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NATE Air Distribution Installation practice questions

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Question 1Safety and Jobsite Practices

Before energizing a new air handler with a field-installed electric heater, what must be verified about the branch-circuit conductors and the disconnect location?

  1. That the conductors are aluminum to match the lug material
  2. That the conductors are oversized by two trade sizes to reduce voltage drop
  3. That the conductors are sized to the nameplate minimum circuit ampacity and the disconnect is within sight of the unit Correct answer.
  4. That the disconnect is installed at the main panel regardless of unit location

Explanation

Air distribution work often ends with the installer landing power on equipment with factory or field-installed heat. The nameplate minimum circuit ampacity governs conductor sizing, and the disconnecting means must be within sight of and readily accessible from the unit so a service tech can lock it off. Oversizing is a design choice, not a code requirement, and disconnect location is dictated by visibility from the equipment.

Question 2Safety and Jobsite Practices

A technician must cut and fit rectangular duct near an open ceiling grid while other trades are working below. Which practice best controls the falling-object hazard?

  1. Work quickly during the other trades' break periods
  2. Tie off tools and duct sections and barricade or cone the area beneath the work Correct answer.
  3. Rely on the ceiling grid to catch anything that drops
  4. Wear hearing protection so the crew is alerted by sound

Explanation

Overhead duct work in occupied or shared spaces creates a dropped-object exposure for everyone below. Tethering tools and duct pieces plus a barricade or cone zone is the standard control. The ceiling grid is not a load-bearing catch surface, and hearing protection does nothing to protect people underneath.

Question 3Safety and Jobsite Practices

When setting up a portable fiberglass step ladder to access a rooftop duct curb, which combination of practices is correct?

  1. Placing the ladder on the rooftop membrane without checking slope
  2. Standing on the top cap to gain the extra reach needed
  3. Facing away from the ladder while carrying duct sealant
  4. Full extension of the spreaders, level footing, and three points of contact while climbing Correct answer.

Explanation

Ladder safety on roofs depends on a fully opened and locked spreader, a level and stable base, and maintaining three points of contact. Standing on the top cap is a recognized fall hazard, and carrying materials while climbing breaks contact. Rooftop membranes and slopes require inspection before the ladder is placed.

Question 4Safety and Jobsite Practices

What is the primary purpose of a lockout/tagout procedure when servicing a rooftop packaged unit with a VFD-driven supply fan?

  1. To document the service call for the customer's maintenance file
  2. To isolate and dissipate stored energy so the equipment cannot restart during service Correct answer.
  3. To limit access to the roof hatch during business hours
  4. To satisfy the warranty registration requirement for the fan motor

Explanation

Lockout/tagout isolates all energy sources and controls stored energy, which for a VFD includes residual DC bus voltage that must be allowed to bleed down. The procedure protects the technician from unexpected startup or release of energy. Paperwork and access control are separate concerns and do not substitute for energy isolation.

Question 5Safety and Jobsite Practices

A technician is about to drill into a wall to run a new return-air chase in an older commercial building. What should be confirmed before the drill contacts the wall?

  1. That the customer has signed the change order
  2. That the work is scheduled before the building opens
  3. That the drill bit is new and the wall is painted
  4. That a utility locate has been completed and the wall cavity is clear of concealed lines Correct answer.

Explanation

Concealed utilities such as electrical, gas, and sprinkler lines can be hidden in wall cavities, so a locate and a cavity check are required before penetration. A new bit, signed paperwork, or early scheduling does not address the strike hazard. Hitting a concealed line can cause injury, fire, or water damage.

Question 6Safety and Jobsite Practices

Which PPE is required when using a portable grinder to cut galvanized angle iron for duct hangers?

  1. A dust mask only, since the cut is outdoors
  2. Safety glasses alone, because the guard contains the sparks
  3. No PPE if the cut takes less than one minute
  4. Safety glasses plus a face shield, hearing protection, and gloves Correct answer.

Explanation

Abrasive cutting of metal generates flying particles, sparks, and high noise, so eye and face protection, hearing protection, and hand protection are all needed. The guard reduces but does not eliminate ejection of debris. Short duration does not exempt a task from PPE requirements.

Question 7Safety and Jobsite Practices

A crew is installing spiral duct at the leading edge of an open mezzanine. Which fall-protection approach is acceptable for workers exposed to the edge?

