Certified Energy Auditor Certification HVAC Systems and Controls 3 — Questions and Answers
Question 1: Demand-controlled ventilation (DCV) uses which sensor to modulate outdoor air intake?
- Relative humidity sensor
- CO₂ sensor (Correct answer)
- Occupancy motion sensor
- Static pressure sensor
Correct answer: CO₂ sensor
DCV uses CO₂ sensors as a proxy for occupancy; when CO₂ rises above setpoint, outdoor airflow increases to dilute occupant-generated contaminants.
Question 2: What is the purpose of a bypass factor in coil selection?
- Fraction of air that bypasses the coil without being conditioned (Correct answer)
- Ratio of supply to return air
- Percentage of refrigerant that bypasses the expansion valve
- Measure of duct leakage
Correct answer: Fraction of air that bypasses the coil without being conditioned
The bypass factor represents the fraction of air that passes through a cooling coil without contacting the coil surface and therefore is not fully conditioned.
Question 3: A two-pipe hydronic system serving both heating and cooling has what significant limitation?
- Cannot use variable speed pumps
- Cannot serve heating and cooling simultaneously in different zones (Correct answer)
- Requires chilled water below 35°F
- Cannot use plate heat exchangers
Correct answer: Cannot serve heating and cooling simultaneously in different zones
A two-pipe system uses the same piping for hot or chilled water, so it can only provide one mode at a time — a problem in buildings with simultaneous heating and cooling needs.
Question 4: What does the term 'integrated part-load value' (IPLV) measure for chillers?
- Efficiency at full load only
- Weighted average efficiency at multiple part-load conditions (Correct answer)
- Thermal storage capacity
- Refrigerant leakage rate
Correct answer: Weighted average efficiency at multiple part-load conditions
IPLV is a weighted average efficiency metric that accounts for part-load operation at 25%, 50%, 75%, and 100% loads, reflecting real-world performance better than full-load ratings.
Question 5: Which type of expansion device provides the best refrigeration system efficiency at varying loads?
- Fixed orifice
- Capillary tube
- Thermostatic expansion valve (TXV)
- Electronic expansion valve (EEV) (Correct answer)
Correct answer: Electronic expansion valve (EEV)
Electronic expansion valves precisely modulate refrigerant flow based on superheat and other inputs, providing superior efficiency across all load conditions.
Question 6: In a direct expansion (DX) system, what does superheat indicate?
- The temperature of liquid refrigerant leaving the condenser
- The temperature rise of refrigerant vapor above its saturation temperature (Correct answer)
- The amount of subcooling at the expansion valve
- The compressor discharge temperature relative to ambient
Correct answer: The temperature rise of refrigerant vapor above its saturation temperature
Superheat is the number of degrees the refrigerant vapor exceeds its saturation (boiling) temperature at a given pressure, ensuring no liquid enters the compressor.
Question 7: What strategy can reduce chiller energy consumption by raising chilled water supply temperature during mild weather?
- Chilled water reset (Correct answer)
- Condenser water bypass
- Compressor unloading
- Hot gas bypass
Correct answer: Chilled water reset
Chilled water reset raises the supply water temperature setpoint when loads are low, reducing the lift the compressor must overcome and improving COP.
Certified Energy Auditor Certification HVAC Systems and Controls 3
Demand-controlled ventilation (DCV) uses which sensor to modulate outdoor air intake?