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NATE Air Distribution Service study materials
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NATE Air Distribution Service practice questions

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Question 1Air Distribution System Components and Terminology

In a residential forced-air system, which component is responsible for moving conditioned air through the supply ductwork?

  1. The condensate pump
  2. The blower assembly Correct answer.
  3. The gas manifold
  4. The condensate drain trap

Explanation

The blower assembly (air handler fan) creates the pressure differential that pushes air through the supply ducts and pulls return air back to the equipment. The other listed parts belong to the refrigerant or combustion side of the system and do not move airstream air.

Question 2Air Distribution System Components and Terminology

What is the primary function of the return air duct system in a typical residential installation?

  1. To exhaust stale air from bathrooms and kitchens
  2. To filter outdoor ventilation air before it enters the building
  3. To distribute heated air to the farthest registers first
  4. To carry air from conditioned spaces back to the air-handling equipment Correct answer.

Explanation

Return ducts complete the air loop by bringing room air back to the blower and coil or heat exchanger for reconditioning. Supply ducts deliver conditioned air, and exhaust or ventilation systems serve a separate purpose.

Question 3Air Distribution System Components and Terminology

Which duct fitting is used to divide one main trunk into two or more branches?

  1. A volume damper
  2. An end cap
  3. A wye or tee fitting Correct answer.
  4. A takeoff collar

Explanation

Wye and tee fittings split a main duct run into multiple branches while managing airflow direction. Takeoffs tap a single branch off a trunk, dampers regulate flow, and end caps simply terminate a run.

Question 4Air Distribution System Components and Terminology

Static pressure in an air distribution system is best described as which of the following?

  1. The temperature difference between supply and return
  2. The outward force air exerts against duct walls Correct answer.
  3. The weight of water vapor in the airstream
  4. The velocity of air measured at a register face

Explanation

Static pressure is the force exerted perpendicular to the direction of flow, acting outward on duct walls. Velocity pressure relates to airflow speed, and the other choices describe temperature or humidity properties.

Question 5Air Distribution System Components and Terminology

A barometric bypass damper in a zoned forced-air system serves which purpose?

  1. It filters return air before the blower
  2. It prevents backdraft at the flue
  3. It measures airflow velocity at each zone
  4. It relieves excess static pressure when zone dampers close Correct answer.

Explanation

In zoning, closing zone dampers reduces available duct area and raises static pressure; a bypass damper opens to route excess air back to the return, protecting the blower and ductwork. Flue backdraft protection is a separate function of draft controls.

Question 6Air Distribution System Components and Terminology

Which flexible connector is commonly installed between the air handler and the main trunk to isolate vibration?

  1. A canvas duct connector Correct answer.
  2. A turning vane
  3. A rigid galvanized elbow
  4. A fire damper

Explanation

A flexible canvas or fabric connector absorbs vibration from the blower and prevents it from transmitting into the rigid duct system. Rigid elbows, fire dampers, and turning vanes perform air routing or safety functions, not vibration isolation.

Question 7Air Distribution System Components and Terminology

A technician measures 0.20 in. w.c. of external static pressure across the air handler. Which instrument is appropriate for this reading?

  1. A clamp meter
  2. A sling psychrometer
  3. A refrigerant gauge set
  4. An inclined or digital manometer Correct answer.

Explanation

External static pressure is measured in inches of water column using a manometer connected to pressure taps on the supply and return plenums. Psychrometers measure humidity, clamp meters measure current, and refrigerant gauges measure system pressures.

Question 8Air Distribution System Components and Terminology

What is the purpose of a turning vane installed inside a rectangular duct elbow?

  1. To reduce turbulence and pressure loss through the bend Correct answer.
  2. To measure airflow velocity at the elbow
  3. To close off the branch during balancing
  4. To filter particulates from the airstream

Explanation

Turning vanes guide air smoothly around a bend, reducing turbulence and the pressure drop associated with abrupt direction changes. They do not filter, measure, or shut off airflow.

Question 9Air Distribution System Components and Terminology

A supply register with adjustable curved blades that can be directed toward a wall or ceiling is best described as which type?

