Certified Energy Auditor Certification Certified Energy Auditor HVAC Systems and Controls 1 — Questions and Answers
Question 1: A variable frequency drive (VFD) is installed on a centrifugal fan operating in an HVAC system. If the fan speed is reduced to 80% of its rated speed, by approximately what percentage does fan power consumption decrease?
- 20%
- 36%
- 49% (Correct answer)
- 64%
Correct answer: 49%
Centrifugal fan power follows the Affinity Laws: power varies as the cube of speed. (0.8)³ = 0.512, meaning power drops to about 51% of original — a reduction of approximately 49%. This is why VFDs yield dramatic energy savings even with modest speed reductions.
Question 2: During an energy audit of a large office building, CO2 concentrations in occupied zones average 1,400 ppm despite ASHRAE 62.1 specifying 1,100 ppm as a guideline threshold above background. The most energy-efficient corrective action is:
- Replace all AHU supply fans with higher-capacity units
- Install demand-controlled ventilation (DCV) using CO2 sensors to modulate outside air (Correct answer)
- Increase fixed outside air damper openings to maximum on all AHUs
- Add supplemental exhaust fans to each zone
Correct answer: Install demand-controlled ventilation (DCV) using CO2 sensors to modulate outside air
Demand-controlled ventilation uses CO2 sensors to modulate outside air intake based on actual occupancy, delivering just enough fresh air to maintain acceptable indoor air quality without conditioning excess outside air. This corrects the IAQ problem while minimizing the energy penalty of over-ventilation.
Question 3: An energy auditor measures the approach temperature of a cooling tower and finds it is 12°F above the design specification of 4°F. Which condition most likely explains this?
- Condenser water supply temperature is too low
- Cooling tower fill is fouled or partially blocked (Correct answer)
- Chilled water return temperature is elevated
- Condenser water flow rate is higher than design
Correct answer: Cooling tower fill is fouled or partially blocked
Approach temperature is the difference between the leaving water temperature and the ambient wet-bulb temperature. Fouled or blocked fill reduces heat and mass transfer between water and air, raising the leaving water temperature and increasing the approach temperature well above design.
Question 4: An air-handling unit serving a hospital serves both interior and perimeter zones simultaneously year-round. An auditor identifies significant reheat energy waste. Which energy recovery technology would best reduce both heating and cooling energy at this AHU?
- Evaporative pre-cooler on the return air stream
- Energy recovery ventilator (ERV) with an enthalpy wheel (Correct answer)
- Static pressure reset control on the supply fan
- Two-pipe fan coil units at each zone
Correct answer: Energy recovery ventilator (ERV) with an enthalpy wheel
An enthalpy wheel ERV transfers both sensible heat and latent moisture between exhaust and supply air streams, pre-conditioning incoming outside air with energy that would otherwise be discarded. In a hospital with high mandatory outside air requirements, this dramatically cuts both heating and cooling loads.
Question 5: During a duct leakage test on a packaged rooftop unit, a technician measures total leakage of 22% of system airflow. ASHRAE Standard 90.1 typically requires sealing when leakage exceeds 4% for new construction. What is the primary energy impact of excessive duct leakage in a supply duct located in an unconditioned attic?
- Increased static pressure causes the supply fan to operate below its design speed
- Conditioned air is lost to an unconditioned space, forcing the system to run longer to meet setpoint (Correct answer)
- Duct leakage improves mixing of return and supply air, reducing zone temperature stratification
- Refrigerant charge is indirectly reduced by the lower airflow across the evaporator coil
Correct answer: Conditioned air is lost to an unconditioned space, forcing the system to run longer to meet setpoint
Supply duct leakage in an unconditioned attic means conditioned air bypasses the occupied zone entirely. The thermostat never sees that delivered energy, so the system runs longer to satisfy the setpoint — directly wasting both fan and compressor energy proportional to the leakage fraction.
Question 6: A building automation system (BAS) log shows that supply air temperature reset is currently disabled and the AHU delivers 55°F supply air continuously regardless of load. An auditor recommends enabling supply air temperature reset. What is the primary energy benefit of this strategy?
- It allows the chiller to operate at a higher leaving chilled water temperature during part-load conditions, improving chiller COP (Correct answer)
- It eliminates the need for economizer controls during mild weather
- It reduces condenser water flow to cooling towers during peak demand
- It lowers the supply fan static pressure setpoint by increasing duct diameter
Correct answer: It allows the chiller to operate at a higher leaving chilled water temperature during part-load conditions, improving chiller COP
When cooling load is low, supply air temperature reset raises the chilled water supply temperature setpoint. Chillers are more efficient at higher evaporator temperatures — each degree of lift reduction typically improves COP by 1–2%. The chiller operates at a higher suction pressure, reducing compressor work significantly during the many hours of part-load operation.
