Navien NP-180 Service Manual page 35

Operation
1. Water Flow Begins
• Water Flow Sensor sends pulses to the PCB.
• PCB senses flow greater than 0.5 GPM (0 GPM for "A" Model) (approximate).
• Firing Sequence begins.
2. Firing Sequence
• PCB monitors Inlet / Outlet water temperature, temperature set point,
and water flow rate.
• Fan Motor energized. Purges combustion chamber.
• Spark igniter begins sparking
• Main Gas control valve opens to minimum fire rate.
• Flame rod proves ignition.
• Spark igniter stops sparking.
3. Normal Operation
• PCB monitors flame rod, fan motor frequency, outlet water temperature,
PCB temperature set point and water flow rate.
• Main Gas control valve modulates gas input to required firing rate.
• Combustion fan speed is adjusted for the required firing rate.
• Water adjustment valve is adjusted as needed.
4. Shut-down Sequence
• PCB senses flow rate less than 0.5 GPM (0 GPM for "A" Model) (approximate).
• Main Gas Control valve closes.
• Water adjustment valve resets to standby position.
• Combustion fan runs for a short period of time at low speed.
5. Standby Mode
• PCB monitors water temperature and remote controls.
• Freeze protection is activated as needed.
©Navien America Inc. 2010
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NR/NP series service Manual Version 2.0