NSS Diagnostics and Troubleshooting 5 — Questions and Answers
Question 1: A Navien NPE unit passes all self-diagnostics but produces lukewarm water at full flow. Thermistors test within spec. What should be investigated next?
- Scale buildup in the heat exchanger reducing thermal transfer (Correct answer)
- PCB firmware version mismatch
- Recirculation pump speed setting
- Condensate trap blockage
Correct answer: Scale buildup in the heat exchanger reducing thermal transfer
Scale deposits on heat exchanger walls act as thermal insulation, reducing heat transfer efficiency and resulting in lower-than-expected outlet temperatures at high flow rates.
Question 2: When a Navien unit's error history shows repeated E016 (false flame) followed by E003 (ignition failure), what is the most likely root cause?
- PCB grounding fault affecting both the ionization rod and igniter circuits (Correct answer)
- Undersized gas supply line
- Blocked condensate drain causing flue gas recirculation
- Failed diverter valve
Correct answer: PCB grounding fault affecting both the ionization rod and igniter circuits
A PCB grounding fault can cause false ionization signals (E016) and simultaneously impair igniter function (E003), as both share the control board's reference ground.
Question 3: A Navien combi unit's space heating supply temperature is 20°F lower than the setpoint during design conditions. Combustion analysis is normal. What is the most likely cause?
- Insufficient glycol concentration in the heating loop
- Unit is undersized for the heating load at design conditions (Correct answer)
- Faulty outdoor reset curve configuration
- Air in the heating circuit causing flow restriction
Correct answer: Unit is undersized for the heating load at design conditions
If combustion is normal but supply temperature falls short of setpoint under peak load, the unit's BTU output is insufficient to meet the system's heat loss at design outdoor temperature.
Question 4: During a Navien NPE service call, the technician notices the venting is PVC and the unit is configured for Category III stainless steel venting. What is the diagnostic concern?
- PVC will cause elevated CO production
- PVC may fail due to exhaust temperatures exceeding its rated limit, creating a safety hazard (Correct answer)
- PVC reduces unit efficiency by 15%
- PVC is acceptable as a temporary repair only
Correct answer: PVC may fail due to exhaust temperatures exceeding its rated limit, creating a safety hazard
Category III units produce higher exhaust temperatures that can exceed PVC's rated limit of approximately 140°F, risking pipe deformation or failure and potential flue gas leaks.
Question 5: A Navien NPN boiler shows E110 (exhaust high limit) only on very cold days. Flue is clear and fan operates correctly. What should be investigated?
- Wind-driven flue gas recirculation at the termination cap on cold, windy days (Correct answer)
- Expansion tank pre-charge pressure drops with temperature
- Cold water inlet causing thermal shock to the heat exchanger
- PCB temperature compensation algorithm failure
Correct answer: Wind-driven flue gas recirculation at the termination cap on cold, windy days
On cold windy days, wind-driven flue gas recirculation at the termination point can cause hot exhaust gases to re-enter the unit, tripping the exhaust high-limit sensor.
Question 6: A Navien unit's delta-T across the heat exchanger is only 10°F during a call for heat, while the expected delta-T is 50°F. Flow rate is confirmed normal. What does this suggest?
- Heat exchanger is fouled and transferring insufficient heat to the water (Correct answer)
- Gas pressure is too high causing overcombustion
- The unit is over-firing due to a stuck-open gas valve
- Incoming water temperature is unusually high
Correct answer: Heat exchanger is fouled and transferring insufficient heat to the water
A low delta-T with normal flow means the heat exchanger is not transferring adequate heat, most commonly due to scale fouling reducing thermal contact between the combustion gases and the water.
Question 7: After completing repairs on a Navien unit, a technician performs a final combustion analysis and finds O2 at 10% and CO2 at 7% on natural gas. What does this indicate?
- Lean combustion — too much excess air in the combustion mix (Correct answer)
- Rich combustion — insufficient air causing incomplete combustion
- Normal combustion within Navien specifications
- Incorrect gas type selected in the installer menu
Correct answer: Lean combustion — too much excess air in the combustion mix
High O2 (above ~5%) and low CO2 (below ~9%) on natural gas indicate excess air in the combustion mix, resulting in a lean flame that reduces efficiency.
NSS Diagnostics and Troubleshooting 5
A Navien NPE unit passes all self-diagnostics but produces lukewarm water at full flow.
Thermistors test within spec.
What should be investigated next?