Certified Energy Auditor Certification HVAC Systems and Performance 3 — Questions and Answers
Question 1: When performing a duct leakage test, what does CFM25 indicate?
- Cubic feet per minute of airflow at 25 Pa pressure differential (Correct answer)
- Cubic feet per minute measured at 25% of design flow
- Duct leakage expressed as 25% of supply volume
- Flow rate at 25°F below design temperature
Correct answer: Cubic feet per minute of airflow at 25 Pa pressure differential
CFM25 is the measured duct leakage airflow in cubic feet per minute at a reference pressure of 25 Pascals.
Question 2: What is the function of an economizer cycle in a commercial HVAC system?
- Reduce chiller lift by modulating condenser water flow
- Use outdoor air for free cooling when conditions allow (Correct answer)
- Recover heat from exhaust air to reduce heating energy
- Lower refrigerant condensing pressure during low-load periods
Correct answer: Use outdoor air for free cooling when conditions allow
An economizer opens outdoor air dampers to use cool outdoor air for free cooling, reducing or eliminating mechanical refrigeration.
Question 3: A building's HVAC system uses a pneumatic control system. What is a major energy-efficiency concern with this type of system?
- High electrical demand from the compressor serving the controls
- Hunting and poor setpoint accuracy increasing energy use (Correct answer)
- Inability to integrate with DDC systems
- Excessive chilled water consumption
Correct answer: Hunting and poor setpoint accuracy increasing energy use
Pneumatic controls are prone to hunting and calibration drift, leading to wider temperature swings and higher energy consumption compared to direct digital control.
Question 4: In a hydronic system, what does 'delta-T' refer to?
- Pressure drop across the pump
- Temperature difference between supply and return water (Correct answer)
- Time delay in control valve response
- Thermal expansion coefficient of piping
Correct answer: Temperature difference between supply and return water
Delta-T (ΔT) in a hydronic system is the temperature difference between the supply water and the return water, indicating heat transfer effectiveness.
Question 5: Which fan law states that fan power varies with the cube of the fan speed?
- First fan law
- Second fan law
- Third fan law (Correct answer)
- Affinity law for pressure
Correct answer: Third fan law
The third fan law (power affinity law) states that power varies as the cube of the speed ratio, which is why VFDs yield dramatic energy savings.
Question 6: What does a high wet-bulb temperature relative to dry-bulb temperature on a psychrometric chart indicate?
- Low relative humidity
- High relative humidity (Correct answer)
- Low enthalpy in the air
- High sensible heat ratio
Correct answer: High relative humidity
When wet-bulb temperature is close to dry-bulb temperature, the air is near saturation, indicating high relative humidity.
Question 7: A facility manager reports that a VAV system's supply air temperature is constant at 55°F year-round. What energy savings opportunity does this represent?
- Compressor cycling to maintain supply temperature
- Supply air temperature reset based on zone demand (Correct answer)
- Economizer lockout optimization
- Chilled water loop delta-T improvement
Correct answer: Supply air temperature reset based on zone demand
Resetting supply air temperature upward when zone cooling loads are reduced allows the chiller to operate at higher suction temperatures, saving compressor energy.
Certified Energy Auditor Certification HVAC Systems and Performance 3
When performing a duct leakage test, what does CFM25 indicate?