Air Handler Units and Coils Flashcards
7 cards from real HVAC practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Air Handler Units and Coils flashcards as text
What is the purpose of a hot gas bypass valve in an air handler system?
Answer: To divert hot refrigerant gas to prevent evaporator coil freezing at low loads
A hot gas bypass valve diverts hot discharge gas back to the evaporator inlet to maintain minimum suction pressure and prevent coil freeze-up during low-load conditions.
What is 'sensible heat ratio' (SHR) and why is it important for coil selection?
Answer: The proportion of total cooling capacity used for temperature reduction vs. dehumidification
SHR is the ratio of sensible cooling (temperature reduction) to total cooling capacity, and it must match the space's load characteristics for proper comfort and humidity control.
What is the typical face velocity (air speed through coil face area) for a residential evaporator coil?
Answer: 300-500 feet per minute
Residential evaporator coils are typically designed for 300-500 feet per minute face velocity, which balances heat transfer efficiency with acceptable airflow resistance and moisture carryover.
What is a 'cased coil' in residential HVAC terminology?
Answer: A coil enclosed in a metal cabinet for installation in a duct system
A cased coil is an evaporator coil enclosed in a sheet metal cabinet or 'case' that can be installed in ductwork, typically above a furnace or in an air handler.
What happens to system efficiency when evaporator coil fins become coated with a thin layer of dirt?
Answer: Efficiency decreases because heat transfer is impaired
A dirt coating on coil fins acts as thermal insulation, significantly reducing heat transfer between the air and refrigerant and decreasing overall system efficiency.
What is the purpose of a secondary (auxiliary) drain pan under an air handler unit?
Answer: To catch water if the primary drain pan or line fails, preventing property damage
A secondary drain pan is a safety device installed under the air handler to collect water if the primary pan overflows or the primary drain line becomes blocked, preventing water damage.
What chemical treatment is commonly applied to drain pans and coil surfaces to prevent biological growth?
Answer: Copper-based biocide tablets or pan strips
Copper-based biocide tablets or pan strips are placed in drain pans to slowly release antimicrobial agents that prevent algae, mold, and bacterial growth.