CEM HVAC and Building Envelope 5 — Questions and Answers
Question 1: In a blower door test, what does a lower ACH50 value indicate?
- Higher ventilation effectiveness
- Better building envelope airtightness (Correct answer)
- Greater stack effect pressure differential
- Lower HVAC system static pressure
Correct 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.
Question 2: What is 'chiller staging' and why is it important for energy efficiency?
- Scheduling chiller maintenance in phases to avoid downtime
- Operating multiple chillers to match load while keeping each near its optimal efficiency point (Correct answer)
- Sequencing chiller start-up to prevent electrical demand spikes
- Rotating chillers to equalize runtime hours
Correct 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.
Question 3: A roof's thermal emittance and solar reflectance index (SRI) are important for which energy strategy?
- Natural ventilation design
- Cool roof implementation to reduce cooling loads (Correct answer)
- Rooftop HVAC equipment sizing
- Daylighting design
Correct 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.
Question 4: What is the 'stack effect' and how does it affect building energy use?
- Duct pressure imbalance caused by variable air volume dampers
- Buoyancy-driven air movement that increases infiltration in tall buildings during winter (Correct answer)
- Heat stratification in high-ceiling spaces that raises thermostat setpoints
- Pressure difference across cooling coils that reduces airflow
Correct 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.
Question 5: Which measure is most effective for reducing peak cooling demand from solar heat gain through east- and west-facing windows?
- Installing high-R-value double-pane glazing
- Adding exterior shading devices such as fins or overhangs (Correct answer)
- Increasing supply air volume to perimeter zones
- Applying interior reflective blinds
Correct 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.
Question 6: What does 'latent heat load' refer to in building cooling calculations?
- Heat conducted through the building envelope
- Energy required to change moisture content of air without changing temperature (Correct answer)
- Radiant heat from occupants transmitted to surfaces
- Sensible heat gain from outdoor air infiltration
Correct 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.
Question 7: 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?
- Below 55°F (13°C)
- Below 32°F (0°C)
- Below 17°F (-8°C) (Correct answer)
- Below 0°F (-18°C)
Correct 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.
CEM HVAC and Building Envelope 5
In a blower door test, what does a lower ACH50 value indicate?