Air Conditioning Repair Airflow and Duct Systems Questions and Answers — 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?
- Increased refrigerant subcooling at the condenser.
- The system will deliver more conditioned air than designed.
- Reduced volumetric airflow (CFM) leading to poor system performance. (Correct answer)
- Over-amping and premature failure of the condenser fan motor.
Correct answer: Reduced volumetric airflow (CFM) leading to poor system performance.
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?
- To mix conditioned air with room air for better circulation.
- To control the velocity of the air leaving the supply duct.
- To prevent the buildup of positive pressure within the conditioned space.
- To cover the return duct opening and, in many cases, hold the air filter. (Correct answer)
Correct answer: To cover the return duct opening and, in many cases, hold the air filter.
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?
- A dirty or clogged air filter. (Correct answer)
- An oversized return duct.
- A refrigerant undercharge.
- A malfunctioning thermostat.
Correct answer: A dirty or clogged air filter.
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?
- The blower motor bearings are failing.
- Excessive air velocity due to restricted outlets. (Correct answer)
- A loose panel on the air handler cabinet is vibrating.
- The ductwork is undersized for the system's airflow.
Correct answer: Excessive air velocity due to restricted outlets.
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?
- The ratio of the duct's total length to its total number of fittings.
- The percentage of airflow lost due to friction within the duct.
- The ratio of the long side of the duct to the short side of the duct. (Correct answer)
- The relationship between the supply air temperature and the return air temperature.
Correct answer: The ratio of the long side of the duct to the short side of the duct.
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.
Question 6: A technician needs to measure the velocity of the air, in feet per minute (FPM), moving through a duct. Which of the following instruments is specifically designed for this purpose?
- A psychrometer.
- A manometer.
- A micron gauge.
- An anemometer. (Correct answer)
Correct answer: An anemometer.
An anemometer is the correct tool for measuring air speed or velocity. A manometer measures pressure differences (like static pressure), a psychrometer measures wet-bulb and dry-bulb temperatures to determine humidity, and a micron gauge measures deep vacuum levels during system evacuation.
Air Conditioning Repair Airflow and Duct Systems Questions and Answers
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?