  1. A baseboard diffuser
  2. A three-way ceiling diffuser Correct answer.
  3. A linear slot diffuser
  4. A fixed stamped-face grille

Explanation

A three-way ceiling diffuser has adjustable deflectors that direct air in multiple directions, allowing the technician to control throw and spread. Fixed grilles cannot be redirected, and linear slot diffusers provide a narrow, directional throw pattern.

Question 10Air Distribution System Components and Terminology

Which term describes the distance air travels from a supply outlet before its velocity drops to a specified terminal value?

  1. Throw Correct answer.
  2. Spread
  3. Drop
  4. Entrainment ratio

Explanation

Throw is the horizontal or vertical distance from the outlet to the point where airstream velocity falls to a defined terminal velocity, often 50 fpm. Spread is the width of the airstream at that point, and drop is the vertical descent of the airstream.

Question 11Air Distribution System Components and Terminology

In duct design, what does the term "equivalent length" account for?

  1. The physical length of duct plus the added resistance of fittings Correct answer.
  2. The total weight of the duct system
  3. The distance between the air handler and the farthest register
  4. The length of flex duct needed for a run

Explanation

Equivalent length converts the pressure loss of fittings, such as elbows and takeoffs, into an equivalent straight-duct length so total system resistance can be compared. It is not a physical measurement of duct length or weight.

Question 12Air Distribution System Components and Terminology

A technician observes that a return grille is undersized relative to the supply capacity. Which symptom is most likely?

  1. Excessive supply airflow and high register velocity
  2. Lower blower motor current draw
  3. Reduced total airflow and elevated return-side static pressure Correct answer.
  4. Increased supply air temperature

Explanation

An undersized return restricts the air available to the blower, lowering total system airflow and raising negative static pressure on the return side. Supply velocity would decrease, not increase, and blower current typically falls with reduced airflow.

Question 13Air Distribution System Components and Terminology

What is the function of a fire damper in a duct system?

  1. To reduce noise transmission through the duct
  2. To balance airflow between zones
  3. To prevent condensation inside the duct
  4. To close automatically when heat is detected and limit fire spread through the duct Correct answer.

Explanation

Fire dampers are rated closures that release and shut when a fusible link melts or heat is detected, blocking fire from traveling through duct penetrations. Balancing, sound attenuation, and condensation control are handled by other components.

Question 14Air Distribution System Components and Terminology

Which duct material is typically used for a high-velocity, low-pressure residential trunk line where rigidity and low friction are priorities?

  1. Uninsulated vinyl flex duct
  2. Galvanized sheet metal Correct answer.
  3. Aluminum foil tape
  4. Fabric duct sock

Explanation

Galvanized sheet metal provides a rigid, smooth interior surface with low friction loss that suits trunk lines. Flex duct has higher resistance, fabric socks are for specialized commercial applications, and foil tape is a sealing material rather than duct.

Question 15Air Distribution System Components and Terminology

A technician is reading a duct sizing chart that lists a friction rate of 0.1 in. w.c. per 100 ft. What does this value represent?

  1. The pressure loss per 100 feet of straight duct at the design airflow Correct answer.
  2. The velocity of air at the supply register
  3. The total external static pressure of the air handler
  4. The allowable leakage rate of the duct system

Explanation

Friction rate expresses pressure loss per unit length of duct at a given airflow and is used with equivalent lengths to size ducts. It is distinct from total external static pressure, register velocity, and leakage limits.

Question 16Air Distribution System Components and Terminology

Which of the following best describes the term "pressure drop" across a filter?

  1. The change in air temperature across the filter
  2. The volume of air bypassing the filter
  3. The amount of dust captured by the filter
  4. The difference in static pressure measured before and after the filter Correct answer.

Explanation

Pressure drop is the difference in static pressure on the upstream and downstream sides of a component, indicating the resistance it adds to airflow. A dirty filter has a higher pressure drop, which is why it must be monitored.

