Air Filtration
Filter ratings, the trade off between filtration and airflow, and how filters actually fail.
Safety
What it is
An air filter removes particles from the air stream. In practice it serves two purposes that are often confused: protecting the equipment, especially the coil and blower, and improving air quality for occupants.
MERV
A rating scale describing how effectively a filter captures particles in defined size ranges. A higher number captures smaller particles.
The trade off
A higher rated filter in the same slot generally means more pressure drop, which means less airflow, which means less capacity and a harder working blower. The correct answer is usually more filter area rather than a thinner high rated filter.
- A four or five inch media cabinet delivers better filtration at lower pressure drop than a one inch filter of the same rating.
- Bypass around a poorly fitting filter defeats the rating entirely.
- Pressure drop across the filter should be measured, not assumed, and rechecked when loaded.
- Very high efficiency filtration retrofitted into a system never designed for it is a common cause of frozen coils and failed blowers.
How to evaluate a filter setup
Pressure drop across the filter with a manometer
Quantifies what the filter costs the system rather than guessing from appearance
Filter fit in the rack and gaps around the frame
Bypass air carries dirt straight to the coil
Coil cleanliness
A dirty coil behind a clean filter means bypass or a history of no filter
Filter face area versus system airflow
Too little area guarantees high pressure drop regardless of the filter chosen
How often the occupant actually changes it
The best filter is the one that gets replaced on schedule
Symptoms
Practice & Mastery
- Questions1
- Air Filtration Troubleshooting Scenarios2
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