HVAC Air Handler Units and Coils 4 — Questions and Answers
Question 1: What is the purpose of a hot gas bypass valve in an air handler system?
- To bypass return air around the coil during defrost
- To divert hot refrigerant gas to prevent evaporator coil freezing at low loads (Correct answer)
- To reheat supply air for dehumidification
- To bypass the compressor during startup
Correct 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.
Question 2: What is 'sensible heat ratio' (SHR) and why is it important for coil selection?
- The ratio of heating to cooling capacity of a coil
- The proportion of total cooling capacity used for temperature reduction vs. dehumidification (Correct answer)
- The efficiency rating of the coil at standard conditions
- The ratio of coil surface area to air volume
Correct 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.
Question 3: What is the typical face velocity (air speed through coil face area) for a residential evaporator coil?
- 100-200 feet per minute
- 300-500 feet per minute (Correct answer)
- 600-900 feet per minute
- 1,000-1,500 feet per minute
Correct 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.
Question 4: What is a 'cased coil' in residential HVAC terminology?
- A coil enclosed in a metal cabinet for installation in a duct system (Correct answer)
- A coil that has been brazed and pressure-tested
- A coil with factory-installed expansion device
- A coil designed specifically for heat pump applications
Correct 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.
Question 5: What happens to system efficiency when evaporator coil fins become coated with a thin layer of dirt?
- Efficiency increases slightly due to added surface roughness
- Efficiency decreases because heat transfer is impaired (Correct answer)
- Efficiency remains the same as long as airflow is maintained
- Efficiency increases because the dirt acts as insulation on the refrigerant side
Correct 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.
Question 6: What is the purpose of a secondary (auxiliary) drain pan under an air handler unit?
- To provide additional condensate capacity during high humidity periods
- To catch water if the primary drain pan or line fails, preventing property damage (Correct answer)
- To collect refrigerant during system servicing
- To hold water for evaporative cooling of the unit
Correct 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.
Question 7: What chemical treatment is commonly applied to drain pans and coil surfaces to prevent biological growth?
- Hydrochloric acid solution
- Copper-based biocide tablets or pan strips (Correct answer)
- Bleach applied to the refrigerant lines
- Ammonia solution sprayed on coil fins
Correct 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.
HVAC Air Handler Units and Coils 4
What is the purpose of a hot gas bypass valve in an air handler system?