Question 17Air Distribution System Components and Terminology

A technician needs to determine whether a supply branch is delivering its design airflow of 150 cfm. Which method provides the most direct measurement?

  1. Read the thermostat setpoint
  2. Measure the return air temperature
  3. Check the blower motor amp draw
  4. Measure the register face velocity with an anemometer and multiply by free area Correct answer.

Explanation

Multiplying measured face velocity by the register's free area yields an airflow estimate in cfm, which can be compared with design. Motor amps, temperature, and thermostat settings do not directly indicate branch airflow.

Question 18Air Distribution System Components and Terminology

What is the primary purpose of a volume damper in a branch duct?

  1. To reduce sound transmission
  2. To regulate and balance the airflow through that branch Correct answer.
  3. To filter the airstream
  4. To prevent fire from spreading through the duct

Explanation

Volume dampers are adjustable blades used during air balancing to set the airflow in each branch to its design value. Fire protection, sound control, and filtration are handled by separate components.

Question 19Air Distribution System Components and Terminology

In a duct system, the term "aspect ratio" refers to which relationship?

  1. The ratio of static pressure to velocity pressure
  2. The ratio of duct length to duct diameter
  3. The ratio of supply airflow to return airflow
  4. The ratio of duct width to duct height Correct answer.

Explanation

Aspect ratio compares the width and height of a rectangular duct; lower ratios reduce friction and improve airflow efficiency. The other ratios describe airflow balance or pressure relationships, not duct geometry.

Question 20Air Distribution System Components and Terminology

A technician finds that a ceiling diffuser produces a draft complaint in an occupied zone. Which adjustment is most appropriate?

  1. Reduce the diffuser's free area with a restrictor plate
  2. Replace the diffuser with a fixed grille
  3. Increase the branch damper opening
  4. Redirect the diffuser blades to increase throw along the ceiling Correct answer.

Explanation

Redirecting the diffuser blades so the airstream clings to the ceiling increases throw and promotes mixing before the air reaches occupants, reducing draft. Opening the damper increases airflow and worsens the draft, and a restrictor plate or fixed grille may not solve the pattern issue.

Question 21Air Distribution System Components and Terminology

A duct system shows high static pressure on the supply side but normal return-side pressure. Which condition is the most likely cause?

  1. An undersized return grille
  2. A dirty return filter
  3. A leaking return duct
  4. A restricted supply trunk or closed supply damper Correct answer.

Explanation

Elevated supply-side static pressure with normal return pressure points to a restriction downstream of the blower, such as a closed damper or blocked trunk. Return-side problems would raise return-side pressure or lower overall airflow.

Question 22Air Distribution System Components and Terminology

Which statement correctly describes the relationship between velocity pressure and airflow velocity in a duct?

  1. Velocity pressure is independent of air velocity
  2. Velocity pressure decreases as air velocity increases
  3. Velocity pressure equals static pressure at all times
  4. Velocity pressure increases with the square of the air velocity Correct answer.

Explanation

Velocity pressure is proportional to the square of air velocity, so doubling velocity quadruples velocity pressure. This relationship underlies the use of pitot tubes and velocity pressure readings to calculate airflow.

Question 23Air Distribution System Components and Terminology

A technician is balancing a zoned system and notices that closing one zone damper causes the blower to operate against a higher static pressure. Which component is designed to mitigate this condition?

  1. A return grille
  2. A bypass damper Correct answer.
  3. A fire damper
  4. A turning vane

Explanation

A bypass damper opens as zone dampers close, routing excess air back to the return and limiting the static pressure rise. Fire dampers, turning vanes, and return grilles do not relieve zone-induced pressure changes.

Question 24Air Distribution System Components and Terminology

What is the purpose of a duct takeoff collar?

  1. To measure static pressure in the trunk
  2. To connect a branch duct to a main trunk at a round opening Correct answer.
  3. To seal the end of a duct run
  4. To support the duct from the ceiling

Explanation

A takeoff collar provides a round connection point where a branch duct attaches to a main trunk. End caps seal duct ends, pressure taps measure static pressure, and hangers provide support.

