Free 313D Airflow and Duct Sizing Questions and Answers 1 — Questions and Answers
Question 1: A technician measures the total external static pressure (TESP) of 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. What is the most likely consequence of this high static pressure reading?
- Increased airflow (CFM) and improved system efficiency.
- Reduced airflow (CFM) and potential for evaporator coil freezing. (Correct answer)
- Normal system operation with no adverse effects.
- Overamping of the indoor blower motor due to reduced load.
Correct answer: Reduced airflow (CFM) and potential for evaporator coil freezing.
High total external static pressure indicates excessive resistance in the duct system. This forces the blower motor to work harder to move less air, resulting in reduced CFM. Low airflow across the evaporator coil can cause its temperature to drop below freezing, leading to ice formation.
Question 2: Which tool is most commonly used by an HVAC technician to quickly determine the required round or rectangular duct size based on a target friction rate and airflow (CFM)?
- A psychrometric chart
- A digital multimeter
- A refrigerant charging scale
- A ductulator (duct slide rule) (Correct answer)
Correct answer: A ductulator (duct slide rule)
A ductulator is a specialized slide rule calculator used in HVAC design. It allows a technician to relate four key variables: airflow (CFM), friction rate, velocity, and duct size, making it the standard tool for quick duct sizing.
Question 3: In duct system design, which of the following best describes the concept of 'equivalent length'?
- The total linear measurement of all straight duct sections in the system.
- The pressure drop caused by a fitting, like an elbow or tee, expressed as the length of straight duct that would cause the same pressure drop. (Correct answer)
- The maximum recommended length for a flexible duct run before it requires support.
- The length of duct required to achieve a specific velocity for proper air mixing.
Correct answer: The pressure drop caused by a fitting, like an elbow or tee, expressed as the length of straight duct that would cause the same pressure drop.
Duct fittings (elbows, tees, transitions) create more turbulence and friction loss than straight duct. 'Equivalent length' is a value assigned to each fitting that represents the length of straight duct that would create an identical amount of pressure loss, allowing designers to calculate the total system resistance accurately.
Question 4: A technician is installing a new 3-ton (36,000 BTU/hr) residential air conditioning system. Using a standard industry rule of thumb, what is the target airflow the duct system should be designed to deliver?
- 800 CFM
- 1050 CFM
- 1200 CFM (Correct answer)
- 1600 CFM
Correct answer: 1200 CFM
The standard industry rule of thumb for residential air conditioning is to provide 400 cubic feet per minute (CFM) of airflow for each ton of cooling capacity. Therefore, a 3-ton system requires 3 tons * 400 CFM/ton = 1200 CFM.
Question 5: Designing a residential duct system with excessively high air velocity is generally avoided primarily because it can lead to which of the following issues?
- Unacceptable noise levels and increased friction loss. (Correct answer)
- Reduced heat transfer at the registers.
- Air stratification within the conditioned space.
- Condensation forming on the outside of the ductwork.
Correct answer: Unacceptable noise levels and increased friction loss.
When air moves too quickly through ductwork, it creates turbulence which results in undesirable noise (whistling or rushing sounds). High velocity also significantly increases the friction rate (a component of static pressure), which reduces overall airflow and makes the fan work harder.
Question 6: A technician needs to verify the airflow (CFM) coming out of a specific supply register to help balance a system. Which of the following instruments is most suitable for this task?
- A static pressure probe (manometer)
- A combustion analyzer
- An airflow capture hood (balometer) (Correct answer)
- A refrigerant leak detector
Correct answer: An airflow capture hood (balometer)
An airflow capture hood, or balometer, is specifically designed to fit over a register or grille and directly measure the volume of air in CFM. While an anemometer can measure velocity (which can then be used to calculate CFM), a capture hood provides a direct and more accurate reading for this purpose.
Free 313D Airflow and Duct Sizing Questions and Answers 1
A technician measures the total external static pressure (TESP) of 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.
What is the most likely consequence of this high static pressure reading?