NSS Diagnostics and Troubleshooting 3 — Questions and Answers
Question 1: A Navien NPE-240A2 shows intermittent E003 errors during peak demand. After verifying gas supply is adequate, what should be checked next?
- Condensate neutralizer pH level
- Spark igniter gap and electrode condition (Correct answer)
- Incoming water temperature
- DHW outlet thermistor resistance
Correct answer: Spark igniter gap and electrode condition
Intermittent ignition failures during high demand often result from a worn or improperly gapped igniter electrode that cannot reliably produce spark.
Question 2: A customer reports their Navien combi unit heats domestic hot water correctly but the radiant floor heating is cold. What is the most likely cause?
- Failed diverter valve not switching to heating mode (Correct answer)
- Blocked air intake screen
- Low gas pressure
- Faulty DHW thermistor
Correct answer: Failed diverter valve not switching to heating mode
A failed diverter valve will prevent the unit from switching flow from the DHW circuit to the central heating circuit, leaving radiant heat cold while DHW works.
Question 3: What resistance value should a properly functioning Navien inlet water thermistor read at approximately 77°F (25°C)?
- 100 ohms
- 10,000 ohms (Correct answer)
- 1,000 ohms
- 47,000 ohms
Correct answer: 10,000 ohms
Navien NTC thermistors typically read approximately 10,000 ohms (10 kΩ) at 25°C; deviation from this value indicates a faulty sensor.
Question 4: A Navien boiler's condensate line is frozen. What is the correct field remedy to prevent recurrence?
- Increase boiler water temperature setpoint
- Insulate and pitch the condensate line correctly, adding heat trace if necessary (Correct answer)
- Replace the condensate trap with a larger model
- Reduce burner modulation range
Correct answer: Insulate and pitch the condensate line correctly, adding heat trace if necessary
Frozen condensate lines must be insulated and properly pitched toward the drain; heat trace tape should be added in areas exposed to freezing temperatures.
Question 5: During startup, a Navien unit's combustion fan runs but the gas valve does not open. What should the technician check first?
- Verify the flow sensor is detecting minimum flow
- Inspect the pressure switch proving fan operation (Correct answer)
- Check DHW thermistor calibration
- Test the ignition control board output voltage
Correct answer: Inspect the pressure switch proving fan operation
The gas valve requires a proven fan signal from the pressure switch before it will open; a faulty pressure switch will prevent gas valve activation even with the fan running.
Question 6: A Navien NPE series unit produces error E030. What condition does this indicate?
- DHW outlet thermistor short circuit
- Exhaust thermistor open circuit (Correct answer)
- Inlet gas pressure too high
- PCB internal fault
Correct answer: Exhaust thermistor open circuit
E030 on Navien units indicates the exhaust thermistor has an open circuit, meaning the sensor wire is broken or the connector is disconnected.
Question 7: When a Navien unit is installed with a recirculation pump and the customer reports occasional cold water at the tap, what is the most likely diagnostic finding?
- Undersized unit BTU rating
- Recirculation loop check valve installed backwards (Correct answer)
- Gas pressure too low at the unit
- Dirty air filter reducing airflow
Correct answer: Recirculation loop check valve installed backwards
A backwards check valve in the recirculation loop prevents proper flow, causing cold water slugs to appear at fixtures before hot water arrives.
NSS Diagnostics and Troubleshooting 3
A Navien NPE-240A2 shows intermittent E003 errors during peak demand.
After verifying gas supply is adequate, what should be checked next?