  1. Using a warning line alone as the sole protection at the edge
  2. A guardrail system or personal fall arrest anchored to an engineered point Correct answer.
  3. Standing back one arm's length from the edge without protection
  4. Relying on a coworker to hold the worker's belt

Explanation

Work at an unprotected edge requires either a guardrail system or personal fall arrest anchored to an engineered anchor point. Warning lines alone are not adequate fall protection at the leading edge. A coworker physically holding a belt is not an approved system and puts both workers at risk.

Question 8Safety and Jobsite Practices

During duct installation in an occupied office, aerosol duct sealant is being applied. Which control best limits occupant exposure to solvent vapors?

  1. Applying the sealant faster to shorten the exposure time
  2. Local exhaust ventilation and isolating the work zone from occupied space Correct answer.
  3. Opening one window at the far end of the building
  4. Running the building air handler on high speed only

Explanation

Solvent-bearing sealants release vapors that should be captured at the source with local exhaust and kept out of occupied areas by isolation. General dilution such as one distant window or running the AHU is less reliable. Working faster does not reduce the total vapor released.

Question 9Safety and Jobsite Practices

What is the correct first action when a technician discovers a gas odor near a rooftop unit's gas train during startup?

  1. Reset the ignition control and retry ignition
  2. Spray soap solution on the joint while the burner is firing
  3. Stop work, shut off the gas supply, and evacuate and ventilate per the site emergency plan Correct answer.
  4. Continue startup and watch for the odor to clear

Explanation

A gas odor indicates a leak that could ignite, so the immediate response is to stop work, shut off the gas, and follow evacuation and ventilation procedures. Continuing startup or retrying ignition can create an ignition source. Leak-checking with soap solution is done only after the gas is off and the area is safe.

Question 10Safety and Jobsite Practices

For a two-worker lift of a long rectangular duct section onto hangers, which practice reduces strain injury?

  1. Agree on a count and lift together with knees bent and load close to the body Correct answer.
  2. Assign the heavier end to the taller worker
  3. Lift with the back rounded and arms extended for reach
  4. Twist at the waist to swing the duct onto the hanger

Explanation

Team lifting requires a coordinated count and proper body mechanics so neither worker takes an unexpected load. Bending the knees and keeping the load close reduces spinal stress. Twisting under load and reaching out with a rounded back are common causes of back injury.

Question 11Safety and Jobsite Practices

A technician is using a scissor lift to install high-side duct in a warehouse aisle. Which practice keeps the lift stable?

  1. Keeping the platform within its rated load and driving with the platform lowered Correct answer.
  2. Extending the platform fully while driving to save repositioning time
  3. Operating the lift on the aisle's sloped drain without checking the grade
  4. Standing on the mid-rail to reach a hanger above

Explanation

Scissor lifts must be operated within rated capacity and driven with the platform lowered per the manufacturer's instructions. Elevated driving, climbing the rails, and ignoring grade limits all raise the risk of tip-over. Load and slope limits are printed on the machine and must be followed.

Question 12Safety and Jobsite Practices

When must a hard hat be replaced rather than reused?

  1. When the worker changes job sites
  2. After a significant impact or when the shell shows cracks, or when it exceeds the manufacturer's service life Correct answer.
  3. Only when the suspension is uncomfortable
  4. Whenever it becomes dusty from duct board fabrication

Explanation

A hard hat that has taken a significant impact or shows cracks may have lost protective capacity and must be replaced. Manufacturers also publish a service life after which the shell is retired. Dust and comfort are not replacement criteria, and changing sites does not require a new hat.

Question 13Safety and Jobsite Practices

A duct crew is about to cut into an existing return plenum that may contain asbestos-containing duct liner in a building constructed in 1965. What is the correct sequence?

  1. Proceed with hand tools because the cut is small
  2. Wet the material and continue without assessment
  3. Ask the building owner verbally whether it contains asbestos and proceed on that answer
  4. Stop, treat the material as presumed asbestos-containing, and have it assessed before any disturbance Correct answer.

Explanation

Pre-1980 building materials must be presumed asbestos-containing until sampling proves otherwise, and disturbance requires assessment by a qualified person. Small cuts and wetting do not remove the regulatory and health requirements. A verbal assurance from an owner is not an acceptable substitute for assessment.

Question 14Safety and Jobsite Practices

Which practice protects workers and occupants when a duct system is opened for modification in a healthcare facility?

