5 flashes = Vacuum switch / draft pressure issue. Different meaning by unit type — atmospheric units indicate inadequate chimney draft; power vent units indicate pressure switch fault.
Atmospheric units — inadequate draft
The vent doesn\'t pull combustion gases out fast enough. Causes:
- Chimney obstruction: bird nest, debris, ice. Visual inspection of chimney top
- Improper chimney height: too short or close to roof line. Code requires 3 ft above roof + 2 ft horizontal clearance
- Liner damage: internal flue liner crack or collapse
- Wind effects: certain wind conditions can prevent draft. Cap modification may help
- Negative pressure in home: kitchen exhaust hood or dryer pulling air. Add make-up air or downsize hood
Power vent units — pressure switch fault
The pressure switch verifies the fan is creating proper draft before the burner fires:
- Switch failed: open or shorted. Replace
- Tubing connection: verify pressure tubing isn\'t kinked or disconnected
- Vent obstruction: PVC sidewall termination blocked by debris, ice, or condensate
- Fan not running: 120V outlet, fan motor, capacitor
Direct vent units
Same as power vent diagnostics — pressure switch verifies sealed-combustion venting is unobstructed.
Reset
- Address the obstruction or fault
- Cycle gas valve (off → pilot → on)
- For power vent: cycle 120V power
- Monitor LED
Bottom line
Atmospheric: inspect chimney. Power vent: inspect vent termination and pressure switch tubing.