CSLB C-20 HVAC ContractorVentilation and Air DistributionMedium
A technician measures static pressure immediately before and after the evaporator coil: +0.25 in. w.c. entering and -0.35 in. w.c. leaving. The manufacturer specifies a maximum coil pressure drop of 0.30 in. w.c. at rated airflow. What is the measured pressure drop across the coil, and what does it indicate?
- A0.10 in. w.c.; within specification
- B0.35 in. w.c.; slightly under specification
- C0.60 in. w.c.; normal for this system's airflow
- D0.60 in. w.c.; exceeds specification, indicating a dirty or restricted coil
Show answer & explanationAnswer & explanation
Correct answer: D. 0.60 in. w.c.; exceeds specification, indicating a dirty or restricted coil
Pressure drop across the coil equals entering pressure minus leaving pressure: 0.25 - (-0.35) = 0.60 in. w.c. This exceeds the manufacturer's 0.30 in. w.c. specification, indicating airflow restriction across the coil, commonly caused by a dirty coil, dirty filter, or undersized ductwork.
Why the other options are wrong
- A. 0.10 in. w.c. does not match the correct subtraction across the coil.
- B. 0.35 in. w.c. does not reflect the correct calculation using both readings.
- C. 0.60 in. w.c. is not normal; it exceeds the manufacturer's stated limit.
Coil Pressure Drop
The difference between static pressure entering and leaving the evaporator coil, used to detect airflow restrictions such as dirty coils or clogged filters when compared to manufacturer specifications.
- Pressure drop = entering pressure − leaving pressure
- Compare measured drop to manufacturer's rated maximum
- Excessive drop often indicates dirty coil, filter, or restricted airflow
Memory trick: Subtract the readings, compare to spec — restriction you'll detect.