HVAC Systems & Building Envelope Flashcards
6 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 6 HVAC Systems & Building Envelope flashcards as text
An energy manager is evaluating the thermal performance of a building's wall assembly. The wall consists of several layers with a combined thermal resistance (R-value) of 25 hr·ft²·°F/Btu. What is the overall heat transfer coefficient (U-value) for this wall assembly?
Answer: 0.04 Btu/hr·ft²·°F
The U-value (or U-factor) is the mathematical reciprocal of the total R-value (U = 1/R). A high R-value indicates good resistance to heat flow, while a low U-value indicates that a material or assembly is a good insulator. In this case, U = 1 / 25 = 0.04 Btu/hr·ft²·°F.
A facility's centrifugal chiller is rated at 400 tons and consumes 240 kW at full load. What is the chiller's full-load energy efficiency rating in kW/ton?
Answer: 0.60 kW/ton
Chiller efficiency is calculated by dividing the power input (kW) by the cooling capacity (tons). A lower kW/ton value indicates higher efficiency. The calculation is: 240 kW / 400 tons = 0.60 kW/ton.
In a hot, sunny climate, which of the following window properties is most critical for reducing solar heat gain and thus minimizing cooling loads?
Answer: A low Solar Heat Gain Coefficient (SHGC)
The Solar Heat Gain Coefficient (SHGC) measures the fraction of solar radiation admitted through a window. A lower SHGC is desirable in cooling-dominated climates to reduce the amount of solar heat entering the building, which in turn reduces the load on the air conditioning system.
A building's air handling unit (AHU) in a temperate climate is equipped with an air-side economizer. Under which condition should the economizer be fully utilized to provide 'free cooling'?
Answer: When the outdoor air is cool and dry enough to meet the cooling demand without mechanical refrigeration.
An air-side economizer provides 'free cooling' by using cool, low-humidity outside air instead of running the mechanical cooling (compressor) system. This is most effective when the outdoor air conditions are favorable (e.g., low temperature and/or low enthalpy) to cool the space directly, reducing or eliminating the need for mechanical refrigeration.
What is the primary negative impact of uncontrolled air infiltration on a building's energy consumption in a cold climate?
Answer: It increases heating loads by allowing cold, unconditioned air to enter the building.
In a cold climate, uncontrolled air infiltration allows cold outdoor air to leak into the building envelope. This forces the heating system to work harder to maintain the desired indoor temperature, thereby increasing heating energy consumption and utility costs.
Which of the following HVAC control strategies directly reduces fan energy consumption in a Variable Air Volume (VAV) system during periods of partial load?
Answer: Static pressure reset
In a VAV system, static pressure reset lowers the duct static pressure setpoint when cooling demand is low. This allows the supply fan, controlled by a VFD, to slow down. According to fan affinity laws, this reduction in speed leads to a significant (cubed relationship) decrease in fan power consumption.