  1. Run the return fan at full speed to dilute contaminants
  2. Cover the opening with a single layer of duct tape at the end of the shift
  3. Prop the duct open and continue other work nearby
  4. Erect containment barriers and maintain negative pressure with HEPA filtration until the system is closed Correct answer.

Explanation

Opening ductwork in a healthcare setting can release dust and microbes into occupied areas, so containment with negative pressure and HEPA filtration is the standard control. Dilution by running the fan can spread contaminants through the building. A taped cover at shift end leaves the opening uncontrolled during work.

Question 15Safety and Jobsite Practices

What is the safe way to handle and store a full cylinder of nitrogen used for pressure testing ductwork?

  1. Store it near the brazing torch for convenience
  2. Leave the regulator attached and the valve open between uses
  3. Lay it horizontally in the truck bed without a cap
  4. Secure it upright with a valve cap in place and keep it away from heat sources Correct answer.

Explanation

Compressed gas cylinders must be secured upright, capped when not in use, and kept away from heat and ignition sources. Laying a cylinder down unsecured allows it to roll and the valve to shear. Storing it near a torch or leaving the valve open with a regulator attached creates a release and ignition hazard.

Question 16Safety and Jobsite Practices

A technician is working alone on a rooftop at night and needs to access a duct curb at the roof edge. Which combination best manages the risk?

  1. Ask a passerby below to watch and call for help if needed
  2. Use a warning line and work alone without notification
  3. Use a personal fall arrest system with an engineered anchor and maintain radio or phone contact per the site plan Correct answer.
  4. Work without fall protection because the task is brief

Explanation

Lone work at height requires both fall protection anchored to an engineered point and a means of summoning help, such as radio or phone check-ins. Brief duration does not justify working unprotected. A warning line is not fall arrest, and an untrained passerby is not a reliable rescue plan.

Question 17Safety and Jobsite Practices

Which is the correct way to verify that a circuit is de-energized before working on a rooftop exhaust fan's controls?

  1. Assume the disconnect is off because the fan stopped
  2. Ask a coworker to confirm the breaker is off
  3. Check that the indicator light on the unit is dark
  4. Test the conductors with a meter rated for the voltage after lockout, verifying the meter on a known live source first Correct answer.

Explanation

Verifying de-energization requires a live-dead-live test with a meter rated for the voltage and proven on a known source first. A stopped fan, a coworker's word, or a dark indicator lamp are not proof of an open circuit. Absence of voltage must be established at the point of work.

Question 18Safety and Jobsite Practices

A worker is fabricating duct board with a utility knife. Which practice best prevents a serious laceration?

  1. Leave the blade extended for the next cut
  2. Hold the board on the lap while cutting toward the torso
  3. Use a longer stroke with extra force to finish the cut quickly
  4. Cut with the blade moving away from the body and change blades when dull Correct answer.

Explanation

Controlled cuts directed away from the body plus a sharp blade reduce the force needed and the chance of the blade jumping. Extra force, cutting toward the torso, and leaving the blade extended all increase laceration risk. A dull blade requires more pressure and is more likely to slip.

Question 19Safety and Jobsite Practices

During duct pressure testing with compressed air, why must the test pressure be limited per the specification and a regulator used?

  1. Test pressure is limited only by the compressor's rating
  2. Higher pressure shortens the test and is always preferred
  3. Excessive pressure can rupture duct joints or seals and injure workers nearby Correct answer.
  4. A regulator is only needed for refrigerant charging

Explanation

Duct systems and their temporary closures have limited pressure capacity, and over-pressurizing can blow a joint or cap loose with enough force to injure someone. A regulator and gauge control the applied pressure and keep it within the specified test limit. Compressor capability is not a safe upper bound.

Question 20Safety and Jobsite Practices

A crew must remove a section of flexible duct that may be contaminated with mold in a school. Which practice is appropriate before removal?

  1. Cut the duct and let the HVAC system carry debris away
  2. Shake the duct out over a trash can in the hallway
  3. Don respiratory and eye protection, bag the duct in place, and contain the work area Correct answer.
  4. Rinse the duct with water in the ceiling space

Explanation

Mold-contaminated duct should be removed without dispersing spores, so the section is bagged in place and the work area contained, with appropriate respiratory and eye protection. Shaking, letting the air system carry debris, or washing in place spreads contamination into occupied areas.

Question 21Duct System Design Fundamentals and Load Considerations

A supply duct carries 1,200 cfm. Using the duct sizing equation A = CFM ÷ V, what nominal round duct diameter is required for a design velocity of 1,000 fpm?

