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Building Code Reference

Got a CMC 305.1 correction?

Gas appliance garage elevation

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 305.1 requires gas appliances installed in garages to be mounted with ignition sources at least 18 inches above the garage floor. The rule covers appliances that have open flames, pilot lights, or exposed heating elements — water heaters and furnaces are the most common. An exception exists for appliances listed as FVIR (flammable vapor ignition resistant), which are tested and certified to handle vapors near floor level without sparking an ignition.

How to fix it

  1. Confirm whether your appliance is FVIR-listed

    Check the manufacturer's listing label on the appliance or the model spec sheet. FVIR-certified water heaters will show 'FVIR' or reference ANSI Z21.10.1 in the listing. If the unit is FVIR-listed, you can note the exception on the plans and skip the pedestal requirement entirely. If no listing is shown, assume the 18-inch elevation rule applies and proceed with the pedestal detail. Most gas water heaters manufactured after 2005 are FVIR-certified by default due to federal safety requirements, but the plans still need to call this out explicitly — reviewers can't assume the listing from the appliance model alone.

  2. Add floor elevation callout to the mechanical plan

    On the mechanical or site plan sheet that shows the garage, add a note stating the appliance ignition source elevation. A callout works: 'Water heater — ignition source at 18" AFF minimum per CMC 305.1' or 'Furnace — burner at 18" AFF minimum per CMC 305.1.' AFF means above finished floor. If the appliance is FVIR-listed, write: 'FVIR-listed unit — CMC 305.1 exception applies, no elevation pedestal required.' The note must appear on the plan sheet the reviewer is checking, not just in a general notes legend that may not be referenced.

  3. Show the pedestal or platform detail on the plan

    If a pedestal is required (non-FVIR unit), include a detail or elevation view showing the platform construction. The platform only needs to raise the ignition source 18 inches above the floor — not the whole appliance cabinet. For most residential water heaters, this means a platform roughly 18 inches tall under the unit. Show the platform material — typically 2x framing with plywood top, or masonry block — the overall height, and how it's anchored to the floor or wall. If seismic strapping is also required (CPC 507.2), show the strapping to the wall on the same detail. Combining both on one detail saves a second correction round.

  4. Address garage-to-living-space separation if applicable

    If the appliance is in a room that opens to a garage — even through a door — confirm the separation meets CRC R302.6 (garage drywall) and CRC R302.5 (rated door). A utility closet that shares a wall with the garage and has an unrated door may still be treated as garage space for CMC 305.1 purposes. If the opening between the closet and garage is properly rated and sealed, note that on the plans so the reviewer sees the separation clearly. On ADU projects where the garage is being converted, this is one of the most common multi-correction sequences: 305.1 flagged alongside R302.5 and R302.6 in the same letter.

  5. Confirm combustion air is noted on the same sheet

    CMC 701.4 requires adequate combustion air for gas appliances in enclosed or semi-enclosed spaces. Plan-checkers reviewing CMC 305.1 will often also flag combustion air if it's missing from the plans. Show the combustion air source — typically louvered vents, an open garage door undercut, or a direct outdoor air connection. Getting both CMC 305.1 and 701.4 addressed on the same resubmittal avoids a back-and-forth cycle that can add another 60 days to your timeline.

  6. Write a clear response letter keyed to the correction number

    Include a written response to the corrections letter that maps to each comment number. For example: 'Comment 7 (CMC 305.1): Revised Sheet M1 — added platform detail at water heater showing 18" AFF elevation. See Sheet M1, Detail A.' Plan-checkers process resubmittals faster when responses map directly to their original comments — they're not hunting through revised sheets to find what changed. A clear letter with specific sheet and detail references can reduce the review cycle to a single round rather than the typical two or three.

Take the CMC 305.1 fix steps with you

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Why you got flagged

The most common reason plan-checkers flag CMC 305.1 is that the elevation requirement simply doesn't appear on the plans. A mechanical plan shows a water heater in the garage with no floor height noted — the reviewer has no way to confirm the 18-inch rule will be met, so they write the correction.

