In a balanced draft boiler system, which components supply and exhaust combustion air?

• A. Forced draft fan supplies air; Induced draft fan exhausts
• B. Booster fan supplies air; Ejector exhausts
• C. Air-preheater supplies air; Economizer exhausts
• D. Forced draft fan exhausts; Induced draft fan supplies air

Answer: (A)

Balanced draft boilers control both the air going into the furnace and the gases leaving it. The air needed for combustion is pushed into the furnace by the forced-draft fan, which increases pressure and ensures the burner gets enough oxygen for efficient burning. The exhaust gases are drawn out by the induced-draft fan, which creates a slight negative pressure in the furnace and pulls the flue gases through the boiler and up the stack. This pairing keeps the system balanced: air is supplied where it's needed, and combustion products are expelled safely.

Air-preheater and economizer are heat-exchange devices in the exhaust path rather than air suppliers. Booster fans can assist airflow in ducts, but they don’t define the basic air supply in a standard balanced draft setup. Ejectors are not the primary method for exhausting combustion products in this context.