  1. 16 in.
  2. 18 in.
  3. 12 in.
  4. 14 in. Correct answer.

Explanation

Area equals 1,200 ÷ 1,000 = 1.2 sq ft, which is 172.8 sq in. Solving A = πd²/4 gives a diameter of about 14.8 in., so a 14 in. duct is the closest nominal size that stays near the target velocity when rounded to standard sizes below the calculated value.

Question 22Duct System Design Fundamentals and Load Considerations

In a low-velocity residential duct system, what is the primary reason supply trunk velocity is typically limited to about 900 fpm or less?

  1. To satisfy combustion air requirements
  2. To keep the static pressure above the fan curve
  3. To prevent condensation on the duct exterior
  4. To control regenerated noise and excessive pressure loss Correct answer.

Explanation

Velocity drives both friction loss and noise generation, so low-velocity residential trunks are held near 600 to 900 fpm to keep the system quiet and the fan static budget reasonable. Higher velocities raise both pressure drop and radiated sound.

Question 23Duct System Design Fundamentals and Load Considerations

What does equivalent length represent when calculating duct system pressure loss?

  1. The developed length plus the insulation thickness
  2. The straight duct length that produces the same pressure drop as the fitting Correct answer.
  3. The length of duct needed to reach terminal velocity
  4. The physical centerline length of the duct run

Explanation

Equivalent length converts a fitting's pressure loss into an equivalent run of straight duct so all losses can be summed in a single friction-rate calculation. It lets designers add fitting losses to straight-run lengths without separate coefficient math.

Question 24Duct System Design Fundamentals and Load Considerations

Which fitting generally has the highest pressure loss coefficient in a duct system?

  1. A 45-degree round elbow
  2. A 90-degree hard square elbow Correct answer.
  3. A gradual round 90-degree elbow
  4. A straight duct section

Explanation

A hard square 90-degree elbow forces an abrupt direction change, producing a large loss coefficient, often well above 1.0. Gradual and 45-degree elbows turn the air more gently and have much lower coefficients, while straight duct has the lowest loss of all.

Question 25Duct System Design Fundamentals and Load Considerations

A room requires a sensible cooling load of 9,000 Btu/h with a design supply-to-room temperature difference of 20°F. Using the sensible heat equation with the standard air constant, what supply airflow is required?

  1. About 415 cfm Correct answer.
  2. About 380 cfm
  3. About 500 cfm
  4. About 450 cfm

Explanation

The sensible heat equation is Qs = 1.08 × CFM × ΔT. Rearranging gives CFM = 9,000 ÷ (1.08 × 20) ≈ 417 cfm, so about 415 cfm is the required airflow.

Question 26Duct System Design Fundamentals and Load Considerations

Why is the friction rate in a duct sizing calculation based on the total available static pressure rather than on a fixed velocity alone?

  1. Velocity alone determines the fan curve shape
  2. Friction rate is independent of duct length
  3. Static pressure only affects the return side
  4. Available static pressure sets the friction rate the system can afford over its total equivalent length Correct answer.

Explanation

The friction rate is the available static pressure divided by the total equivalent length of the longest run. This ties duct size to what the fan can actually deliver, so the run balances at the design airflow.

Question 27Duct System Design Fundamentals and Load Considerations

Which factor most directly increases the static pressure drop across a filter in a duct system?

  1. Lower return air temperature
  2. Higher face velocity through the filter media Correct answer.
  3. Longer supply trunk length
  4. Smaller duct insulation thickness

Explanation

Filter pressure drop rises with face velocity because the media resistance increases with airflow through a given area. Designers size filter area to keep face velocity within the manufacturer's limit.

Question 28Duct System Design Fundamentals and Load Considerations

What is the purpose of a manual balancing damper in a branch duct?

  1. To shut off airflow during unoccupied periods
  2. To reduce duct noise at the terminal
  3. To modulate airflow automatically with room temperature
  4. To adjust branch airflow so each zone receives its design cfm Correct answer.

Explanation

Manual balancing dampers are set during commissioning to trim branch resistance so each terminal delivers its design airflow. They are adjusted once and locked, not used for automatic control.

Question 29Duct System Design Fundamentals and Load Considerations

A designer must select a round duct size for 800 cfm at a friction rate of 0.10 in. w.c. per 100 ft. Which approach correctly applies a duct sizing chart?

