be within 12 inches of the ceiling, the lower within 12 inches of the floor. Third, the plans show the opening but don’t label the minimum free area or show the calculation. Plan-checkers can’t approve what they can’t verify. Add the calculation table and label the openings with their minimum and provided free area.
Does the calculation change if the furnace is a high-efficiency sealed-combustion unit?
Yes, significantly. A sealed-combustion (direct-vent) appliance draws all its combustion air through a dedicated sealed pipe from outdoors, so CMC 701.4’s room-air-supply requirements don’t apply to that appliance. The calculation only covers appliances that draw air from the room. If you’re replacing a natural-draft furnace with a 95%-efficiency sealed unit, the water heater may be the only appliance left pulling room air, and the calculation drops to the water heater’s BTU input alone. Make sure the plan notes explicitly identify each appliance as direct-vent or natural-draft so the plan-checker can follow your reasoning.
Does CMC 701.4 apply to an ADU utility closet?
Yes. ADU mechanical rooms get no carve-out — the same CMC 701.4 requirements apply regardless of whether the structure is a primary dwelling or an accessory unit. ADU projects often get flagged because the designer scaled down the utility closet to save square footage and then forgot that even a small tankless water heater needs combustion air in an enclosed space if it’s a natural-draft unit. If the ADU uses an all-electric mechanical system or a sealed-combustion tankless, CMC 701.4 doesn’t apply. That’s worth noting on the plans: ‘All appliances are electric / direct-vent — CMC 701.4 N/A.’

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