A blocked draft is most readily detected by which instrument showing increased negative pressure?

• A. Compound/vacuum gauge
• B. Temperature gauge
• C. Pressure gauge
• D. Flow meter

Answer: (A)

Blocked draft creates a drop in pressure in the intake, i.e., negative pressure or vacuum. The instrument that shows this most clearly is a gauge that can read vacuum—specifically a compound gauge, which measures both positive pressure and negative pressure. When the draft is blocked, the gauge will register increased negative pressure (vacuum), signaling the problem. Temperature gauges, standard pressure gauges, and flow meters don’t directly reveal this negative-pressure condition: temperature gauges monitor heat, a normal pressure gauge shows positive pressure and may not display vacuum clearly, and a flow meter indicates how much air is moving rather than the pressure state in the intake.