FREE 313D Airflow and Duct Sizing 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?

Accepted Answer

Reduced airflow (CFM) and potential for evaporator coil freezing.

Answer

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)?

Accepted Answer

A ductulator (duct slide rule)

Answer

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'?

Accepted 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.

Answer

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?

Accepted Answer

1200 CFM

Answer

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?

Accepted Answer

Unacceptable noise levels and increased friction loss.

Answer

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.

(313D) Residential Air Conditioning Systems Mechanic Certification Practice Test

(313D) Residential Air Conditioning Systems Mechanic Certification Practice Test

FREE 313D Airflow and Duct Sizing Questions and Answers