Which air is supplied to a furnace to control the rate of combustion?

• A. Tertiary
• B. Makeup air
• C. Primary
• D. Auxiliary

Answer: (C)

Primary air is the air that mixes with the fuel at the burner before combustion begins. This initial fuel–air mix sets the rate of burning because the fuel–air ratio determines how quickly and completely the fuel can burn. Adjusting how much primary air is supplied changes the flame temperature and overall combustion rate: too little primary air leads to incomplete combustion and smoky flames, while too much can cool the flame and reduce efficiency. Other air streams, like secondary air, are introduced later to finish combustion and improve flame quality, but they don’t control the rate in the same direct way as the primary air. Makeup air refers to ventilation air for the space and isn’t used to regulate the burner’s rate.