Furnace Pressure Switch Vacuum Test — Professional Diagnostic
Published: July 23, 2026 — BC Wide Home Services Ltd, doing business as BC Wide Heating & Air Conditioning — Greater Vancouver, BC
How the Test Works
A technician uses a manometer to measure the vacuum (negative pressure) created by the inducer motor. The manometer hose is connected to the pressure switch port while the inducer runs. The reading shows how much draft the inducer is producing. The pressure switch has a specified setpoint — it should close when the vacuum reaches that level.
Normal Readings
Typical vacuum readings: -0.5 to -1.5 inches of water column (depending on the furnace model). If the reading is below the switch setpoint, the switch will not close. Causes of low vacuum: blocked vent pipe, cracked heat exchanger, weak inducer motor, or restricted combustion air intake.
What the Test Reveals
If vacuum is low — the inducer or venting is the problem. If vacuum is normal but the switch does not close — the switch is faulty and needs replacement. If the switch closes but the error code returns — there may be an intermittent vent blockage or switch condition. The vacuum test is the definitive diagnostic for pressure switch issues.