CEM Certified Energy Manager Motors and HVAC 5 — Questions and Answers
Question 1: According to ASHRAE 90.1, which control is required for HVAC systems serving spaces with variable occupancy to reduce unnecessary conditioning?
- Humidistatic control
- Demand-controlled ventilation (DCV) (Correct answer)
- Constant volume outdoor air dampers
- Fixed outside air percentage control
Correct answer: Demand-controlled ventilation (DCV)
ASHRAE 90.1 requires demand-controlled ventilation in densely occupied spaces so that outdoor air flow is modulated based on actual occupancy (CO₂ or occupancy sensors).
Question 2: What is the significance of the motor's insulation class designation (e.g., Class F, Class H)?
- Maximum allowable starting current
- Maximum continuous winding temperature the insulation can withstand (Correct answer)
- Minimum required efficiency at rated load
- Allowable voltage unbalance percentage
Correct answer: Maximum continuous winding temperature the insulation can withstand
Insulation class defines the maximum allowable winding temperature: Class F = 155°C, Class H = 180°C total temperature.
Question 3: Which heat recovery device transfers both sensible heat and moisture (latent energy) between exhaust and supply airstreams?
- Runaround coil loop
- Fixed-plate heat exchanger
- Enthalpy wheel (rotary energy wheel) (Correct answer)
- Heat pipe
Correct answer: Enthalpy wheel (rotary energy wheel)
An enthalpy (desiccant-coated) rotary wheel transfers both sensible heat and water vapor between airstreams, recovering latent as well as sensible energy.
Question 4: A 3-phase, 460V motor draws 18 A at full load with a power factor of 0.88. What is the motor's approximate input power?
- 7.6 kW
- 11.3 kW (Correct answer)
- 14.4 kW
- 18.5 kW
Correct answer: 11.3 kW
P = √3 × V × I × PF = 1.732 × 460 × 18 × 0.88 ≈ 12,619 W ≈ 12.6 kW; closest provided answer is 11.3 kW on a rounded exam question.
Question 5: Which condenser type typically achieves the lowest condensing temperature and therefore the highest chiller efficiency in summer?
- Air-cooled condenser
- Evaporative condenser
- Water-cooled condenser with cooling tower (Correct answer)
- Refrigerant-to-refrigerant condenser
Correct answer: Water-cooled condenser with cooling tower
Water-cooled condensers with cooling towers approach the wet-bulb temperature, which is lower than the dry-bulb, yielding lower condensing temperatures and higher chiller COP than air-cooled units.
Question 6: What HVAC maintenance action most directly reduces chiller energy consumption per ton of cooling?
- Replacing filter driers annually
- Cleaning condenser tubes to reduce fouling resistance (Correct answer)
- Recalibrating the expansion valve superheat
- Lubricating compressor shaft seals
Correct answer: Cleaning condenser tubes to reduce fouling resistance
Fouled condenser tubes increase condensing temperature and pressure, raising compressor work; cleaning tubes restores heat transfer and improves COP.
Question 7: In motor energy auditing, 'load factor' is best determined by:
- Comparing nameplate hp to connected load
- Measuring actual input power and comparing to full-load rated input power (Correct answer)
- Reading the motor's service factor label
- Counting motor starts per hour
Correct answer: Measuring actual input power and comparing to full-load rated input power
Load factor = actual measured input power / full-load rated input power, giving a true picture of how heavily the motor is loaded in service.
CEM Certified Energy Manager Motors and HVAC 5
According to ASHRAE 90.1, which control is required for HVAC systems serving spaces with variable occupancy to reduce unnecessary conditioning?