CEM HVAC Systems & Building Envelope 4 — Questions and Answers
Question 1: A facility engineer notes that cooling tower approach temperature has increased over several months. What is the most likely cause?
- Higher chilled water supply setpoint
- Fouled or scaled fill media reducing heat transfer (Correct answer)
- Increased chiller compressor speed
- Lower condenser water flow rate setpoint
Correct answer: Fouled or scaled fill media reducing heat transfer
Scale or biological fouling on the tower fill reduces wetted surface area and heat/mass transfer effectiveness, increasing the approach temperature.
Question 2: Under ASHRAE 90.1, what is the primary purpose of the building envelope compliance path using the prescriptive method?
- Size HVAC equipment based on calculated peak loads
- Verify that envelope component U-values meet tabulated maximum requirements (Correct answer)
- Demonstrate that whole-building EUI is below a reference building
- Set minimum ventilation rates for each occupancy type
Correct answer: Verify that envelope component U-values meet tabulated maximum requirements
The prescriptive path requires that individual envelope components (roof, walls, windows) meet maximum U-factors and minimum R-values specified in ASHRAE 90.1 tables by climate zone.
Question 3: Which refrigerant property most directly determines the operating pressures of a vapor-compression system?
- Latent heat of vaporization
- Saturation pressure-temperature relationship (Correct answer)
- Global warming potential (GWP)
- Molecular weight of the refrigerant
Correct answer: Saturation pressure-temperature relationship
The saturation pressure-temperature curve dictates the evaporating and condensing pressures at given temperatures, defining compressor lift and system design.
Question 4: A building automation system (BAS) shows supply air temperature consistently 3°F above setpoint. What is the most energy-efficient corrective action to investigate first?
- Lower the chilled water plant setpoint by 3°F
- Calibrate or replace the supply air temperature sensor (Correct answer)
- Add a second cooling coil section in the AHU
- Increase outdoor air damper opening
Correct answer: Calibrate or replace the supply air temperature sensor
A faulty sensor is the lowest-cost root cause; sensor drift or failure will cause the control system to maintain incorrect conditions wasting energy before mechanical changes are made.
Question 5: What does the term 'thermal mass' refer to in the context of building envelope performance?
- Weight of insulation material per square foot of wall
- Capacity of building materials to absorb and store heat energy (Correct answer)
- Rate of heat gain through glazing per unit area
- Maximum allowable surface temperature of interior wall finishes
Correct answer: Capacity of building materials to absorb and store heat energy
Thermal mass describes materials with high heat capacity (concrete, masonry) that absorb heat during the day and release it later, moderating indoor temperature swings.
Question 6: Which HVAC system type delivers 100% outdoor air to each zone and exhausts 100% of return air, making it common in healthcare settings?
- Fan-coil unit system
- Dedicated outdoor air system (DOAS) (Correct answer)
- Induction unit system
- Multi-zone rooftop unit
Correct answer: Dedicated outdoor air system (DOAS)
A DOAS provides 100% outdoor air (no recirculation) to meet ventilation and infection-control requirements, often paired with radiant panels or fan-coils for sensible load.
Question 7: In a hot and humid climate, which building envelope strategy most effectively reduces latent (moisture) load on the HVAC system?
- Increasing wall R-value with fibrous batt insulation
- Installing a continuous exterior vapor retarder on the warm side (Correct answer)
- Using high-SHGC glazing to raise indoor temperature
- Adding thermal mass concrete flooring
Correct answer: Installing a continuous exterior vapor retarder on the warm side
A continuous vapor retarder on the warm-humid exterior side blocks moisture-laden outdoor air from diffusing into the wall assembly and reaching the cooled interior.
CEM HVAC Systems & Building Envelope 4
A facility engineer notes that cooling tower approach temperature has increased over several months.
What is the most likely cause?