Pressure Switch Logic Tree / Parts Testing

Pressure Switch Diagnostic (Selection Flow)

0) Safety circuits — Any high limit or flame rollout open / board flashing limit code?

What the readings mean

Across pressure switch (VAC): Open ≈ 24 VAC; Closed ≈ 0–1 VAC
Continuity (Ω): Open = OL; Closed ≈ 0 Ω
Manometer: Must meet/exceed stamped setpoint (e.g., “-0.90 in. w.c. CLOSE”)
Safety circuits: High Limit or Rollout open will often show board fault labels like “Limit open” or “Rollout open”.

Low draft causes: tubing kink/water, plugged pressure taps, clogged condensate trap/hoses, vent/intake blockage, weak inducer, collector or secondary HX restriction, improper vent length/slope.