CEM HVAC and Building Envelope 3 — Questions and Answers
Question 1: What is the purpose of demand-controlled ventilation (DCV) in commercial buildings?
- Maintain constant outdoor airflow regardless of occupancy
- Modulate outdoor air intake based on CO2 or occupancy sensors (Correct answer)
- Increase supply air temperature during peak occupancy
- Pressurize stairwells for fire egress
Correct answer: Modulate outdoor air intake based on CO2 or occupancy sensors
DCV reduces outdoor air ventilation when spaces are less occupied, cutting heating and cooling energy while maintaining acceptable indoor air quality.
Question 2: Which of the following best describes 'thermal bridging' in a wall assembly?
- Heat stored in high-mass wall materials
- A highly conductive element that bypasses insulation layers (Correct answer)
- The transfer of latent heat through a wall cavity
- Radiant heat transfer across an air gap
Correct answer: A highly conductive element that bypasses insulation layers
Thermal bridges are elements like metal studs or fasteners that conduct heat more easily than surrounding insulation, reducing the effective R-value of the assembly.
Question 3: What is the Integrated Part Load Value (IPLV) used to evaluate?
- Annual energy use of a boiler at multiple firing rates
- Chiller efficiency across a range of part-load operating conditions (Correct answer)
- Fan energy at variable speed drive set points
- Heat pump performance at varying outdoor temperatures
Correct answer: Chiller efficiency across a range of part-load operating conditions
IPLV is a weighted average efficiency metric for chillers that accounts for the fact that they rarely operate at full load, providing a more realistic performance comparison.
Question 4: In a cooling load calculation, which load component accounts for heat from occupants, lighting, and equipment?
- Envelope conduction load
- Ventilation load
- Internal heat gains (Correct answer)
- Infiltration load
Correct answer: Internal heat gains
Internal heat gains are heat generated inside the conditioned space by people, lighting, and plug loads, all of which must be removed by the cooling system.
Question 5: What does an air-side economizer's 'high-limit shutoff' control?
- Maximum supply air temperature leaving the AHU
- Outdoor air temperature or enthalpy threshold above which economizing is disabled (Correct answer)
- Maximum fan speed during economizer operation
- Minimum outdoor air damper position for code compliance
Correct answer: Outdoor air temperature or enthalpy threshold above which economizing is disabled
The high-limit shutoff closes the outdoor air damper to minimum position when outdoor conditions (temperature or enthalpy) exceed a threshold where free cooling is no longer beneficial.
Question 6: Which insulation material has the highest R-value per inch of thickness?
- Fiberglass batt
- Expanded polystyrene (EPS)
- Spray polyurethane foam (closed-cell) (Correct answer)
- Mineral wool board
Correct answer: Spray polyurethane foam (closed-cell)
Closed-cell spray polyurethane foam achieves approximately R-6 to R-7 per inch, the highest commonly used insulation, due to its low-conductivity blowing agent.
Question 7: A building's heating system is sized using outdoor design conditions. What temperature is typically used for heating design in the U.S.?
- Annual minimum recorded temperature
- 99% or 99.6% heating dry-bulb from ASHRAE weather data (Correct answer)
- Average January temperature for the location
- Lowest 10-year average winter temperature
Correct answer: 99% or 99.6% heating dry-bulb from ASHRAE weather data
ASHRAE recommends using the 99% or 99.6% heating design dry-bulb temperature, meaning outdoor air exceeds this cold temperature only 1% or 0.4% of annual hours.
CEM HVAC and Building Envelope 3
What is the purpose of demand-controlled ventilation (DCV) in commercial buildings?