Certified Energy Auditor Certification HVAC Systems and Controls 5 — Questions and Answers
Question 1: Which metric best quantifies the overall efficiency of a chilled water plant including pumps, tower fans, and auxiliary equipment?
- Chiller COP
- EER of the compressor
- kW/ton of the entire plant (Correct answer)
- SEER of the condensing unit
Correct answer: kW/ton of the entire plant
Plant-level kW/ton accounts for all energy consumers in the chilled water plant, providing a more accurate picture of total system efficiency than chiller-only metrics.
Question 2: What is the purpose of a hot gas bypass in a refrigeration system?
- Recover heat from compressor discharge for domestic hot water
- Maintain minimum compressor load during light cooling loads (Correct answer)
- Increase system efficiency at full load
- Preheat the refrigerant before the expansion valve
Correct answer: Maintain minimum compressor load during light cooling loads
Hot gas bypass artificially loads the compressor by diverting discharge gas back to the suction side, preventing compressor shutdown during very light loads — though it wastes energy compared to variable capacity compressors.
Question 3: An energy auditor finds that a building's AHU static pressure setpoint is fixed at 1.5 in. w.g. A better control strategy would be:
- Raise the setpoint to 2.0 in. w.g. for safety margin
- Use supply air temperature reset instead
- Reset static pressure down until one VAV box is fully open (trim-and-respond) (Correct answer)
- Install a pressure-independent VAV box
Correct answer: Reset static pressure down until one VAV box is fully open (trim-and-respond)
Trim-and-respond static pressure reset lowers the setpoint until the most-open VAV box is nearly fully open, minimizing fan energy while still meeting zone loads.
Question 4: What is the significance of a high wet-bulb temperature when evaluating cooling tower performance?
- It limits how cold the condenser water can get, reducing chiller efficiency (Correct answer)
- It increases airflow resistance through the tower fill
- It indicates low humidity and better evaporative cooling potential
- It has no effect on chiller performance
Correct answer: It limits how cold the condenser water can get, reducing chiller efficiency
Cooling towers can only cool condenser water to approximately the outdoor wet-bulb temperature; high wet-bulb limits cooling tower performance and raises condensing pressure in the chiller.
Question 5: When auditing HVAC controls, finding a 'deadband' of only 1°F between heating and cooling setpoints is a problem because:
- It prevents the system from reaching setpoint
- It causes frequent short-cycling and simultaneous operation (Correct answer)
- It violates ASHRAE 90.1 minimum efficiency standards
- It increases outdoor air intake unnecessarily
Correct answer: It causes frequent short-cycling and simultaneous operation
A narrow deadband causes heating and cooling to compete — a small temperature swing triggers one system then the other, wasting energy and causing equipment wear.
Question 6: Which control sequence in a dual-duct HVAC system would most reduce energy waste?
- Keep both hot and cold decks at fixed temperatures year-round
- Reset hot deck temperature upward and cold deck temperature upward in winter (Correct answer)
- Operate only the cold deck in summer and only the hot deck in winter
- Increase supply fan speed to improve mixing
Correct answer: Reset hot deck temperature upward and cold deck temperature upward in winter
Resetting hot deck temperature up and cold deck temperature up in cold weather reduces the energy needed for both heating and cooling the respective air streams.
Question 7: A retrocommissioning study reveals that a chiller is consistently operating at 110% of its rated capacity. The best corrective action is:
- Override the high-pressure cutout to allow more capacity
- Add a second chiller or upgrade to larger capacity
- Increase chilled water supply temperature setpoint to reduce load (Correct answer)
- Reduce cooling tower fan speed to raise head pressure
Correct answer: Increase chilled water supply temperature setpoint to reduce load
Raising the chilled water supply temperature setpoint reduces the cooling load the chiller must handle and may bring operation back within rated capacity without capital expense.
Certified Energy Auditor Certification HVAC Systems and Controls 5
Which metric best quantifies the overall efficiency of a chilled water plant including pumps, tower fans, and auxiliary equipment?