TheoryHVAC NATE
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30 free HVAC NATE questions
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Airflow & Ductwork
What is the primary purpose of a supply duct in an HVAC system?
Correct — A. Supply ducts carry conditioned (heated or cooled) air from the air-handling unit to registers in the occupied space. Return ducts perform the reverse function, pulling room air back to be reconditioned. -
Airflow & Ductwork
Which unit is used to express airflow volume in HVAC ductwork?
Correct — C. Airflow volume is measured in cubic feet per minute (CFM). FPM measures velocity, and in. w.c. measures static pressure — both related but distinct from volume. -
Airflow & Ductwork
What does 'static pressure' represent in a duct system?
Correct — B. Static pressure acts outward in all directions on duct walls and is what drives leakage through seams and holes. It is distinct from velocity pressure, which acts only in the direction of airflow. -
Airflow & Ductwork
Velocity pressure in a duct is best described as:
Correct — D. Velocity pressure (VP) results from the kinetic energy of air in motion. It acts only in the direction of airflow and equals (V/4005)² in. w.c., where V is velocity in FPM at standard air density. -
Airflow & Ductwork
Total pressure in a duct is the sum of which two pressures?
Correct — A. Total pressure = static pressure + velocity pressure (Bernoulli). Knowing total pressure is essential for understanding how fans and ducts interact at any operating point. -
Airflow & Ductwork
The basic airflow equation CFM = Area × Velocity requires area in which unit?
Correct — B. When velocity is in feet per minute (FPM), area must be in square feet to produce CFM directly. Convert square inches to square feet by dividing by 144. -
Airflow & Ductwork
A 3-ton residential air conditioner requires 400 CFM per ton. What total airflow should the system deliver?
Correct — C. 400 CFM/ton × 3 tons = 1,200 CFM. The 400 CFM/ton rule of thumb is a standard residential design target ensuring adequate sensible heat removal and coil performance. -
Airflow & Ductwork
Room-by-room CFM requirements are determined during which ACCA design procedure?
Correct — D. ACCA Manual J calculates room-by-room heating and cooling loads, which are converted to CFM requirements. Manual D uses those CFM values to size ducts; Manual S selects the equipment. -
Airflow & Ductwork
A supply plenum feeds three branch ducts carrying 300, 450, and 250 CFM respectively. The main trunk must carry at least how much CFM?
Correct — A. The main trunk must carry the sum of all branches: 300 + 450 + 250 = 1,000 CFM. Conservation of mass (continuity equation) requires airflow entering a junction to equal airflow leaving. -
Airflow & Ductwork
A space has a sensible cooling load of 10,800 BTU/h with a supply-air temperature of 55°F and a room design temperature of 75°F. What supply airflow is required?
Correct — B. CFM = Q ÷ (1.08 × ΔT) = 10,800 ÷ (1.08 × 20) = 10,800 ÷ 21.6 = 500 CFM. The constant 1.08 = 0.075 lb/ft³ × 0.24 BTU/(lb·°F) × 60 min/hr, applicable at standard air density. -
Airflow & Ductwork
Which factor does NOT directly affect friction loss in a straight duct run?
Correct — C. Friction loss depends on velocity, duct size (hydraulic diameter), and surface roughness per the Darcy-Weisbach equation. Exterior color has no effect on internal airflow resistance. -
Airflow & Ductwork
If duct velocity doubles while all other variables remain constant, friction loss changes by approximately:
Correct — B. Friction pressure loss varies with the square of velocity (Darcy-Weisbach). Doubling velocity squares to ×4, so friction loss quadruples — a key reason high-velocity systems consume significantly more fan energy. -
Airflow & Ductwork
A duct section has a friction loss rate of 0.08 in. w.c. per 100 feet. What is the friction loss over 250 feet of equivalent length?
