HVAC Systems & Building Envelope Flashcards
7 cards from real CEM practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 HVAC Systems & Building Envelope flashcards as text
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?
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.
What is the purpose of a thermal break in a window frame or curtain wall system?
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.
In psychrometrics, what does the wet-bulb temperature primarily indicate?
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.
Which strategy best addresses the 'stack effect' that drives air infiltration in tall buildings during winter?
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.
A CEM is evaluating heat recovery from exhaust air. Which device achieves both sensible and latent energy recovery?
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.
Which metric is used to rate the seasonal efficiency of a central air-conditioning system in the US?
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.
What is the main energy penalty of reheat in a constant-volume dual-duct or reheat HVAC system?
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.