HVAC Energy Efficiency 4 — Questions and Answers
Question 1: What is the primary energy efficiency advantage of a two-stage compressor over a single-stage unit?
- It operates at reduced capacity during mild conditions, consuming less energy (Correct answer)
- It uses two different refrigerants for better performance
- It requires no condenser fan during low-stage operation
- It eliminates the need for an expansion device
Correct answer: It operates at reduced capacity during mild conditions, consuming less energy
Two-stage compressors can run at lower capacity during mild weather, matching actual load and reducing energy consumption compared to a single-stage unit cycling on and off.
Question 2: Which type of duct leakage has the greatest negative impact on HVAC efficiency?
- Supply duct leakage into unconditioned spaces (Correct answer)
- Return duct leakage drawing in unconditioned air
- Leakage between adjacent conditioned zones
- Leakage at the air handler cabinet
Correct answer: Supply duct leakage into unconditioned spaces
Supply duct leakage into unconditioned spaces (like attics or crawlspaces) directly wastes conditioned air that never reaches the occupied space, causing significant energy loss.
Question 3: What is the significance of the AHRI 210/240 test standard for residential HVAC equipment?
- It establishes the test conditions used to measure and rate cooling and heating efficiency (SEER2, HSPF2) (Correct answer)
- It sets refrigerant charge requirements for field installation
- It defines maximum allowable sound levels
- It certifies technician competency for equipment installation
Correct answer: It establishes the test conditions used to measure and rate cooling and heating efficiency (SEER2, HSPF2)
AHRI 210/240 defines the standardized test conditions and procedures used to rate residential unitary HVAC equipment efficiency, ensuring consistent manufacturer comparisons.
Question 4: A geothermal heat pump has a much higher COP than an air-source heat pump in winter. Why?
- Ground temperatures remain relatively stable year-round, providing a more favorable heat source than cold outdoor air (Correct answer)
- Geothermal systems use larger compressors
- Underground refrigerant lines are shorter
- Geothermal systems use a different refrigerant cycle
Correct answer: Ground temperatures remain relatively stable year-round, providing a more favorable heat source than cold outdoor air
Ground temperatures at depth stay near 50–55°F year-round, giving geothermal heat pumps a much warmer heat source than outdoor air during cold winters, greatly improving COP.
Question 5: What does 'integrated part load value' (IPLV) measure for commercial chillers?
- Weighted average efficiency across multiple part-load operating points to reflect real-world performance (Correct answer)
- The chiller's efficiency at 100% design load only
- The electrical demand at the point of peak cooling
- The ratio of chilled water flow to condenser water flow
Correct answer: Weighted average efficiency across multiple part-load operating points to reflect real-world performance
IPLV is a weighted average of chiller efficiency at 100%, 75%, 50%, and 25% loads, better representing annual energy use than full-load efficiency alone.
Question 6: Which control strategy best reduces energy use in a multi-zone VAV system?
- Supply air temperature reset based on zone demand (Correct answer)
- Fixed supply air temperature with constant volume at each zone
- Increasing static pressure setpoint to ensure all zones are satisfied
- Bypassing excess supply air back to the return
Correct answer: Supply air temperature reset based on zone demand
Resetting supply air temperature upward when cooling loads are low reduces refrigeration energy and allows VAV boxes to open wider, lowering fan energy simultaneously.
Question 7: How do low-e window coatings contribute to HVAC energy efficiency?
- They reflect infrared radiation, reducing solar heat gain in summer and heat loss in winter (Correct answer)
- They improve the structural insulation value of the glass
- They reduce visible light transmission to lower lighting loads
- They prevent condensation that could damage HVAC ductwork
Correct answer: They reflect infrared radiation, reducing solar heat gain in summer and heat loss in winter
Low-e coatings selectively reflect long-wave infrared radiation, reducing unwanted solar heat gain in summer and slowing heat escape through windows in winter.
HVAC Energy Efficiency 4
What is the primary energy efficiency advantage of a two-stage compressor over a single-stage unit?