TL;DR
Combustion air is the oxygen supply a fuel-burning appliance needs to burn its gas or oil completely and to keep flue gases moving up the vent. Furnaces, boilers, and water heaters in tight mechanical rooms must draw it through openings, ducts, or louvers sized by the fuel gas code, roughly tied to the total BTU input of the appliances served.
What it means
Combustion air is the oxygen supply a fuel-burning appliance needs to burn its gas or oil completely and to keep flue gases moving up the vent. Furnaces, boilers, and water heaters in tight mechanical rooms must draw it through openings, ducts, or louvers sized by the fuel gas code, roughly tied to the total BTU input of the appliances served. Starve an appliance of it, by enclosing a furnace closet or weatherizing aggressively, and the result is sooting, flame rollout, or carbon monoxide spillage.
Where it sits in the glossary
Combustion air is part of the Trade jargon group inside the ProFix Directory glossary. Browse every term in this category from the glossary index.
Why Ohio homeowners should know it
This is a term Ohio homeowners encounter when reading contractor quotes, hiring paperwork, or inspection reports. Understanding it well enough to ask one good follow-up question is usually all the protection a homeowner needs.
ProFix Directory keeps definitions short on the index page and saves the longer context — Ohio-specific rules, where the term comes from, and which ProFix tools touch it — for these per-term pages so the term is easy to cite and easy to share.
ProFix tools that touch this term
See also
License: CC-BY-4.0 — quote freely with attribution to ProFix Editorial Team / ProFix Directory.