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Ductwork Flashcards

7 cards from real HVAC practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

Read the first 7 Ductwork flashcards as text

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

    Answer: 1 inch

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

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

    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.

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

    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.

  4. A plenum-rated duct is required in which location?

    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.

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

    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.

  6. In HVAC ductwork, what does the 'equivalent length' method account for when sizing ducts?

    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.

  7. Which type of duct joint sealant is NOT acceptable for sealing supply ducts carrying conditioned air?

    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.