Correct — D. Friction loss = (rate ÷ 100) × length = (0.08 ÷ 100) × 250 = 0.20 in. w.c. Friction loss scales linearly with duct length at constant velocity and duct size. -
Airflow & Ductwork
According to ASHRAE and ACCA guidelines, the typical design friction rate for residential low-velocity ductwork is:
Correct — A. Residential systems are typically designed at 0.08–0.10 in. w.c./100 ft. Higher rates suit commercial high-velocity systems but produce excessive noise and energy use in homes. -
Airflow & Ductwork
What is 'equivalent length' as used in duct system design?
Correct — C. Equivalent length converts each fitting's resistance into feet of equivalent straight duct at the same velocity, allowing total system resistance to be calculated as a single combined value for sizing the fan. -
Airflow & Ductwork
A 90° round elbow has an equivalent length of 14 feet in a duct using a friction rate of 0.08 in. w.c. per 100 feet. What pressure loss does this elbow add?
Correct — D. Pressure loss = (friction rate ÷ 100) × equivalent length = (0.08 ÷ 100) × 14 = 0.0112 in. w.c. ≈ 0.011 in. w.c. The equivalent-length method converts fitting resistance to an easy-to-add duct-length equivalent. -
Airflow & Ductwork
The equal-friction method of duct sizing means that:
Correct — A. The equal-friction method sizes each duct so the pressure drop per 100 feet of duct is the same throughout the system, simplifying design and providing natural self-balancing tendencies. -
Airflow & Ductwork
Using the velocity method of duct sizing, which section typically carries the highest design velocity?
Correct — B. In velocity-method design, velocity is highest in the main trunk closest to the air handler (carrying maximum CFM) and reduced progressively in branches carrying smaller CFM. -
Airflow & Ductwork
A rectangular duct is 16 inches wide and 10 inches tall. What is its cross-sectional area in square feet?
Correct — C. Area = (16 × 10) ÷ 144 = 160 ÷ 144 = 1.11 sq ft. Dividing by 144 converts square inches to square feet. -
Airflow & Ductwork
A duct must carry 600 CFM at a design velocity of 900 FPM. What cross-sectional area is required?
Correct — D. Area = CFM ÷ Velocity = 600 ÷ 900 = 0.667 sq ft. This rearrangement of CFM = Area × Velocity is the fundamental duct-sizing calculation. -
Airflow & Ductwork
What 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 — A. 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. -
Airflow & Ductwork
Which duct material has the lowest internal surface roughness and therefore the least friction loss per foot at a given velocity?
Correct — B. 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. -
Airflow & Ductwork
Fiberglass duct board is primarily used in HVAC because it:
Correct — C. 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. -
Airflow & Ductwork
When flexible duct is installed with sags and tight bends rather than fully extended, what happens to its effective friction loss?
Correct — D. 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. -
Airflow & Ductwork
What SMACNA standard governs the construction and installation of HVAC sheet-metal ductwork?
Correct — A. 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. -
Airflow & Ductwork
What is the recommended method for sealing duct joints and seams in residential systems?
Correct — B. 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. -
Airflow & Ductwork
Duct leakage in a typical unsealed residential system can account for what percentage of total conditioned airflow loss?
Correct — C. 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. -
Airflow & Ductwork
Which testing method is used to quantify duct leakage in a residential system?
Correct — D. 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. -
Airflow & Ductwork
ENERGY STAR v3.1 requires total duct leakage of no more than what value for new residential construction?
Correct — A. 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. -
Airflow & Ductwork
Why must ducts located in unconditioned attics or crawlspaces be insulated?
Correct — B. 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.
HVAC NATE sample questions
Tap any question below to reveal the answer and a plain-English explanation.
Airflow & Ductwork IECC 2021 requires R-8 minimum for supply ducts in unconditioned attics. Using R-4 insulation instead in a hot climate would primarily cause:
A. Supply air temperature to decrease due to reverse heat transfer
B. Condensation inside the duct to be eliminated
C. Roughly double the heat gain into the duct, increasing cooling load ✓
D. Fan energy use to increase due to higher airflow resistance
Correct — C. Halving the R-value (R-8 → R-4) roughly doubles the heat gain into the cold supply duct in summer, degrading delivery temperature and increasing the cooling load on the equipment.