Question 25Air Distribution System Components and Terminology

Which of the following best describes "entrainment" in the context of supply air distribution?

  1. The reversal of airflow in a return duct
  2. The condensation of water vapor on duct surfaces
  3. The leakage of air through duct seams
  4. The induction of room air into the supply airstream Correct answer.

Explanation

Entrainment is the process by which a moving supply jet induces surrounding room air into its airstream, increasing total airflow and improving mixing. Duct leakage, condensation, and return airflow reversal are unrelated phenomena.

Question 26Air Distribution System Components and Terminology

In air distribution terminology, what does the abbreviation CFM stand for?

  1. Cubic flow measure
  2. Cubic foot mass
  3. Cubic feet per meter
  4. Cubic feet per minute Correct answer.

Explanation

CFM is the standard volumetric airflow unit in North American HVAC work, meaning cubic feet per minute. It quantifies how much air volume moves through a duct or outlet each minute and is the basis for equipment and register sizing.

Question 27Air Distribution System Components and Terminology

Which component is installed at the end of a duct run to distribute conditioned air into a room with a controlled pattern?

  1. Volume damper
  2. Supply diffuser Correct answer.
  3. Fire damper
  4. Turning vane

Explanation

A supply diffuser is the terminal device that discharges air into the occupied space and shapes its throw pattern. Dampers and vanes are located inside the duct system and do not serve as room air outlets.

Question 28Air Distribution System Components and Terminology

A return air grille is best described by which function?

  1. It balances the supply airflow to each branch.
  2. It provides an opening through which room air is drawn back to the air handler. Correct answer.
  3. It measures the static pressure of the return system.
  4. It filters the air before it enters the return duct.

Explanation

A return grille is the intake opening that allows room air to enter the return ductwork and travel back to the air handler. It may include a filter rack, but its defining role is to serve as the return-air entry point.

Question 29Air Distribution System Components and Terminology

Which term describes the pressure exerted by the fan that must overcome the resistance of the duct system, filters, and coils?

  1. Velocity pressure
  2. Differential pressure
  3. Total external static pressure Correct answer.
  4. Static pressure

Explanation

Total external static pressure is the combined resistance the blower must overcome on both the supply and return sides of the system, excluding the internal pressure drops of the air handler itself. It is the value used to compare measured system resistance with the fan performance curve.

Question 30Air Distribution System Components and Terminology

A technician is selecting a rectangular duct size for a branch that must carry 400 cfm at a friction rate of 0.10 in. w.c. per 100 ft. Which duct sizing method is being applied?

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

Explanation

The equal friction method sizes ductwork so that each section has the same friction loss per unit length, commonly 0.10 in. w.c. per 100 ft. The duct dimensions are then read from a friction chart at the required airflow.

Question 31Air Distribution System Components and Terminology

What does a duct-mounted smoke detector do when it senses smoke in the airstream?

  1. It increases fan speed to dilute the smoke.
  2. It opens a bypass to exhaust smoke outdoors.
  3. It activates the cooling coil to suppress the smoke.
  4. It closes a damper to block airflow and signals the alarm system. Correct answer.

Explanation

A duct smoke detector responds to smoke in the duct by shutting down or isolating the air-handling unit and sending an alarm signal to the fire alarm system. This prevents smoke from being distributed through the occupied spaces.

Question 32Air Distribution System Components and Terminology

Which fitting is used to connect a round branch duct to a rectangular trunk duct at a right angle?

  1. Reducer
  2. Takeoff Correct answer.
  3. Wye
  4. Elbow

Explanation

A takeoff is a fitting that branches from the side of a main duct, commonly used to connect a round branch to a rectangular trunk. It provides the transition and often includes a balancing damper.

Question 33Air Distribution System Components and Terminology

A technician measures a velocity pressure of 0.25 in. w.c. in a duct. Using the relationship VP = (V/4005)^2, what is the approximate air velocity?