  1. Enter the chart at the duct length and read velocity only
  2. Enter the chart at the fan rpm and read the diameter
  3. Enter the chart at 800 cfm, move to the 0.10 friction line, and read the resulting diameter Correct answer.
  4. Enter the chart at 0.10 friction and read the diameter at zero cfm

Explanation

On a standard friction chart, airflow and friction rate are the two known inputs used to find diameter and velocity. Entering at the cfm value and following it to the desired friction line yields the diameter directly.

Question 30Duct System Design Fundamentals and Load Considerations

A rectangular duct measures 12 in. by 20 in. What is its cross-sectional area in square feet?

  1. 1.67 sq ft Correct answer.
  2. 3.33 sq ft
  3. 2.40 sq ft
  4. 1.20 sq ft

Explanation

Area equals 12 × 20 = 240 sq in., and dividing by 144 gives 1.67 sq ft. This area is then used with the design cfm to find velocity.

Question 31Duct System Design Fundamentals and Load Considerations

Why do designers often convert a rectangular duct to its equivalent round diameter when checking velocity and friction?

  1. Friction charts and loss data are commonly tabulated for round duct Correct answer.
  2. Round duct always has lower material cost
  3. Equivalent diameter eliminates the need for fittings
  4. Rectangular ducts cannot carry the design airflow

Explanation

Most friction charts and fitting loss data are based on round duct, so a rectangular duct is converted to an equivalent round diameter for comparison. The conversion preserves the same friction loss per unit length at the same airflow.

Question 32Duct System Design Fundamentals and Load Considerations

A supply run has 60 ft of straight duct and fittings with a combined equivalent length of 40 ft. If the available static pressure for the run is 0.25 in. w.c., what friction rate should be used?

  1. 0.40 in. w.c. per 100 ft
  2. 0.25 in. w.c. per 100 ft Correct answer.
  3. 0.10 in. w.c. per 100 ft
  4. 0.60 in. w.c. per 100 ft

Explanation

Total equivalent length is 60 + 40 = 100 ft. Dividing the available 0.25 in. w.c. by 100 ft (one 100-ft unit) gives 0.25 in. w.c. per 100 ft.

Question 33Duct System Design Fundamentals and Load Considerations

A 14 in. round duct carries 1,000 cfm. What is the approximate velocity in the duct?

  1. About 700 fpm
  2. About 1,200 fpm
  3. About 935 fpm Correct answer.
  4. About 1,500 fpm

Explanation

A 14 in. duct has an area of π(0.583)² ≈ 1.07 sq ft. Dividing 1,000 cfm by 1.07 sq ft gives about 935 fpm.

Question 34Duct System Design Fundamentals and Load Considerations

Which condition best justifies increasing supply duct velocity above typical residential limits in a commercial design?

  1. The system uses a variable-speed blower
  2. The building has a low occupancy density
  3. The duct will be installed in a conditioned ceiling plenum
  4. Space constraints make larger ductwork impractical and the fan has adequate static pressure Correct answer.

Explanation

Higher velocity is acceptable when physical space is limited and the fan can overcome the added pressure loss, provided noise criteria are still met. Velocity limits are a design trade-off between space, noise, and pressure.

Question 35Duct System Design Fundamentals and Load Considerations

A designer adds a long radius 90-degree elbow in place of a hard square elbow on a main trunk. What is the expected effect on system performance?

  1. Higher pressure loss with unchanged airflow
  2. Lower pressure loss and reduced noise at the same airflow Correct answer.
  3. Higher airflow with lower fan static pressure
  4. No change because both fittings turn 90 degrees

Explanation

A long radius elbow turns the air more gradually, lowering the loss coefficient compared with a mitered square elbow. The reduced resistance translates into lower pressure drop and less regenerated noise.

Question 36Duct System Design Fundamentals and Load Considerations

Why is the longest equivalent-length run, rather than the shortest, used to establish the friction rate for a duct system?

  1. The longest run carries the least airflow
  2. Friction rate is independent of run length
  3. The longest run has the greatest resistance and governs the available static pressure Correct answer.
  4. The shortest run always has the highest velocity

Explanation

The run with the greatest total equivalent length experiences the most resistance, so it determines the friction rate the fan must support. Sizing to that run ensures every branch can achieve design airflow.

Question 37Duct System Design Fundamentals and Load Considerations

A room has a total cooling load of 12,000 Btu/h and a sensible load of 9,600 Btu/h. What is the sensible heat ratio?

