Section 303.3/Combustion Air Requirements
UMC 303.3 covers combustion air supply for fuel-burning appliances to ensure safe and complete combustion.
Fuel-burning appliances must have an adequate supply of combustion air. The UMC provides three methods for supplying combustion air: from indoor spaces of sufficient volume (50 cubic feet per 1,000 BTU/h input for confined spaces), from outdoor air through two permanent openings (one within 12 inches of the top and one within 12 inches of the bottom of the enclosure), or through a combination of indoor and outdoor air. Mechanical combustion air systems are permitted as an alternative. Direct-vent and sealed combustion appliances draw combustion air directly from outdoors through a dedicated duct and are exempt from the room ventilation requirements.
Why this section exists
A fuel-burning appliance starved for combustion air produces carbon monoxide, a colorless and odorless gas that causes illness and death. As modern buildings become more airtight, the natural infiltration that historically provided combustion air is no longer sufficient. Without dedicated combustion air provisions, a furnace, boiler, or water heater in a tight mechanical room can deplete the oxygen in the space, leading to incomplete combustion and carbon monoxide production.
What plan reviewers look for
Plan reviewers check the mechanical drawings for combustion air provisions at every fuel-burning appliance. They verify the method used (indoor volume, outdoor openings, or mechanical supply). For outdoor air, they check that two openings of adequate size are shown on the plan. For appliances in confined spaces, they verify the combustion air calculation. They check that direct-vent appliances are identified on the equipment schedule.