Certified Energy Auditor Certification HVAC Systems and Performance 5 — Questions and Answers
Question 1: What is 'thermal stratification' in a large commercial space and why does it matter for energy efficiency?
- Uneven insulation causing cold spots that increase heating loads
- Temperature layering where hot air accumulates near the ceiling, increasing heating energy if not destratified (Correct answer)
- Condensation on cold surfaces causing latent heat gains
- Uneven airflow that creates comfort complaints but has no energy impact
Correct answer: Temperature layering where hot air accumulates near the ceiling, increasing heating energy if not destratified
Warm air rises and stratifies near the ceiling in tall spaces; heating energy is wasted warming unconditioned upper zones unless destratification fans mix the air.
Question 2: Which control strategy can reduce chiller plant energy use by raising chilled water supply temperature during low-load conditions?
- Chilled water temperature reset (Correct answer)
- Condenser water flow reset
- Chiller sequencing with lead-lag control
- Cooling tower fan speed modulation
Correct answer: Chilled water temperature reset
Chilled water supply temperature reset raises the setpoint when cooling loads are low, allowing the chiller to operate at a higher suction pressure and lower kW/ton.
Question 3: An auditor finds that a 50-hp supply fan motor runs 8,760 hours/year at full speed. Installing a VFD that reduces average speed to 80% would reduce annual kWh consumption by approximately how much?
- 20%
- 36%
- 49%
- 51% (Correct answer)
Correct answer: 51%
By the cube law, power at 80% speed = 0.8³ = 0.512, so energy savings ≈ 1 – 0.512 = 48.8%, approximately 51%.
Question 4: What is 'integrated part-load value' (IPLV) used to characterize?
- The chiller's efficiency at a single full-load ARI test condition
- A weighted average chiller efficiency across multiple part-load conditions (Correct answer)
- Annual energy use index for the entire HVAC plant
- Seasonal efficiency of rooftop packaged units only
Correct answer: A weighted average chiller efficiency across multiple part-load conditions
IPLV is a weighted average of chiller COP or kW/ton measured at 100%, 75%, 50%, and 25% load conditions, reflecting real-world part-load operation.
Question 5: A maintenance log shows a split-system air conditioner's condenser coils are heavily fouled with dirt. What is the most direct energy impact?
- Reduced airflow across the evaporator coil lowers sensible cooling capacity
- Increased condensing temperature raises compressor power and reduces efficiency (Correct answer)
- Refrigerant charge is lost through micro-leaks at fouled coil joints
- Evaporator superheat decreases, causing liquid slugging of the compressor
Correct answer: Increased condensing temperature raises compressor power and reduces efficiency
Fouled condenser coils restrict airflow and heat transfer, raising condensing temperature and pressure, which forces the compressor to work harder and reduces efficiency.
Question 6: Under ASHRAE 90.1, what is required for single-zone HVAC systems serving spaces greater than 10,000 ft²?
- Variable speed drives on all fans
- An airside economizer unless exempted by climate zone or other exception (Correct answer)
- Demand-controlled ventilation at all times
- Heat recovery on 100% of exhaust air
Correct answer: An airside economizer unless exempted by climate zone or other exception
ASHRAE 90.1 requires airside economizers on single-zone systems serving spaces over 10,000 ft² (with climate-zone-based exemptions and other exceptions).
Question 7: What does a superheat measurement at the evaporator outlet indicate about refrigerant charge and metering device performance?
- High superheat suggests overcharge or a stuck-open expansion valve
- Low superheat suggests undercharge or a restricted expansion valve
- High superheat may indicate undercharge or an underfeeding expansion valve (Correct answer)
- Superheat is unrelated to charge and only reflects ambient temperature
Correct answer: High superheat may indicate undercharge or an underfeeding expansion valve
High evaporator outlet superheat typically indicates insufficient refrigerant (undercharge) or an expansion valve that is not feeding enough refrigerant into the evaporator.
Certified Energy Auditor Certification HVAC Systems and Performance 5
What is 'thermal stratification' in a large commercial space and why does it matter for energy efficiency?