Draft value for residential oil burners, measured over the fire with the burner in operation is approximately:

• A. -.20
• B. -.02
• C. 0
• D. +.02

Answer: (B)

A small negative draft is desired here. Draft is the slight suction in the burner area that pulls in air for combustion and moves the flue gases up the chimney. For a residential oil burner operating normally, the typical value is a tiny negative pressure, about -0.02 inches of water column. This keeps the air supply balanced and ensures good combustion and venting. If the draft were much stronger (like -0.20), the flame could become unstable or lose efficiency; if there were no draft (0) or a positive draft (+0.02), combustion would be poor or exhaust could be pushed back. So the approximate draft value is -0.02 inches of water column.