  1. 0.75
  2. 0.80 Correct answer.
  3. 0.85
  4. 0.60

Explanation

Sensible heat ratio equals sensible load divided by total load, or 9,600 ÷ 12,000 = 0.80. This ratio guides equipment selection and supply air temperature.

Question 38Duct System Design Fundamentals and Load Considerations

What is the primary function of a volume damper installed at a duct takeoff rather than at the terminal?

  1. To provide fire protection at the penetration
  2. To balance the branch before the air reaches individual terminals Correct answer.
  3. To filter particulates from the airstream
  4. To reduce the duct static pressure at the fan

Explanation

A takeoff damper balances the whole branch, allowing the designer to set branch airflow before final terminal adjustment. Terminal dampers then fine-tune individual outlets.

Question 39Duct System Design Fundamentals and Load Considerations

A duct system is designed with a total external static pressure of 0.50 in. w.c. If the supply duct accounts for 0.20 in. w.c. and the return accounts for 0.15 in. w.c., what static pressure remains for the coil and accessories?

  1. 0.05 in. w.c.
  2. 0.20 in. w.c.
  3. 0.35 in. w.c.
  4. 0.15 in. w.c. Correct answer.

Explanation

Subtracting the supply and return losses from the total gives 0.50 − 0.20 − 0.15 = 0.15 in. w.c. remaining for the coil and accessories.

Question 40Duct System Design Fundamentals and Load Considerations

Which duct design method sizes each segment so the velocity decreases progressively toward the terminals?

  1. Static regain method
  2. Equal friction method
  3. Constant velocity method
  4. Velocity reduction method Correct answer.

Explanation

The velocity reduction method assigns a lower velocity to each successive downstream segment, reducing noise and pressure loss near terminals. The equal friction method holds friction rate constant, and static regain uses pressure recovery at fittings.

Question 41Duct System Design Fundamentals and Load Considerations

A designer is comparing two layouts for a main trunk: one with three 90-degree hard elbows and one with three 45-degree elbows plus a short offset. Which statement best reflects the pressure-loss comparison?

  1. Both layouts have identical loss because they turn the same total angle
  2. The hard-elbow layout always has lower loss because it is shorter
  3. The extra offset eliminates all fitting losses
  4. The layout with 45-degree elbows generally has lower total loss despite the extra offset Correct answer.

Explanation

Each 45-degree elbow has a much lower loss coefficient than a hard 90-degree elbow, so three of them plus a short offset usually total less loss than three hard 90s. Layout comparisons must sum coefficients, not count elbows.

Question 42Duct System Design Fundamentals and Load Considerations

A 20 in. by 12 in. rectangular duct carries 1,800 cfm. What is the approximate velocity in the duct?

  1. About 1,080 fpm Correct answer.
  2. About 750 fpm
  3. About 900 fpm
  4. About 1,350 fpm

Explanation

The area is 240 sq in., or 1.67 sq ft. Dividing 1,800 cfm by 1.67 sq ft gives about 1,080 fpm.

Question 43Duct System Design Fundamentals and Load Considerations

A branch serving a conference room is short in equivalent length but must meet a strict NC-30 noise criterion. How should the designer resolve the conflict between low pressure loss and low noise?

  1. Size the branch for lower velocity and add acoustic treatment, accepting a slightly larger duct Correct answer.
  2. Remove the balancing damper to reduce turbulence
  3. Reduce the terminal static pressure by increasing fan speed
  4. Size the branch at the highest velocity allowed by the fan to minimize duct size

Explanation

Noise criteria are met by limiting velocity and controlling regenerated sound, even when the short run could tolerate higher velocity. The designer accepts a larger duct and acoustic treatment to satisfy NC-30.

Question 44Duct System Design Fundamentals and Load Considerations

A designer calculates a friction rate of 0.12 in. w.c. per 100 ft for a run with 150 ft of total equivalent length. What total friction loss should be expected for that run?

  1. 0.12 in. w.c.
  2. 0.24 in. w.c.
  3. 0.08 in. w.c.
  4. 0.18 in. w.c. Correct answer.

Explanation

Multiply the friction rate by the equivalent length in 100-ft units: 0.12 × 1.5 = 0.18 in. w.c. This is the friction loss the run consumes from the available static pressure.

Question 45Duct System Design Fundamentals and Load Considerations

Why does the static regain method often produce a more energy-efficient trunk than the equal friction method on long runs with many branches?

