CEM HVAC Systems & Building Envelope 2 — Questions and Answers
Question 1: What is the primary purpose of an economizer cycle in an HVAC system?
- Reduce refrigerant charge
- Use outdoor air for free cooling when conditions allow (Correct answer)
- Increase supply air temperature during winter
- Bypass the air handler during peak loads
Correct answer: Use outdoor air for free cooling when conditions allow
An economizer uses outdoor air for cooling when outdoor conditions are favorable, reducing or eliminating mechanical cooling energy.
Question 2: A building's R-value measures which property of the building envelope?
- Air infiltration rate
- Thermal resistance to heat flow (Correct answer)
- Solar heat gain coefficient
- Moisture vapor permeability
Correct answer: Thermal resistance to heat flow
R-value quantifies the thermal resistance of a material or assembly; higher R-values indicate better insulation performance.
Question 3: Which HVAC control strategy staggers equipment startup times to reduce peak electrical demand?
- Duty cycling
- Demand limiting
- Optimal start (Correct answer)
- Load shedding
Correct answer: Optimal start
Optimal start calculates the latest possible startup time to reach setpoint by occupancy, spreading equipment activation and reducing demand spikes.
Question 4: In a chilled water system, what does the term 'chiller plant delta-T' refer to?
- Outdoor air temperature swing
- Temperature difference between chilled water supply and return (Correct answer)
- Condenser water temperature rise across the cooling tower
- Difference between chiller setpoint and zone setpoint
Correct answer: Temperature difference between chilled water supply and return
Delta-T in a chilled water system is the difference between return and supply water temperatures; a low delta-T syndrome indicates system inefficiency.
Question 5: Which window property most directly controls the amount of solar radiation entering a building?
- U-factor
- Visible light transmittance
- Solar Heat Gain Coefficient (SHGC) (Correct answer)
- Air leakage rating
Correct answer: Solar Heat Gain Coefficient (SHGC)
SHGC indicates the fraction of incident solar radiation admitted through a window; lower values reduce cooling loads in warm climates.
Question 6: What is the function of a thermal expansion valve (TXV) in a vapor-compression refrigeration system?
- Compress refrigerant vapor to high pressure
- Condense refrigerant from vapor to liquid
- Meter refrigerant flow and drop pressure before the evaporator (Correct answer)
- Separate liquid from vapor in the suction line
Correct answer: Meter refrigerant flow and drop pressure before the evaporator
The TXV reduces refrigerant pressure and controls mass flow into the evaporator to maintain proper superheat.
Question 7: Which blower door test result metric quantifies building envelope air tightness?
- ACH50 (air changes per hour at 50 Pa) (Correct answer)
- CFM per ton of cooling
- Inches of water column static pressure
- Effective leakage area in square inches
Correct answer: ACH50 (air changes per hour at 50 Pa)
ACH50 expresses the number of whole-house air changes per hour when the building is pressurized to 50 Pascals, providing a standardized infiltration metric.
CEM HVAC Systems & Building Envelope 2
What is the primary purpose of an economizer cycle in an HVAC system?