The desired draft, measured over the fire with the burner in operation is what?

• A. -.20
• B. 0
• C. -.02
• D. +.02

Answer: (C)

Draft is the slight suction created over the flame that pulls combustion gases up the vent while drawing the right amount of air for burning. When the burner is running, you want a small negative draft, not zero or positive, so the gases leave the furnace safely and the flame stays stable. A value of about -0.02 inches of water (negative sign shows suction) is the typical, appropriate level—small enough to avoid blowing air through the flame or pulling it apart, but enough to keep the exhaust moving upward. If the draft were much stronger, like -0.20, it would pull in too much air and could cause flame instability or excessive cooling. Zero draft wouldn’t reliably move gases out and could lead to poor venting, while a positive draft would push gases back toward the burner and disrupt safe operation.