HVAC Ductwork 2 — Questions and Answers
Question 1: What is the minimum clearance required between uninsulated supply ducts and combustible materials in most US building codes?
- 1 inch (Correct answer)
- 3 inches
- 6 inches
- 12 inches
Correct answer: 1 inch
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?
- Square
- Rectangular
- Round (Correct answer)
- Oval
Correct answer: Round
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?
- Total duct volume
- Maximum allowable leakage per 100 sq ft of duct surface (Correct answer)
- Duct material thickness rating
- Pressure drop across the duct
Correct answer: Maximum allowable leakage per 100 sq ft of duct surface
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?
- Inside a conditioned living space
- In an unconditioned attic
- In a return air plenum above a drop ceiling (Correct answer)
- Underground below the slab
Correct answer: In a return air plenum above a drop ceiling
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)?
- Measure airflow at each register
- Quantify total duct leakage under pressurized conditions (Correct answer)
- Check duct insulation R-value
- Verify duct sizing is correct
Correct answer: Quantify total duct leakage under pressurized conditions
A duct blaster pressurizes the duct system to a reference pressure and measures total CFM leakage, revealing how airtight the duct system is.
Question 6: In HVAC ductwork, what does the 'equivalent length' method account for when sizing ducts?
- Total duct weight
- Added friction loss from fittings converted to equivalent straight duct length (Correct answer)
- Number of supply registers
- Thermal loss through duct walls
Correct answer: Added friction loss from fittings converted to equivalent straight duct length
Equivalent length converts the friction resistance of fittings (elbows, tees, transitions) into feet of straight duct for easier pressure drop calculations.
Question 7: Which type of duct joint sealant is NOT acceptable for sealing supply ducts carrying conditioned air?
- Mastic sealant
- Foil-backed tape (UL 181B rated)
- Standard cloth duct tape (non-listed) (Correct answer)
- Aerosol-injected sealant (Aeroseal)
Correct answer: Standard cloth duct tape (non-listed)
Standard cloth duct tape deteriorates over time and is not listed under UL 181; it is not acceptable for permanent duct sealing in conditioned air systems.
HVAC Ductwork 2
What is the minimum clearance required between uninsulated supply ducts and combustible materials in most US building codes?