Certified Energy Auditor HVAC Systems and Controls 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?

Accepted Answer

Implement a duct static pressure reset strategy.

Answer

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?

Accepted Answer

To provide "free cooling" by using cool outside air instead of mechanical refrigeration.

Answer

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?

Accepted Answer

49%

Answer

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?

Accepted Answer

Total plant efficiency in kW/ton.

Answer

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?

Accepted Answer

Cause: Oversized chilled water pumps or control valves failing open; Impact: Decreased plant efficiency due to higher pumping energy and inefficient chiller loading.

Answer

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.

Certified Energy Auditor Certification Practice Test

Certified Energy Auditor Certification Practice Test

Certified Energy Auditor HVAC Systems and Controls Questions and Answers