CEM HVAC and Building Envelope 2 — Questions and Answers
Question 1: What is the primary purpose of an economizer cycle in an HVAC system?
- Reduce compressor wear by limiting run time
- Use outdoor air for free cooling when conditions allow (Correct answer)
- Preheat outdoor air using exhaust air heat recovery
- Reduce humidity levels during humid outdoor conditions
Correct answer: Use outdoor air for free cooling when conditions allow
An economizer uses cool outdoor air directly for space cooling when outdoor conditions are favorable, reducing or eliminating mechanical refrigeration energy.
Question 2: Which metric best describes the overall thermal resistance of a building's entire envelope assembly?
- U-factor
- R-value (Correct answer)
- SHGC
- Thermal mass index
Correct answer: R-value
R-value measures thermal resistance; higher R-values indicate better insulating performance across the full assembly.
Question 3: A chiller's COP is 4.5. How many kW of cooling does it deliver per kW of electrical input?
- 1.0 kW
- 3.5 kW
- 4.5 kW (Correct answer)
- 5.5 kW
Correct answer: 4.5 kW
COP (Coefficient of Performance) equals cooling output divided by energy input, so a COP of 4.5 means 4.5 kW of cooling per 1 kW of electricity consumed.
Question 4: What does 'infiltration' refer to in building energy analysis?
- Intentional fresh air introduced by the ventilation system
- Uncontrolled air leakage through gaps in the building envelope (Correct answer)
- Moisture diffusion through wall insulation
- Duct leakage within conditioned space
Correct answer: Uncontrolled air leakage through gaps in the building envelope
Infiltration is the uncontrolled flow of outdoor air into a building through cracks, gaps, and openings in the envelope, increasing heating and cooling loads.
Question 5: Which HVAC system type delivers conditioned air at variable airflow rates to match changing loads?
- Constant Air Volume (CAV)
- Variable Air Volume (VAV) (Correct answer)
- Fan Coil Unit (FCU)
- Packaged Terminal Air Conditioner (PTAC)
Correct answer: Variable Air Volume (VAV)
VAV systems modulate supply air volume to zones based on demand, reducing fan energy significantly at part-load conditions.
Question 6: What is the dew point temperature in the context of HVAC psychrometrics?
- Temperature at which air becomes fully saturated and condensation begins (Correct answer)
- Outdoor design temperature used for cooling load calculations
- Supply air temperature leaving the AHU cooling coil
- Minimum thermostat setpoint allowed by ASHRAE 90.1
Correct answer: Temperature at which air becomes fully saturated and condensation begins
Dew point is the temperature at which moist air becomes saturated (100% RH) and water vapor begins to condense into liquid water.
Question 7: A building owner is considering replacing single-pane windows with double-pane low-e windows. What is the primary energy benefit?
- Increased daylighting and reduced lighting loads
- Reduced solar heat gain and improved thermal resistance (Correct answer)
- Improved natural ventilation potential
- Lower window frame conductive losses only
Correct answer: Reduced solar heat gain and improved thermal resistance
Double-pane low-e windows reduce both conductive heat transfer (lower U-factor) and solar heat gain (lower SHGC), cutting both heating and cooling loads.
CEM HVAC and Building Envelope 2
What is the primary purpose of an economizer cycle in an HVAC system?