HVAC Systems & Building Envelope Flashcards

What is the primary purpose of an economizer cycle in an HVAC system?

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.

A building's R-value measures which property of the building envelope?

Answer: Thermal resistance to heat flow

R-value quantifies the thermal resistance of a material or assembly; higher R-values indicate better insulation performance.

Which HVAC control strategy staggers equipment startup times to reduce peak electrical demand?

Answer: Optimal start

Optimal start calculates the latest possible startup time to reach setpoint by occupancy, spreading equipment activation and reducing demand spikes.

In a chilled water system, what does the term 'chiller plant delta-T' refer to?

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.

Which window property most directly controls the amount of solar radiation entering a building?

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.

What is the function of a thermal expansion valve (TXV) in a vapor-compression refrigeration system?

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.

Which blower door test result metric quantifies building envelope air tightness?

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.