HVAC Energy Efficiency 2 — Questions and Answers
Question 1: What does the term 'COP' stand for in HVAC system performance?
- Coefficient of Performance (Correct answer)
- Cooling Output Percentage
- Compressor Operating Pressure
- Capacity Output Parameter
Correct answer: Coefficient of Performance
COP (Coefficient of Performance) is the ratio of useful heat or cooling energy output to the work energy input.
Question 2: A heat pump has a heating COP of 3.5. What does this mean?
- It delivers 3.5 units of heat for every 1 unit of electrical energy consumed (Correct answer)
- It uses 3.5 kW of power at full load
- Its efficiency is 350% above baseline
- It operates at 3.5 tons of capacity
Correct answer: It delivers 3.5 units of heat for every 1 unit of electrical energy consumed
A COP of 3.5 means the heat pump delivers 3.5 units of thermal energy for every 1 unit of electrical energy input.
Question 3: Which refrigerant property most directly impacts system energy efficiency?
- Latent heat of vaporization (Correct answer)
- Molecular weight
- Boiling point color code
- Container pressure rating
Correct answer: Latent heat of vaporization
High latent heat of vaporization allows a refrigerant to absorb or release large amounts of heat during phase change, improving efficiency.
Question 4: What is the purpose of an economizer cycle in a commercial HVAC system?
- To use outdoor air for free cooling when conditions allow (Correct answer)
- To recover heat from exhaust gases
- To reduce refrigerant charge
- To cycle the compressor on and off
Correct answer: To use outdoor air for free cooling when conditions allow
An economizer uses cool outdoor air directly for space cooling, reducing or eliminating compressor operation when outdoor conditions are favorable.
Question 5: What HVAC measure is most effective at reducing energy use in large commercial buildings during partial-load conditions?
- Variable frequency drives (VFDs) on fans and pumps (Correct answer)
- Increasing refrigerant charge by 10%
- Using larger fixed-speed compressors
- Adding more supply air diffusers
Correct answer: Variable frequency drives (VFDs) on fans and pumps
VFDs reduce motor speed proportionally to load, cutting fan and pump energy consumption dramatically at partial loads.
Question 6: Which of the following best describes 'sensible heat ratio' (SHR) in HVAC?
- The fraction of total cooling load attributable to sensible (temperature) heat vs. total heat (Correct answer)
- The ratio of supply air temperature to return air temperature
- The efficiency of the heating coil
- The percentage of outdoor air in the supply stream
Correct answer: The fraction of total cooling load attributable to sensible (temperature) heat vs. total heat
SHR is the ratio of sensible cooling capacity to total cooling capacity (sensible + latent), indicating how much of the load is temperature-related vs. moisture-related.
Question 7: How does subcooling refrigerant before the expansion valve improve system efficiency?
- It increases the refrigerant's capacity to absorb heat in the evaporator (Correct answer)
- It raises the compressor discharge pressure
- It reduces superheat at the suction line
- It increases the refrigerant flow rate through the condenser
Correct answer: It increases the refrigerant's capacity to absorb heat in the evaporator
Subcooling ensures the refrigerant enters the expansion valve as a full liquid, increasing the enthalpy difference across the evaporator and improving system capacity.
HVAC Energy Efficiency 2
What does the term 'COP' stand for in HVAC system performance?