HVAC NATE Exam Questions & Answers 2026 (11–20)

Question 1

Which factor does NOT directly affect friction loss in a straight duct run?

TheoryPractice · Accepted Answer

The exterior color of the duct. 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.

Question 2

If duct velocity doubles while all other variables remain constant, friction loss changes by approximately:

TheoryPractice · Accepted Answer

It quadruples (×4). 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.

Question 3

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?

TheoryPractice · Accepted Answer

0.20 in. w.c.. 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.

Question 4

According to ASHRAE and ACCA guidelines, the typical design friction rate for residential low-velocity ductwork is:

TheoryPractice · Accepted Answer

0.08 to 0.10 in. w.c. per 100 feet. 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.

Question 5

What is 'equivalent length' as used in duct system design?

TheoryPractice · Accepted Answer

The length of straight duct producing the same pressure loss as a fitting. 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.

Question 6

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?

TheoryPractice · Accepted Answer

0.011 in. w.c.. 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.

Question 7

The equal-friction method of duct sizing means that:

TheoryPractice · Accepted Answer

The friction loss per unit length is kept constant throughout the system. 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.

Question 8

Using the velocity method of duct sizing, which section typically carries the highest design velocity?

TheoryPractice · Accepted Answer

The main trunk leaving the air handler. 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.

Question 9

A rectangular duct is 16 inches wide and 10 inches tall. What is its cross-sectional area in square feet?

TheoryPractice · Accepted Answer

1.11 sq ft. Area = (16 × 10) ÷ 144 = 160 ÷ 144 = 1.11 sq ft. Dividing by 144 converts square inches to square feet.

Question 10

A duct must carry 600 CFM at a design velocity of 900 FPM. What cross-sectional area is required?

TheoryPractice · Accepted Answer

0.67 sq ft. Area = CFM ÷ Velocity = 600 ÷ 900 = 0.667 sq ft. This rearrangement of CFM = Area × Velocity is the fundamental duct-sizing calculation.

HVAC NATE Exam Questions & Answers 2026 (11–20)

More HVAC NATE practice questions

Free practice here. Timed mocks when you are ready.