/ Hub · FBC Fuel Gas 304 · Combustion air
Combustion air is not in the Mechanical code. That is where most sets go wrong.
Chapter 7 of the Mechanical code is two sections long — 701.1 Scope and 701.2 Dampered openings — and it carries no sizing method. It defers combustion and dilution air for gas-fired appliances to the fuel gas code: IFGC Section 304, adopted in Florida as the Florida Building Code, Fuel Gas. A set that cites “IMC 703” for an opening size is citing a section that does not carry the requirement, and the reviewer who catches it now knows the set was not checked.
Open the combustion-air method check · Packet: 304 PDF · ES.
Which document governs, by fuel
| Appliance | Where the combustion-air criteria actually live |
|---|---|
| Gas-fired | IFGC Section 304 — in Florida, the Florida Building Code, Fuel Gas. Indoor air 304.5 (standard method, plus the known-air-infiltration-rate method that becomes mandatory below 0.40 ACH), outdoor openings 304.6, combination 304.7, engineered 304.8, mechanical supply 304.9. |
| Oil-fired | NFPA 31, Standard for the Installation of Oil-Burning Equipment. |
| Solid-fuel | The listed manufacturer installation instructions. |
| Direct-vent | Outside the chapter’s methods entirely — the appliance brings its own air through its listed system. Confirm the listing before assuming it. |
| Any of the above, with a dampered opening | Mechanical 701.2 still applies: the damper interlocks with the firing cycle, manual dampers are prohibited in combustion-air ducts, and an undampered duct through rated construction goes in a shaft. 701 proof-of-open. |
Miami-Dade reviews the Florida Building Code, Fuel Gas alongside the Mechanical volume, 8th Edition (2023). Confirm the adopted edition and local amendments before applying any of this to a permit set.
What the drawings should identify
- Appliance type, fuel, and whether it is indoor, outdoor, or direct-vent
- The governing document, stated on the sheet — fuel gas code, NFPA 31, or the listing — before any opening is sized
- Combustion-air method: indoor volume, outdoor openings, combination, ducts, or engineered
- Opening size as free area — not nominal louver size
- Dampered openings that interlock with the appliance, not gravity “hope”
- Rated-assembly penetrations and coastal / HVHZ opening protection
- Existing-room constraints when a larger or tighter appliance replaces the original
Citing the wrong book
“Combustion air per IMC 703” is the most common version of this error. There is no 703. Chapter 7 stops at 701.2. Name the fuel gas code section, the NFPA standard, or the listing — whichever actually governs the appliance on the schedule.
Replacement unit, original closet
Sealed-combustion marketing copy does not automatically retire the requirement. Confirm the listing and the actual air path before the closet is framed.
Louver free area ignored
A 12×12 louver is not 144 in² of free area. The grille, screen, and damper eat the opening. Plan review will ask.
Daily Code Talk
- IMC 701 combustion air — scope and dampered openings
- IMC chapter map · Combustion Air Field Guide (IFGC 304)
What to send
Appliance cut sheets, mechanical-room or closet dimensions, existing opening locations, and whether this is new construction or a replacement in a sealed room.
Send the appliance and the room
Cut sheets plus the closet or mechanical-room dimensions. That is enough to start a written scope.
Send Project Background+1 (786) 398-6940 · osmany.portal@masterbuildconsulting.com
MasterBuild provides engineering services under a written scope. Nothing on this page is project-specific engineering advice, a code interpretation for your project, or a representation about permit approval.