313D Air Distribution and Ducting Questions and Answers 1 — Questions and Answers
Question 1: A homeowner complains of a persistent whistling noise coming from a supply air register, which becomes louder when other registers in the house are closed. What is the most probable cause of this issue?
- The return air filter is oversized.
- The air velocity through the register is too high. (Correct answer)
- The thermostat is located too close to the supply register.
- The ductwork is oversized for the system's CFM output.
Correct answer: The air velocity through the register is too high.
A whistling noise at a register is a classic sign of excessive air velocity. When air is forced through a small opening (the register) at a high speed, it creates turbulence and noise. Closing other registers increases the static pressure in the system, forcing even more air through the remaining open registers at a higher velocity, which explains why the noise gets louder.
Question 2: In rectangular duct design, what does the term 'aspect ratio' describe?
- The ratio of the duct's total length to its total CFM.
- The ratio of static pressure to velocity pressure.
- The ratio of the duct's longer side to its shorter side. (Correct answer)
- The ratio of supply airflow to return airflow.
Correct answer: The ratio of the duct's longer side to its shorter side.
Aspect ratio is a fundamental term in duct design that refers to the ratio of the long side to the short side of a rectangular duct. For example, a 16x8 inch duct has an aspect ratio of 2:1. Lower aspect ratios (closer to 1:1, or square) are generally more efficient for airflow and minimize friction loss, while very high aspect ratios can lead to increased static pressure and potential for noise.
Question 3: A technician is inspecting ductwork in an attic and identifies a type of duct constructed with a helical wire coil core, covered by a polymer plastic liner, a layer of insulation, and an outer vapor barrier jacket. Which type of duct material is this?
- Sheet metal duct
- Fiberglass duct board
- Flexible duct (Flex duct) (Correct answer)
- PVC duct
Correct answer: Flexible duct (Flex duct)
This description accurately portrays the construction of a flexible duct, often called 'flex duct'. It consists of a spring-like wire coil that forms the tube, an inner plastic liner, a layer of fiberglass insulation, and a protective outer jacket, which is often foil or polyethylene.
Question 4: A technician measures the total external static pressure (TESP) of a residential air handler and gets a reading of 0.95 in. w.c. (inches of water column). The equipment's data plate specifies a maximum TESP of 0.5 in. w.c. Which of the following is a direct consequence of this high static pressure?
- The system will provide excess airflow, overcooling the space.
- The blower motor will work less, consuming less electricity.
- The compressor will short-cycle due to rapid heat exchange.
- The blower motor will be strained, leading to reduced airflow and potential premature failure. (Correct answer)
Correct answer: The blower motor will be strained, leading to reduced airflow and potential premature failure.
High external static pressure is resistance against which the blower motor must push air. When the TESP exceeds the manufacturer's maximum rating, the blower cannot move the intended volume of air (CFM). This strains the blower motor, increases its energy consumption, reduces its lifespan, and results in insufficient airflow to the conditioned spaces, leading to comfort issues and reduced system efficiency.
Question 5: According to ACCA Manual D, what is the primary objective of the residential duct design process?
- To calculate the total heating and cooling load for the structure.
- To select the specific brand and model of HVAC equipment.
- To size the duct system to deliver the correct CFM to each room with minimal pressure loss. (Correct answer)
- To determine the R-value of insulation required for the building envelope.
Correct answer: To size the duct system to deliver the correct CFM to each room with minimal pressure loss.
ACCA Manual D is the industry standard for designing and sizing residential duct systems. Its core purpose is to ensure the ductwork is correctly sized to deliver the required airflow (CFM), as determined by the Manual J load calculation, to each room quietly and efficiently by properly managing friction and pressure losses throughout the system.
Question 6: Which of the following duct fittings will typically create the highest amount of friction loss (equivalent length) in an air distribution system?
- A 45-degree round elbow.
- A 90-degree smooth radius elbow.
- A straight 20-foot section of flexible duct.
- A 90-degree square-throated elbow without turning vanes. (Correct answer)
Correct answer: A 90-degree square-throated elbow without turning vanes.
Sharp, abrupt changes in direction cause the most turbulence and therefore the highest friction loss. A 90-degree square-throated elbow without turning vanes forces the air to make a hard, immediate turn, creating significant pressure drop. Smooth radius elbows and 45-degree elbows are designed to guide the air more gradually, resulting in much lower friction loss.
313D Air Distribution and Ducting Questions and Answers 1
A homeowner complains of a persistent whistling noise coming from a supply air register, which becomes louder when other registers in the house are closed.
What is the most probable cause of this issue?