CEA Lighting & HVAC Energy Analysis 4 — Questions and Answers
Question 1: What is the 'cube law' as applied to variable speed HVAC fans and pumps?
- Energy use doubles when speed doubles
- Energy use varies with the square of speed
- Energy use varies with the cube of speed (Correct answer)
- Energy use is independent of speed for centrifugal equipment
Correct answer: Energy use varies with the cube of speed
The affinity law states that fan/pump power varies as the cube of speed, so reducing speed to 80% reduces power to approximately 51% (0.8³ = 0.512).
Question 2: Which lighting metric represents the ratio of lumens emitted by a luminaire to lumens produced by the bare lamp(s) inside?
- Color Rendering Index (CRI)
- Luminaire Efficacy Rating (LER)
- Light Output Ratio (LOR) (Correct answer)
- Scotopic/photopic ratio
Correct answer: Light Output Ratio (LOR)
Light Output Ratio (LOR) compares the lumens exiting the fixture to those produced by its lamps, accounting for optical losses within the housing.
Question 3: A retail store has 200 track lights at 75W each. The utility charges a demand charge of $12/kW-month for the peak 15-minute interval. What is the annual demand charge from these lights alone?
- $2,160 (Correct answer)
- $2,520
- $3,600
- $4,200
Correct answer: $2,160
Peak demand = 200 × 75W = 15,000W = 15 kW; annual demand charge = 15 kW × $12/kW-month × 12 months = $2,160.
Question 4: In HVAC energy analysis, what does 'latent load' refer to?
- Heat transferred through walls and roofs by conduction
- Energy required to change moisture content (humidity) of air (Correct answer)
- Cooling needed to overcome solar gain through windows
- Heat generated by occupants and equipment
Correct answer: Energy required to change moisture content (humidity) of air
Latent load is the energy required to add or remove moisture from air without changing its temperature, critical in humid climates.
Question 5: A demand-controlled ventilation (DCV) system reduces outdoor air flow based on which sensor input?
- Supply air temperature
- CO₂ concentration (Correct answer)
- Relative humidity
- Occupant badge access data
Correct answer: CO₂ concentration
DCV uses CO₂ sensors as a proxy for occupancy; higher CO₂ levels trigger increased outdoor air supply, reducing ventilation energy when spaces are less occupied.
Question 6: What is the Energy Efficiency Ratio (EER) of a 5-ton cooling unit that draws 6 kW at full load? (1 ton = 12,000 BTU/hr)
- 8.33
- 10.0 (Correct answer)
- 12.0
- 14.4
Correct answer: 10.0
EER = BTU/hr ÷ Watts = (5 × 12,000) ÷ 6,000 = 60,000 ÷ 6,000 = 10.0.
Question 7: Which lamp type produces light by exciting mercury vapor to emit ultraviolet radiation, which then excites a phosphor coating to produce visible light?
- Incandescent lamp
- LED lamp
- Fluorescent lamp (Correct answer)
- High-pressure sodium lamp
Correct answer: Fluorescent lamp
Fluorescent lamps pass electrical current through mercury vapor, producing UV that activates the phosphor coating on the tube interior to emit visible light.
CEA Lighting & HVAC Energy Analysis 4
What is the 'cube law' as applied to variable speed HVAC fans and pumps?