HVAC and 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 and Building Envelope flashcards as text
In a blower door test, what does a lower ACH50 value indicate?
Answer: Better building envelope airtightness
ACH50 is air changes per hour at 50 Pascals of pressure; a lower value means less air leaks through the envelope, indicating a tighter, more energy-efficient building.
What is 'chiller staging' and why is it important for energy efficiency?
Answer: Operating multiple chillers to match load while keeping each near its optimal efficiency point
Proper staging ensures chillers operate at high part-load ratios where efficiency is greatest, rather than running one chiller at very low load where efficiency degrades.
A roof's thermal emittance and solar reflectance index (SRI) are important for which energy strategy?
Answer: Cool roof implementation to reduce cooling loads
High SRI roofs reflect solar radiation and emit absorbed heat efficiently, reducing roof surface temperature and cutting cooling loads for the conditioned space below.
What is the 'stack effect' and how does it affect building energy use?
Answer: Buoyancy-driven air movement that increases infiltration in tall buildings during winter
The stack effect occurs when warm indoor air rises and escapes at the top of a building, drawing cold outdoor air in at lower levels, increasing heating loads particularly in tall buildings.
Which measure is most effective for reducing peak cooling demand from solar heat gain through east- and west-facing windows?
Answer: Adding exterior shading devices such as fins or overhangs
Exterior shading intercepts solar radiation before it strikes the glass, far more effective than interior shading which allows heat to enter and be trapped between blind and glass.
What does 'latent heat load' refer to in building cooling calculations?
Answer: Energy required to change moisture content of air without changing temperature
Latent load is the energy needed to condense moisture out of air (dehumidification); it is associated with occupants, outdoor air, and infiltration but does not change air temperature.
A CEM candidate is evaluating a heat pump system replacing a gas furnace. At what outdoor temperature does a standard air-source heat pump typically lose significant heating capacity?
Answer: Below 17°F (-8°C)
Standard air-source heat pumps experience significant capacity and efficiency degradation below about 17°F (-8°C), often requiring supplemental electric resistance heat at lower temperatures.