CEM HVAC Systems & Building Envelope 3 — Questions and Answers
Question 1: A variable-air-volume (VAV) system reduces airflow to a zone at part-load conditions. What energy benefit does this provide over a constant-volume system?
- Eliminates the need for duct insulation
- Fan energy decreases cubically with airflow reduction (Correct answer)
- Chiller efficiency improves at higher condensing temperatures
- Boiler fuel consumption increases proportionally
Correct answer: Fan energy decreases cubically with airflow reduction
Fan power follows the affinity laws: reducing airflow to 50% cuts fan power to approximately 12.5% (cube of flow ratio), yielding large energy savings.
Question 2: What is the purpose of a thermal break in a window frame or curtain wall system?
- Seal gaps against air infiltration
- Interrupt conductive heat transfer through the metal frame (Correct answer)
- Increase structural rigidity of the glazing system
- Reduce solar reflectance of the frame surface
Correct answer: Interrupt conductive heat transfer through the metal frame
A thermal break is a low-conductivity material inserted in the frame to interrupt the metal conduction path, reducing heat transfer and condensation risk.
Question 3: In psychrometrics, what does the wet-bulb temperature primarily indicate?
- Dry sensible temperature of the air
- Moisture content of air in grains per pound
- Evaporative cooling potential and enthalpy of air (Correct answer)
- Dew point temperature of the air sample
Correct answer: Evaporative cooling potential and enthalpy of air
Wet-bulb temperature reflects the air's evaporative cooling potential and correlates closely with air enthalpy, making it critical for cooling tower and economizer analysis.
Question 4: Which strategy best addresses the 'stack effect' that drives air infiltration in tall buildings during winter?
- Increasing supply air temperature at the AHU
- Pressurizing lobby and lower floors with conditioned air (Correct answer)
- Installing operable windows on upper floors
- Reducing chilled water flow to perimeter coils
Correct answer: Pressurizing lobby and lower floors with conditioned air
Pressurizing the lower building (where cold outdoor air infiltrates) counteracts the stack effect-driven pressure differential that draws cold air in at the base.
Question 5: A CEM is evaluating heat recovery from exhaust air. Which device achieves both sensible and latent energy recovery?
- Run-around glycol coil loop
- Rotary enthalpy wheel (Correct answer)
- Fixed-plate sensible heat exchanger
- Heat pipe heat exchanger
Correct answer: Rotary enthalpy wheel
A rotary enthalpy wheel uses a desiccant-coated rotating matrix to transfer both sensible heat and moisture (latent energy) between exhaust and incoming air streams.
Question 6: Which metric is used to rate the seasonal efficiency of a central air-conditioning system in the US?
- COP (Coefficient of Performance)
- EER (Energy Efficiency Ratio)
- SEER (Seasonal Energy Efficiency Ratio) (Correct answer)
- IPLV (Integrated Part Load Value)
Correct answer: SEER (Seasonal Energy Efficiency Ratio)
SEER accounts for efficiency across a range of outdoor temperatures and part-load conditions over a cooling season, making it the standard US rating for residential and light commercial AC.
Question 7: What is the main energy penalty of reheat in a constant-volume dual-duct or reheat HVAC system?
- Increased fan motor current draw
- Simultaneous cooling and heating of the same air stream (Correct answer)
- Higher outdoor air infiltration rates
- Reduced latent heat removal from conditioned spaces
Correct answer: Simultaneous cooling and heating of the same air stream
Reheat systems cool air to handle humidity, then heat it back up to supply temperature — wasting both the cooling energy and the reheat fuel simultaneously.
CEM HVAC Systems & Building Envelope 3
A variable-air-volume (VAV) system reduces airflow to a zone at part-load conditions.
What energy benefit does this provide over a constant-volume system?