Combustion Air and Makeup Air
Why fuel fired appliances need a reliable air supply, and what happens in a tight building without one.
Safety
What it is
Combustion air is the air a burner consumes. Beyond that, appliances in a confined space may need dilution and ventilation air, and a building with large exhaust equipment may need makeup air so the exhaust does not depressurize the house.
Why it matters
Older leaky buildings supplied combustion air by accident. Modern tight construction does not. Add a powerful range hood, a clothes dryer and bathroom fans, and a mechanical room can go negative enough to reverse the draft on a natural draft appliance.
- Insufficient air produces incomplete combustion and carbon monoxide.
- Negative pressure can back draft a water heater or furnace into the living space.
- Direct vent and sealed combustion appliances take air from outdoors and are far less vulnerable to this.
Approaches
| Approach | How it works |
|---|---|
| Air from inside the building | Openings between the appliance space and adjoining rooms, only valid where the building volume is adequate |
| Air from outdoors | Ducts or openings sized to the appliance input, a far more reliable approach in tight construction |
| Direct vent | Combustion air ducted directly to a sealed burner, isolating the appliance from the building |
| Powered makeup air | A fan supplies air to offset large exhaust equipment, often interlocked with the exhaust device |
Test under worst case
Local requirements
Combustion air opening sizes, duct configurations, permitted methods and makeup air thresholds vary by jurisdiction, adopted code edition, building construction and the manufacturer instructions.
Local Code Notice
Practice & Mastery
- Combustion Air and Makeup Air Troubleshooting Scenarios2
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