Gas Heat & Combustion What are the three essential elements required for combustion to occur?
A. Fuel, oxygen, and carbon dioxide
B. Fuel, nitrogen, and heat
C. Fuel, oxygen, and heat ✓
D. Oxygen, carbon monoxide, and ignition
Correct — C. Combustion requires the 'fire triangle': fuel, oxygen (oxidizer), and heat (ignition energy). Remove any one element and combustion cannot be sustained.
Controls & Sequencing On a standard 24 V heating thermostat, which terminal is the common (return) leg of the control circuit?
A. C ✓
B. R
C. W
D. G
Correct — A. The 'C' terminal is the common (negative) side of the 24 V transformer secondary. R carries the 24 V hot leg, W calls for heat, and G calls for the fan.
Electrical & Motors What is the unit of electrical resistance?
A. Ohm ✓
B. Volt
C. Ampere
D. Watt
Correct — A. Resistance is measured in Ohms (Ω), as defined by Ohm's Law. Volts measure voltage, Amperes measure current, and Watts measure power.
Heat Pumps In a heat pump operating in heating mode, which component absorbs heat from the outdoor air?
A. Outdoor coil (evaporator) ✓
B. Indoor coil (condenser)
C. Compressor
D. Reversing valve
Correct — A. In heating mode the outdoor coil acts as an evaporator, absorbing heat energy from the outdoor air and transferring it to the refrigerant. The indoor coil then acts as a condenser, releasing that heat into the building.
Refrigerant Management (EPA) Which federal regulation prohibits the knowing venting of refrigerants that contain CFCs or HCFCs during maintenance, service, repair, or disposal of refrigeration equipment?
A. Section 608 of the Clean Air Act ✓
B. Section 609 of the Clean Air Act
C. Title IV of the Resource Conservation and Recovery Act
D. ASHRAE Standard 34
Correct — A. Section 608 of the Clean Air Act establishes the requirements for technician certification, refrigerant recovery, and prohibition of venting for stationary refrigeration and air-conditioning equipment. Section 609 applies specifically to motor vehicle air conditioning.
Refrigeration Cycle What are the four basic components of a vapor-compression refrigeration cycle?
A. Compressor, condenser, metering device, evaporator ✓
B. Compressor, condenser, receiver, evaporator
C. Compressor, condenser, filter-drier, evaporator
D. Compressor, accumulator, metering device, evaporator
Correct — A. The standard vapor-compression cycle consists of a compressor, condenser, metering (expansion) device, and evaporator. These four components complete the refrigerant loop and allow heat transfer from the low-pressure to the high-pressure side.
Troubleshooting & Safety A technician finds the suction pressure is lower than normal and the discharge pressure is normal on a residential AC system. What is the most likely cause?
A. Low refrigerant charge ✓
B. Overcharged refrigerant system
C. Faulty condenser fan motor
D. Restricted discharge line
Correct — A. Low suction pressure with normal discharge pressure is a classic indicator of undercharge (refrigerant leak). With too little refrigerant, the evaporator cannot absorb enough heat, causing suction pressure to drop while discharge pressure remains relatively unaffected.
About the HVAC NATE test
Pass your HVAC NATE with confidence. HVAC NATE Exam Prep gives you 750+ realistic practice questions with clear, detailed explanations for every answer.
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Refrigeration Cycle, Electrical & Motors, Airflow & Ductwork, Heat Pumps, Gas Heat & Combustion, Refrigerant Management (EPA) — and more.
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Topics in this question bank
Core formulas and field calculations
Codes, standards and compliance
Safe work practices and PPE
Tools, materials and on-site reference
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