CEA Building Systems & Energy Efficiency 5 — Questions and Answers
Question 1: What is the primary purpose of retro-commissioning (RCx) an existing building?
- Designing new HVAC systems to replace outdated equipment
- Identifying and correcting operational deficiencies to restore or improve original design performance (Correct answer)
- Certifying a building for LEED Operations & Maintenance
- Replacing controls hardware with smart building systems
Correct answer: Identifying and correcting operational deficiencies to restore or improve original design performance
Retro-commissioning systematically investigates existing building systems to find operational and maintenance problems — such as stuck dampers, wrong setpoints, or failed sensors — and corrects them, typically yielding 5–15% energy savings.
Question 2: A simple payback period of 3.2 years means:
- The project earns a 3.2% annual return
- The initial capital cost is recovered through energy savings in 3.2 years (Correct answer)
- The project has a net present value of 3.2 times the investment
- Energy savings exceed costs after 3.2 months
Correct answer: The initial capital cost is recovered through energy savings in 3.2 years
Simple payback = initial cost ÷ annual savings; a 3.2-year payback means savings accumulate to equal the upfront investment in 3.2 years, after which the savings are net positive.
Question 3: Which of the following best describes 'whole-building energy use intensity' (EUI)?
- Total annual energy consumption in kWh
- Annual energy use per square foot of conditioned floor area (kBtu/ft²/yr) (Correct answer)
- Peak demand in kW per occupant
- Monthly utility cost per square foot
Correct answer: Annual energy use per square foot of conditioned floor area (kBtu/ft²/yr)
EUI normalizes total annual energy consumption by conditioned floor area, enabling meaningful comparisons across buildings of different sizes and uses.
Question 4: In a building automation system (BAS), an 'optimal start' algorithm saves energy by:
- Keeping HVAC running 24/7 to avoid morning ramp-up energy
- Calculating the latest possible start time to reach setpoint by occupancy, minimizing pre-conditioning hours (Correct answer)
- Reducing fan speed during morning warm-up
- Locking out economizer operation during startup
Correct answer: Calculating the latest possible start time to reach setpoint by occupancy, minimizing pre-conditioning hours
Optimal start learns how long the building takes to reach setpoint under different conditions and delays system startup as long as possible, reducing hours of unoccupied HVAC operation.
Question 5: A facility installs a variable frequency drive (VFD) on a 50 hp cooling tower fan currently running at full speed 8,760 hours/year. If the fan can operate at 80% speed for 70% of hours, what is the approximate energy reduction for those hours?
- 20%
- 36%
- 51% (Correct answer)
- 64%
Correct answer: 51%
At 80% speed, power consumption ≈ (0.80)³ = 51.2% of full-load power, meaning approximately 49% savings; the question asks for approximate reduction, which is ~51% reduction in those hours.
Question 6: What does 'air sealing' address in building envelope performance?
- Reducing radiant heat transfer through walls
- Controlling uncontrolled air leakage (infiltration/exfiltration) that bypasses insulation (Correct answer)
- Improving window solar heat gain coefficients
- Increasing thermal mass of exterior walls
Correct answer: Controlling uncontrolled air leakage (infiltration/exfiltration) that bypasses insulation
Air sealing addresses infiltration — the uncontrolled movement of air through cracks and gaps — which can account for 25–40% of heating and cooling energy loss even in well-insulated buildings.
Question 7: A hospital's domestic hot water system uses a recirculation loop. The main energy benefit of insulating the recirculation piping is:
- Faster hot water delivery to fixtures
- Reduced standing heat losses from piping, lowering boiler runtime (Correct answer)
- Elimination of Legionella risk
- Lower pump head requirements
Correct answer: Reduced standing heat losses from piping, lowering boiler runtime
Uninsulated recirculation pipes continuously lose heat to the surrounding space, forcing the water heater to fire more frequently; insulation cuts these standing losses and reduces boiler energy by 10–20%.
CEA Building Systems & Energy Efficiency 5
What is the primary purpose of retro-commissioning (RCx) an existing building?