NSS Diagnostics and Troubleshooting 5

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?

Accepted Answer

Scale buildup in the heat exchanger reducing thermal transfer

Answer

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?

Accepted Answer

PCB grounding fault affecting both the ionization rod and igniter circuits

Answer

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?

Accepted Answer

Unit is undersized for the heating load at design conditions

Answer

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?

Accepted Answer

PVC may fail due to exhaust temperatures exceeding its rated limit, creating a safety hazard

Answer

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?

Accepted Answer

Wind-driven flue gas recirculation at the termination cap on cold, windy days

Answer

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.

(NSS) Certified Navien Service Specialists Practice Test

(NSS) Certified Navien Service Specialists Practice Test

NSS Diagnostics and Troubleshooting 5