NCT Installation Standards & Best Practices 2 — Questions and Answers
Question 1: When installing a filter bank in a horizontal airflow application, which edge of the filter frame should face the incoming air stream?
- The downstream edge
- The upstream edge with the gasket side (Correct answer)
- Either edge, as orientation does not matter
- The edge marked with the manufacturer's logo
Correct answer: The upstream edge with the gasket side
Filters installed in horizontal airflow must present the gasketed upstream face toward the incoming air to ensure a proper seal and prevent bypass.
Question 2: What is the primary purpose of a filter bypass seal (gasket) in a holding frame?
- To increase filter MERV rating
- To prevent unfiltered air from bypassing the filter media (Correct answer)
- To reduce static pressure drop
- To allow easier filter removal
Correct answer: To prevent unfiltered air from bypassing the filter media
Gaskets seal the interface between the filter and frame to eliminate bypass leakage, which would allow unfiltered air into the clean air stream.
Question 3: According to NAFA best practices, how much space should typically be maintained between the final filter bank and the fan inlet?
- At least one fan wheel diameter (Correct answer)
- No space is required
- At least 6 inches regardless of fan size
- At least three duct diameters
Correct answer: At least one fan wheel diameter
NAFA recommends maintaining at least one fan wheel diameter of space between the final filter bank and the fan inlet for uniform airflow distribution across the filter face.
Question 4: What installation defect is indicated when a differential pressure gauge reads significantly lower than the manufacturer's clean filter resistance specification?
- Filter media is too dense
- Air bypass around the filter is occurring (Correct answer)
- The fan speed is too high
- The filter is loaded with particulate
Correct answer: Air bypass around the filter is occurring
Lower-than-expected differential pressure typically indicates that air is bypassing the filter rather than passing through the media, pointing to a sealing problem.
Question 5: Which factor most significantly affects face velocity uniformity across a filter bank?
- Filter color
- Upstream duct transitions and obstructions (Correct answer)
- Filter weight
- Manufacturer country of origin
Correct answer: Upstream duct transitions and obstructions
Duct transitions, elbows, or obstructions directly upstream create turbulent or non-uniform airflow that causes uneven face velocity distribution across the filter bank.
Question 6: In a multi-stage filtration system, pre-filters should be installed at what MERV range relative to the final filters?
- The same MERV rating as the final filter
- A lower MERV rating than the final filter (Correct answer)
- A higher MERV rating than the final filter
- MERV rating is irrelevant for pre-filters
Correct answer: A lower MERV rating than the final filter
Pre-filters with a lower MERV rating capture larger particles to extend the life of higher-efficiency final filters downstream.
Question 7: What NAFA installation practice is required when stacking two standard filters to achieve a thicker bed?
- The two filters may be stacked freely without any modification
- A special double-deep holding frame or adapter must be used (Correct answer)
- Only pleated filters can be stacked
- The downstream filter should face reverse direction
Correct answer: A special double-deep holding frame or adapter must be used
Stacking filters requires a purpose-built double-deep frame to maintain proper support and sealing; improvised stacking creates bypass gaps and structural failure.
NCT Installation Standards & Best Practices 2
When installing a filter bank in a horizontal airflow application, which edge of the filter frame should face the incoming air stream?