HVAC Systems & Building Envelope Flashcards

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?

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.

What is the energy implication of a low 'delta-T syndrome' in a chilled water system?

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.

Which of the following roof assembly types provides the greatest reduction in urban heat island effect and cooling load?

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.

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?

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.

In ASHRAE climate zone 7 (very cold), which wall assembly location for the insulation layer is generally recommended to control moisture?

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.

What energy savings mechanism makes demand-controlled ventilation (DCV) effective in high-occupancy spaces?

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.

Which parameter is most important when selecting insulation for a below-grade foundation wall to prevent moisture-related performance degradation?

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.