  1. It sizes downstream segments to convert velocity pressure into static pressure at each branch Correct answer.
  2. It holds velocity constant so the fan operates at a single point
  3. It sizes all segments to the smallest available duct
  4. It ignores fitting losses entirely

Explanation

Static regain sizing reduces velocity after each takeoff so that velocity pressure is recovered as static pressure, offsetting friction loss. This can lower total fan energy on long trunks with many branches compared with holding a constant friction rate.

Question 46Duct System Design Fundamentals and Load Considerations

A supply plenum transitions from a rectangular main to a round branch. Which fitting loss coefficient should be used when the branch takeoff is a straight, radiused shoe rather than a square boot?

  1. The coefficient for a radiused shoe, which is lower because the gradual turning reduces turbulence and separation Correct answer.
  2. The coefficient for a square boot, because takeoff type does not affect branch pressure drop
  3. The coefficient for a fully open volume damper, because the damper governs branch loss
  4. The coefficient for a 90-degree mitered elbow, because both fittings turn the airflow 90 degrees

Explanation

Fitting loss coefficients are tabulated by fitting geometry, and a radiused shoe turns the air gradually so it separates less and produces a lower loss coefficient than a square boot. The takeoff fitting type therefore directly changes the branch pressure drop used in the equal friction or static regain calculation.

Question 47Duct System Design Fundamentals and Load Considerations

A designer is laying out a trunk that must pass through a congested ceiling plenum with limited depth. Which duct characteristic most directly drives the required depth of a rectangular trunk at a fixed airflow?

  1. The number of volume dampers in the branches
  2. The sensible heat ratio of the conditioned space
  3. The aspect ratio of the duct cross section Correct answer.
  4. The total equivalent length of the longest run

Explanation

For a given airflow and velocity, a rectangular duct's depth is set by its aspect ratio; a higher aspect ratio yields a shallower but wider duct. Equivalent length, damper count, and sensible heat ratio affect pressure loss or load, not the physical depth of the duct.

Question 48Duct System Design Fundamentals and Load Considerations

A duct system serves a zone with a design cooling load of 18,000 Btu/h and a design supply-to-room temperature difference of 20°F. Using the sensible heat equation, what supply airflow is required?

  1. 830 cfm Correct answer.
  2. 1,500 cfm
  3. 1,000 cfm
  4. 1,250 cfm

Explanation

Airflow is found from Q = 1.08 × cfm × ΔT, so cfm = 18,000 ÷ (1.08 × 20) ≈ 833 cfm, closest to 830 cfm. This airflow then sets the duct size and velocity used in the distribution layout.

Question 49Duct System Design Fundamentals and Load Considerations

A designer must choose between the equal friction method and the static regain method for a long, high-velocity commercial trunk with many branches. Which statement best describes a design consequence of selecting static regain?

  1. Branch dampers are eliminated because the method self-balances the entire system
  2. Trunk segments are sized to recover velocity pressure, so downstream sizes grow and fan energy can drop Correct answer.
  3. Every segment is sized to the same friction rate, so downstream velocity rises as airflow drops
  4. Duct sizes are set by the longest run's equivalent length rather than by velocity recovery

Explanation

Static regain sizes each downstream segment to convert velocity pressure into static pressure, which generally increases duct size toward the terminals and can lower fan energy on long runs. The tradeoff is larger ducts and more careful fitting selection compared with equal friction.

Question 50Duct System Design Fundamentals and Load Considerations

A designer is comparing two supply layouts for the same zone. Layout A uses a short, direct run with two tight square elbows. Layout B uses a longer run with four long-radius elbows but the same total equivalent length. Which conclusion about friction rate and fitting loss is most defensible?

  1. Layout B will have the lower total pressure loss because long-radius elbows reduce fitting losses despite more fittings Correct answer.
  2. Layout A will have the lower loss because square elbows are cheaper and therefore more efficient
  3. Both layouts will have identical pressure loss because their total equivalent lengths are equal
  4. Layout A will have the lower total pressure loss because its physical length is shorter

Explanation

Equivalent length already accounts for fitting losses, so equal total equivalent length means equal friction loss at a given friction rate. However, long-radius elbows have much lower loss coefficients than square elbows, so Layout B's fittings contribute less loss per fitting and its straight-duct portion is longer at the same total. The defensible conclusion is that the long-radius fittings reduce fitting loss, favoring Layout B when comparing fitting performance.