NCT System Maintenance & Troubleshooting 2 — Questions and Answers
Question 1: A technician notices that filter differential pressure readings are unusually low even though the filters appear dirty. What is the most likely cause?
- Manometer tubing is blocked or disconnected (Correct answer)
- Filters have been installed correctly
- Airflow velocity has increased
- Filter media is too dense
Correct answer: Manometer tubing is blocked or disconnected
Blocked or disconnected manometer tubing prevents accurate differential pressure measurement, resulting in falsely low readings despite dirty filters.
Question 2: During routine inspection, a bag filter shows signs of fabric tears near the seal area. What corrective action should be taken first?
- Continue operating until the next scheduled maintenance
- Apply duct tape as a temporary patch
- Replace the damaged filter immediately to prevent bypass (Correct answer)
- Increase fan speed to compensate for leakage
Correct answer: Replace the damaged filter immediately to prevent bypass
Tears near the seal allow unfiltered air to bypass the media, so the filter must be replaced immediately to maintain system integrity.
Question 3: Which parameter should a technician verify first when a HEPA filter system experiences a sudden increase in face velocity?
- Filter MERV rating
- Upstream damper position (Correct answer)
- Filter media manufacturer
- Downstream duct color
Correct answer: Upstream damper position
An incorrectly positioned upstream damper can increase the volume of air directed through the filter, causing elevated face velocity.
Question 4: What does a rising differential pressure trend over successive maintenance cycles indicate about filter loading?
- Filters are being changed too frequently
- Filters are reaching end-of-life faster than expected (Correct answer)
- System airflow has decreased
- The filter housing is leaking
Correct answer: Filters are reaching end-of-life faster than expected
A consistently steeper rise in differential pressure with each cycle suggests contaminant loading is accelerating, indicating filters are reaching end-of-life sooner.
Question 5: A HVAC technician finds microbial growth on the downstream side of a cooling coil filter. What maintenance action is most appropriate?
- Reduce filter efficiency rating
- Clean only the visible growth and reinstall
- Replace the filter and disinfect the downstream surfaces (Correct answer)
- Increase system humidity to dilute contaminants
Correct answer: Replace the filter and disinfect the downstream surfaces
Microbial growth on downstream surfaces requires both filter replacement and disinfection of contaminated ductwork to prevent continued proliferation.
Question 6: When troubleshooting a filter system with excessive dust bleed-through, which design factor should be evaluated first?
- Color of the filter frame
- Filter efficiency rating relative to particle size (Correct answer)
- Number of pleats in the media
- Filter housing paint quality
Correct answer: Filter efficiency rating relative to particle size
Bleed-through typically indicates the filter efficiency is insufficient for the particle sizes present, requiring a higher-rated filter.
Question 7: After replacing filters, the system's differential pressure is higher than the manufacturer's initial resistance specification. What should the technician check?
- Whether filters are installed in the wrong direction of airflow (Correct answer)
- Whether the building envelope is sealed
- Whether the motor bearings are lubricated
- Whether the ductwork color matches specifications
Correct answer: Whether filters are installed in the wrong direction of airflow
Installing filters against the intended airflow direction increases resistance significantly above rated initial pressure drop.
NCT System Maintenance & Troubleshooting 2
A technician notices that filter differential pressure readings are unusually low even though the filters appear dirty.
What is the most likely cause?