Air Conditioning Repair Airflow and Duct Systems Questions and Answers

Question 1

A technician measures the total external static pressure (TESP) on a residential air handler and finds it to be 1.1 inches of water column (in. w.c.). The manufacturer's data plate specifies a maximum TESP of 0.5 in. w.c. Which of the following is the MOST likely consequence of this condition?

Accepted Answer

Reduced volumetric airflow (CFM) leading to poor system performance.

Answer

Total external static pressure (TESP) is the resistance the blower fan must overcome to move air through the duct system. When TESP is significantly higher than the manufacturer's specification, it indicates excessive resistance. The blower cannot move its rated volume of air (CFM) against this high resistance, resulting in reduced airflow, which can lead to insufficient cooling, frozen evaporator coils, or a furnace overheating.

Question 2

What is the primary purpose of the return air grille in a forced-air HVAC system?

Accepted Answer

To cover the return duct opening and, in many cases, hold the air filter.

Answer

The return air grille is the entry point for air from the conditioned space to be pulled back into the HVAC unit for reconditioning. Its primary functions are to cover the duct opening for safety and aesthetics and, in most residential systems, to provide a location for the air filter for easy access and replacement.

Question 3

Which of the following is the most common cause of reduced airflow across the evaporator coil in a residential air conditioning system?

Accepted Answer

A dirty or clogged air filter.

Answer

A dirty air filter creates a significant obstruction in the return air path, increasing the static pressure the blower must work against. This is the most frequent cause of reduced airflow in residential systems. This condition can lead to an iced-over evaporator coil, reduced cooling capacity, and increased energy consumption.

Question 4

A homeowner reports a high-pitched whistling sound coming from the supply registers, which becomes more noticeable when some vents are closed. What is the most likely cause of this noise?

Accepted Answer

Excessive air velocity due to restricted outlets.

Answer

Whistling or high-pitched noises from registers are typically caused by high air velocity. When several supply registers are closed, the total airflow from the blower is forced through fewer, smaller openings. This constriction increases the speed of the air moving through the open registers and grilles, creating the whistling sound.

Question 5

In rectangular duct design and fabrication, what does the term "aspect ratio" describe?

Accepted Answer

The ratio of the long side of the duct to the short side of the duct.

Answer

Aspect ratio is a key parameter in duct design that defines the ratio of the long side to the short side of a rectangular duct. For example, a 24-inch by 12-inch duct has an aspect ratio of 2:1. Lower aspect ratios (closer to 1:1, or square) are generally more efficient for airflow as they minimize friction for a given cross-sectional area.

Air Conditioning Repair Practice Test

Air Conditioning Repair Practice Test

Air Conditioning Repair Airflow and Duct Systems Questions and Answers