HVAC Ductwork 2

Question 1

What is the minimum clearance required between uninsulated supply ducts and combustible materials in most US building codes?

Accepted Answer

1 inch

Answer

Most US codes require at least 1 inch of clearance between uninsulated supply ducts and combustible materials to reduce fire risk.

Question 2

Which duct shape provides the highest airflow efficiency per unit of cross-sectional area?

Accepted Answer

Round

Answer

Round ducts have the lowest surface-area-to-volume ratio, which minimizes friction losses and makes them the most efficient shape for airflow.

Question 3

What does the term 'duct leakage class' (CL) measure in duct systems?

Accepted Answer

Maximum allowable leakage per 100 sq ft of duct surface

Answer

Duct leakage class (CL) defines the maximum allowable CFM of leakage per 100 square feet of duct surface area at a reference pressure.

Question 4

A plenum-rated duct is required in which location?

Accepted Answer

In a return air plenum above a drop ceiling

Answer

Plenum-rated ducts (meeting UL 181) are required when installed in return air plenums above drop ceilings because those spaces carry circulated air and must not off-gas toxic fumes in a fire.

Question 5

What is the primary purpose of a duct static pressure test (using a duct blaster)?

Accepted Answer

Quantify total duct leakage under pressurized conditions

Answer

A duct blaster pressurizes the duct system to a reference pressure and measures total CFM leakage, revealing how airtight the duct system is.

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HVAC Ductwork 2