NSS Maintenance and Repair Techniques 2 — Questions and Answers
Question 1: When descaling a Navien condensing tankless water heater, which solution is recommended for flushing the heat exchanger?
- Muriatic acid diluted 10:1
- White vinegar or food-grade citric acid solution (Correct answer)
- Bleach diluted 1:10
- Sodium bicarbonate solution
Correct answer: White vinegar or food-grade citric acid solution
White vinegar or food-grade citric acid solution safely dissolves mineral scale in Navien heat exchangers without damaging internal components.
Question 2: A Navien NPE-240A displays error code E003. What does this indicate?
- Ignition failure (Correct answer)
- Outlet water temperature sensor fault
- Exhaust blockage
- Cold water inlet pressure too low
Correct answer: Ignition failure
Error code E003 on Navien units indicates an ignition failure, meaning the burner did not ignite after the allotted number of attempts.
Question 3: What is the minimum recommended annual maintenance task for Navien combi-boiler systems?
- Replacing the gas valve
- Inspecting and cleaning the cold water inlet filter screen (Correct answer)
- Replacing the pressure relief valve
- Flushing the condensate trap with bleach
Correct answer: Inspecting and cleaning the cold water inlet filter screen
Navien recommends inspecting and cleaning the cold water inlet filter screen annually to prevent flow restriction and performance issues.
Question 4: When replacing a Navien flow sensor, which precaution is most critical before starting the procedure?
- Switching the unit to summer mode
- Isolating the gas supply and depressurizing the water lines (Correct answer)
- Increasing the set temperature to maximum
- Enabling the freeze protection mode
Correct answer: Isolating the gas supply and depressurizing the water lines
Before replacing the flow sensor, the gas supply must be isolated and water lines depressurized to prevent hazardous leaks or flooding.
Question 5: A Navien NPN-199 boiler shows recurrent short-cycling. After verifying no errors are present, what should a technician check first?
- The recirculation pump speed setting
- System expansion tank pre-charge pressure and condition (Correct answer)
- The outdoor reset curve
- Condensate line slope
Correct answer: System expansion tank pre-charge pressure and condition
A waterlogged or improperly sized expansion tank is a leading cause of short-cycling in hydronic systems, causing rapid pressure fluctuation.
Question 6: Which tool is required to perform a proper combustion analysis on a Navien condensing unit in the field?
- A digital multimeter only
- A combustion analyzer measuring O2, CO, and CO2 in flue gases (Correct answer)
- A manometer and soap bubbles
- A thermal imaging camera
Correct answer: A combustion analyzer measuring O2, CO, and CO2 in flue gases
A calibrated combustion analyzer is required to measure O2, CO, and CO2 levels and verify the air-fuel mixture is within Navien specifications.
Question 7: When a Navien tankless unit produces lukewarm water at high flow rates but performs normally at low flow rates, what is the most likely cause?
- A faulty gas valve modulation (Correct answer)
- Scale buildup on the secondary heat exchanger reducing transfer efficiency
- A defective outlet thermistor reading high
- Low incoming gas pressure only at high demand
Correct answer: A faulty gas valve modulation
At high flow rates, if the gas valve cannot modulate to full output, the unit cannot add sufficient BTUs to reach the setpoint temperature.
NSS Maintenance and Repair Techniques 2
When descaling a Navien condensing tankless water heater, which solution is recommended for flushing the heat exchanger?