CEM HVAC Systems & Building Envelope 5 — Questions and Answers
Question 1: A chiller's kW/ton rating is 0.55 at full load and 0.72 at 25% load. What does this indicate about part-load performance?
- The chiller is more efficient at part load than full load
- The chiller consumes more energy per ton of cooling at part load (Correct answer)
- The chiller capacity increases at reduced load
- The compressor speed remains constant regardless of load
Correct answer: The chiller consumes more energy per ton of cooling at part load
A higher kW/ton at part load means the chiller uses more electricity per unit of cooling produced at reduced capacity, a common characteristic of centrifugal chillers at very low loads.
Question 2: What is the energy implication of a low 'delta-T syndrome' in a chilled water system?
- Chiller lift is reduced, improving COP
- Pumping energy increases because higher flow rates are needed to meet load (Correct answer)
- Cooling tower approach temperature decreases
- Refrigerant charge must be increased to compensate
Correct answer: Pumping energy increases because higher flow rates are needed to meet load
Low delta-T means chilled water returns nearly as cold as it left, so much more water must be pumped to deliver the same cooling tonnage, wasting pump energy.
Question 3: Which of the following roof assembly types provides the greatest reduction in urban heat island effect and cooling load?
- Dark-membrane built-up roof with gravel ballast
- White TPO membrane cool roof with high solar reflectance (Correct answer)
- Copper standing-seam metal roof
- Asphalt shingle roof with attic ventilation
Correct answer: White TPO membrane cool roof with high solar reflectance
A white or light-colored membrane (high solar reflectance and thermal emittance) reflects solar radiation rather than absorbing it, reducing roof surface temperatures and cooling loads.
Question 4: A CEM is reviewing a proposed geothermal heat pump system. What is the primary advantage of using the ground as a heat source/sink compared to air?
- Ground loops eliminate the need for a refrigerant circuit
- Ground temperatures are more stable year-round than outdoor air temperatures (Correct answer)
- Geothermal systems do not require electricity to operate
- Ground-source systems always have lower installed cost than air-source systems
Correct answer: Ground temperatures are more stable year-round than outdoor air temperatures
Ground temperatures remain relatively constant (50–60°F in most US locations) year-round, allowing the heat pump to operate at consistently higher efficiency than air-source systems that face extreme winter or summer air temperatures.
Question 5: In ASHRAE climate zone 7 (very cold), which wall assembly location for the insulation layer is generally recommended to control moisture?
- All insulation on the interior side of the stud cavity
- Continuous rigid insulation on the exterior with vapor retarder near the interior face (Correct answer)
- Vapor-permeable insulation between studs only, no exterior layer
- Spray foam insulation on the exterior masonry only
Correct answer: Continuous rigid insulation on the exterior with vapor retarder near the interior face
In very cold climates, placing rigid insulation outside the structure keeps the sheathing warm (above the dew point) and a vapor retarder near the interior prevents warm moist indoor air from reaching cold surfaces.
Question 6: What energy savings mechanism makes demand-controlled ventilation (DCV) effective in high-occupancy spaces?
- DCV reduces supply fan speed based on outdoor temperature
- DCV modulates outdoor air intake based on CO2 levels, reducing over-ventilation when occupancy is low (Correct answer)
- DCV bypasses the cooling coil when CO2 is elevated
- DCV increases recirculation during occupied periods to save fan energy
Correct answer: DCV modulates outdoor air intake based on CO2 levels, reducing over-ventilation when occupancy is low
By tracking CO2 as a proxy for occupancy, DCV delivers only as much outdoor air as needed, avoiding the conditioning energy cost of over-ventilating partially occupied spaces.
Question 7: Which parameter is most important when selecting insulation for a below-grade foundation wall to prevent moisture-related performance degradation?
- High compressive strength only
- Low water absorption and moisture resistance (Correct answer)
- High vapor permeability to allow drying
- Low density for ease of installation
Correct answer: Low water absorption and moisture resistance
Below-grade insulation is in contact with soil moisture, so materials like closed-cell spray foam or extruded polystyrene (XPS) that resist water absorption maintain their R-value when wet.
CEM HVAC Systems & Building Envelope 5
A chiller's kW/ton rating is 0.55 at full load and 0.72 at 25% load.
What does this indicate about part-load performance?