Certified Energy Auditor HVAC Systems and Controls Questions and Answers — Questions and Answers
Question 1: An energy auditor is reviewing the Building Automation System (BAS) trend data for a multi-zone Variable Air Volume (VAV) system. The auditor notes that the supply fan maintains a constant duct static pressure of 2.5 inches w.c. at all times, regardless of the building load. What is the most effective energy conservation measure to recommend?
- Implement a duct static pressure reset strategy. (Correct answer)
- Increase the chilled water supply temperature setpoint.
- Calibrate all the VAV box damper actuators.
- Replace the existing supply fan with a larger one.
Correct answer: Implement a duct static pressure reset strategy.
Maintaining a high, constant static pressure forces the fan to work harder than necessary, especially at part-load conditions when VAV boxes are closed. A static pressure reset strategy dynamically lowers the pressure setpoint when demand is low, saving significant fan energy according to the fan affinity laws. While calibrating VAV boxes is good practice and increasing the chilled water setpoint saves chiller energy, neither addresses the primary inefficiency of constant high fan pressure. Replacing the fan with a larger one would likely increase energy use.
Question 2: What is the primary function of an air-side economizer in an HVAC system?
- To pre-heat ventilation air using exhaust air.
- To increase the humidity of the supply air in dry conditions.
- To provide "free cooling" by using cool outside air instead of mechanical refrigeration. (Correct answer)
- To mix return air with exhaust air before venting to the outside.
Correct answer: To provide "free cooling" by using cool outside air instead of mechanical refrigeration.
An air-side economizer's main purpose is to reduce the need for energy-intensive mechanical cooling (compressors/chillers) by using cool, low-humidity outdoor air when conditions are favorable. This practice is often called "free cooling." Heat recovery ventilators pre-heat air, and humidifiers add humidity; these are different system components.
Question 3: An energy audit recommends installing a variable frequency drive (VFD) on a constant volume supply fan motor and reducing the fan speed by 20%. According to the fan affinity laws, what is the approximate theoretical reduction in fan power consumption?
- 36%
- 49% (Correct answer)
- 20%
- 64%
Correct answer: 49%
The fan affinity laws state that power is proportional to the cube of the speed change. A 20% speed reduction means the new speed is 80% (or 0.8) of the original speed. The new power will be (0.8)^3 = 0.512 of the original power. This represents a power reduction of 1 - 0.512 = 0.488, which is approximately 49%.
Question 4: When evaluating the overall energy efficiency of a central water-cooled chiller plant, which of the following metrics is the most comprehensive performance indicator?
- Cooling tower approach temperature.
- Chilled water delta-T (difference between supply and return temperature).
- Chiller coefficient of performance (COP) at full load.
- Total plant efficiency in kW/ton. (Correct answer)
Correct answer: Total plant efficiency in kW/ton.
Total plant efficiency in kW/ton is the most comprehensive metric as it accounts for the energy consumption of all major components of the system—chillers, chilled water pumps, condenser water pumps, and cooling tower fans—relative to the cooling load delivered. The other metrics are important indicators for individual components (chiller, cooling tower, distribution) but do not represent the entire system's combined performance.
Question 5: During an HVAC system assessment, an auditor measures the chilled water supply temperature at 44°F and the return temperature at 50°F, resulting in a 6°F delta-T. The system was designed for a 12°F delta-T. Which of the following is a likely cause of this "low delta-T syndrome," and what is its primary energy impact?
- Cause: A faulty cooling tower fan; Impact: Increased chiller head pressure.
- Cause: Oversized chilled water pumps or control valves failing open; Impact: Decreased plant efficiency due to higher pumping energy and inefficient chiller loading. (Correct answer)
- Cause: Clogged air filters on the air handling unit; Impact: Reduced airflow, but improved chiller efficiency.
- Cause: The outdoor air economizer is stuck open; Impact: Increased load on the cooling coil, but improved delta-T.
Correct answer: Cause: Oversized chilled water pumps or control valves failing open; Impact: Decreased plant efficiency due to higher pumping energy and inefficient chiller loading.
Low delta-T syndrome is often caused by chilled water flowing too quickly through the cooling coils, not allowing enough time for proper heat transfer. This is a common result of oversized pumps or control valves failing open. It forces more water to be pumped than necessary (wasting pump energy) and can cause chillers to run inefficiently at low loads or require more chillers to be staged on to meet the flow demand.
Question 6: Which control strategy adjusts the amount of outdoor ventilation air delivered to a space based on the real-time number of occupants, often by using CO2 sensors?
- Supply Air Temperature Reset
- Optimal Start/Stop
- Demand-Controlled Ventilation (DCV) (Correct answer)
- Enthalpy Economizer Control
Correct answer: Demand-Controlled Ventilation (DCV)
Demand-Controlled Ventilation (DCV) uses sensors (typically CO2 sensors) to estimate the number of occupants in a space and modulates the outdoor air damper to provide the required amount of ventilation. This avoids the energy waste associated with conditioning excessive amounts of outdoor air when a space has low or no occupancy.
Certified Energy Auditor HVAC Systems and Controls Questions and Answers
An energy auditor is reviewing the Building Automation System (BAS) trend data for a multi-zone Variable Air Volume (VAV) system.
The auditor notes that the supply fan maintains a constant duct static pressure of 2.5 inches w.c. at all times, regardless of the building load.
What is the most effective energy conservation measure to recommend?