A second common scenario involves a standard water heater being shown without any FVIR notation. If the unit isn't labeled as FVIR-listed, the reviewer defaults to requiring the 18-inch pedestal. Contractors who install FVIR-certified units but don't call that out on the plans get flagged even though the installation would comply.

Garage conversions and ADUs with mechanical rooms create a third scenario. When a garage is converted to living space, the existing water heater may end up in an adjacent utility closet that opens to the garage. Plan-checkers treat any room that communicates with the garage as subject to the same elevation requirement unless the opening is fire-rated and sealed.

Furnaces installed in garages are flagged when the combustion air supply or the burner access panel is shown at floor level. Even if the main cabinet sits on a platform, reviewers will note the correction if the plans don't show the burner elevation clearly meeting the 18-inch minimum.

The official code

California Mechanical Code (CMC) — Part 4, State-Published Edition via DGS Building Standards Commission

Read the source →

Common questions

Does CMC 305.1 apply to electric water heaters in the garage?
No. CMC 305.1 only covers appliances with open flames, pilot lights, or exposed heating elements that could ignite flammable vapors. An electric resistance water heater has no ignition source, so it doesn't need the 18-inch pedestal under CMC 305.1. If you're not sure whether your unit qualifies as electric-only, check the listing label — fully electric units show no gas connection and no flame icon in the listing. Heat pump water heaters are also electric and are not subject to this rule.
What is an FVIR water heater and do I still need an 18-inch pedestal?
FVIR stands for flammable vapor ignition resistant. FVIR-certified water heaters are tested under ANSI Z21.10.1 and contain the ignition zone so that vapors near the floor can't reach an open flame. If your water heater is FVIR-listed, CMC 305.1 does not require the 18-inch pedestal — the listing itself satisfies the code. Most gas water heaters sold since 2005 are FVIR-listed by default due to federal CPSC regulations. To confirm: check the label on the side of the unit or the spec sheet from the manufacturer. On your plans, note 'FVIR-listed unit per ANSI Z21.10.1 — CMC 305.1 exception applies' and the correction will typically be cleared.
Can I build a wood-framed platform to meet the 18-inch requirement?
Yes. A wood-framed platform with a plywood top is an accepted way to meet the elevation requirement. Build it from 2x lumber, make it structurally stable under the full appliance weight, and show it on the plans with a detail — height, materials, and anchoring method. For water heaters, also show the seismic strapping to the wall (CPC 507.2) on the same detail. Some inspectors in Orange County prefer masonry block platforms because they're non-combustible, but a wood-framed platform is code-compliant. Either way, the detail needs to show the ignition source elevation explicitly.
Does CMC 305.1 apply to a furnace installed in the garage?
Yes, if the furnace has a gas burner with an ignition source. The 18-inch elevation requirement applies to the ignition source — typically the burner assembly inside the unit. For most upflow furnaces, the burner is near the bottom of the cabinet, so the cabinet needs to sit on a platform that puts the burner at 18 inches AFF. Show the platform and the burner elevation on the mechanical plan. Sealed combustion furnaces that draw outside air directly may qualify for an exception, but confirm with your local plan-checker before relying on it.
What if the garage is being converted to living space — does CMC 305.1 still apply?
It depends on where the appliance ends up. If the water heater or furnace is relocated to a mechanical closet that no longer communicates with the garage, CMC 305.1 would not apply to that closet. But if the closet has any opening to what was the garage — even a rated door — plan-checkers will apply the garage rule until the separation is clearly documented. Show the fire rating on the wall and door separating the closet from the former garage. Once that separation is confirmed, the 18-inch rule does not apply to the mechanical space.
How do I show the 18-inch elevation clearly enough on the plans?
The clearest approach is an elevation detail or section cut on the mechanical plan sheet showing the appliance on the platform, with the platform height dimensioned. A text note alone — without a dimension or detail — often gets flagged again because the reviewer can't confirm compliance from words alone. Dimension the platform height at 18 inches minimum, label the finished floor level, and reference the detail bubble on the floor plan. If the appliance is FVIR-listed, a note with the listing reference is sufficient — no detail needed.

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