HVAC NATE practice questions and answers 2026. Tap an option to test yourself — you'll see the correct answer and a plain-English explanation for every question. Free, no login.
Q21What is the circular-equivalent diameter for a 20×12-inch rectangular duct (using ASHRAE formula De = 1.30 × (a·b)^0.625 / (a+b)^0.25)?
✓ Correct answer: A. 15.9 inches
De = 1.30 × (20×12)^0.625 / (20+12)^0.25 = 1.30 × (240)^0.625 / (32)^0.25 ≈ 1.30 × 41.5 / 2.38 ≈ 15.9 in. This diameter is used to enter standard friction-loss charts based on round duct.
Topic: Airflow & Ductwork
Q22Which duct material has the lowest internal surface roughness and therefore the least friction loss per foot at a given velocity?
✓ Correct answer: B. Galvanized sheet metal (smooth interior)
Galvanized sheet metal has a smooth interior surface (roughness factor ε ≈ 0.0003 ft), producing less friction than fibrous or corrugated surfaces. Flexible duct, especially when compressed or kinked, has significantly higher resistance.
Topic: Airflow & Ductwork
Q23Fiberglass duct board is primarily used in HVAC because it:
✓ Correct answer: C. Provides both thermal insulation and acoustic attenuation in one product
Duct board combines the duct wall, thermal insulation, and sound absorption into a single product, simplifying installation and reducing labor compared to sheet metal with separately applied insulation.
Topic: Airflow & Ductwork
Q24When flexible duct is installed with sags and tight bends rather than fully extended, what happens to its effective friction loss?
✓ Correct answer: D. It increases significantly due to higher turbulence and added equivalent length
Compressed or kinked flex duct dramatically increases turbulence and adds resistance beyond what the charts predict. ACCA Manual D and SMACNA require flex duct to be fully extended and supported to minimize friction losses.
Topic: Airflow & Ductwork
Q25What SMACNA standard governs the construction and installation of HVAC sheet-metal ductwork?
✓ Correct answer: A. SMACNA HVAC Duct Construction Standards
SMACNA publishes the HVAC Duct Construction Standards, specifying gauge, reinforcement, seam types, and pressure classifications for sheet-metal ductwork. NFPA 90A covers fire safety; ASHRAE 90.1 covers energy efficiency.
Topic: Airflow & Ductwork
Q26What is the recommended method for sealing duct joints and seams in residential systems?
✓ Correct answer: B. Mastic duct sealant or UL 181-listed foil tape
Mastic (brushed on) or UL 181-listed foil tape maintain a durable seal over time. Standard cloth 'duct tape' adhesive degrades rapidly in the duct environment and typically fails within a few years.
Topic: Airflow & Ductwork
Q27Duct leakage in a typical unsealed residential system can account for what percentage of total conditioned airflow loss?
✓ Correct answer: C. 20 to 30%
Studies by LBNL and ACCA show unsealed duct systems commonly leak 20–30% of conditioned airflow, significantly increasing energy use and degrading comfort. Proper sealing can recover most of this loss.
Topic: Airflow & Ductwork
Q28Which testing method is used to quantify duct leakage in a residential system?
✓ Correct answer: D. Duct leakage test (duct blaster)
A duct leakage test (duct blaster) pressurizes the duct system to 25 Pa and measures airflow through all leakage paths, reporting CFM25. The blower door tests the building envelope, not the duct system.
Topic: Airflow & Ductwork
Q29ENERGY STAR v3.1 requires total duct leakage of no more than what value for new residential construction?
✓ Correct answer: A. 4 CFM25 per 100 sq ft of conditioned floor area
ENERGY STAR Certified Homes v3.1 requires total duct leakage ≤ 4 CFM25 per 100 sq ft of conditioned floor area. This stringent requirement promotes well-sealed duct systems in new construction.
Topic: Airflow & Ductwork
Q30Why must ducts located in unconditioned attics or crawlspaces be insulated?
✓ Correct answer: B. To minimize heat transfer between conditioned supply air and the surrounding unconditioned environment
Ducts in unconditioned spaces exchange heat with the hot or cold surrounding air. Insulation reduces conductive heat transfer, preserving the conditioned air temperature and reducing energy waste.
Topic: Airflow & Ductwork
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