  1. 2000 fpm Correct answer.
  2. 2500 fpm
  3. 4000 fpm
  4. 1000 fpm

Explanation

The formula VP = (V/4005)^2 can be rearranged to V = 4005 times the square root of VP. The square root of 0.25 is 0.5, so V = 4005 × 0.5, which is about 2000 fpm.

Question 34Air Distribution System Components and Terminology

Which statement correctly describes the function of a barometric relief damper in a building's air distribution system?

  1. It measures the airflow in the main return duct.
  2. It prevents reverse airflow through the supply fan when the fan stops.
  3. It maintains a constant supply air temperature by modulating the cooling coil.
  4. It relieves excess building pressure by allowing air to exhaust when the pressure rises. Correct answer.

Explanation

A barometric relief damper is a pressure-actuated device that opens to exhaust air when building pressure exceeds its setpoint, helping to prevent overpressurization from outdoor air intake. It does not control temperature or measure airflow.

Question 35Air Distribution System Components and Terminology

What is the primary purpose of a plenum in an air distribution system?

  1. To serve as a large chamber that distributes air to multiple ducts with low velocity and pressure loss Correct answer.
  2. To increase air velocity before the branch ducts
  3. To measure the static pressure of the supply system
  4. To filter the air before it enters the coil

Explanation

A plenum is a large, often box-like chamber that receives air from the fan and distributes it into several branch ducts. Its large cross section lowers velocity and reduces pressure loss at the transition to the branches.

Question 36Air Distribution System Components and Terminology

Which type of duct system is designed to operate at a relatively high velocity and is often used in commercial buildings where space is limited?

  1. Low-velocity duct system
  2. High-velocity duct system Correct answer.
  3. Flexible duct system
  4. Fabric duct system

Explanation

High-velocity duct systems use smaller-diameter ducts and higher air speeds to fit within tight ceiling spaces. They are common in commercial retrofit and multi-story applications where duct routing space is constrained.

Question 37Air Distribution System Components and Terminology

A technician is troubleshooting a system and finds that the supply duct has a high static pressure reading while the return duct static pressure is normal. Which condition is the most likely cause?

  1. An oversized supply duct
  2. A clogged return air filter
  3. A closed or restricted supply damper Correct answer.
  4. A failed return fan

Explanation

High supply-side static pressure with normal return-side pressure points to a restriction on the supply side, such as a closed damper or crushed duct. The blower builds pressure against the restriction while the return side remains unaffected.

Question 38Air Distribution System Components and Terminology

In duct design, what does the term "pressure drop" refer to?

  1. The decrease in static pressure as air flows through a duct, fitting, or device Correct answer.
  2. The increase in air velocity as air moves through a reducer
  3. The change in air temperature across the cooling coil
  4. The difference in elevation between the supply and return ducts

Explanation

Pressure drop is the loss of static pressure that occurs as air moves through ductwork, fittings, filters, and coils due to friction and turbulence. It is a key value used to select equipment and size duct sections.

Question 39Air Distribution System Components and Terminology

A technician is balancing a duct system and needs to measure the airflow in a branch without permanently installing a flow station. Which instrument is most appropriate for a spot reading in the branch?

  1. A clamp-on ammeter
  2. A digital manometer connected to a pitot tube Correct answer.
  3. A thermocouple thermometer
  4. A refrigerant gauge manifold

Explanation

A pitot tube used with a digital manometer measures velocity pressure, which can be converted to velocity and then to airflow using the duct area. This is the standard field method for obtaining a spot airflow reading in a duct.

Question 40Air Distribution System Components and Terminology

In air distribution terminology, what does the term "throw" describe?

  1. The total volume of air delivered by a diffuser per minute
  2. The pressure loss across a diffuser
  3. The angle at which air leaves a diffuser
  4. The distance air travels from a supply outlet before its velocity drops to a specified value Correct answer.

Explanation

Throw is the horizontal or vertical distance from the outlet to the point where the airstream velocity falls to a specified terminal value, typically 50 or 100 fpm. It is used to evaluate whether conditioned air will reach the occupied zone without causing drafts.

Question 41Duct Design, Sizing, and Static Pressure

When sizing a rectangular galvanized steel trunk duct with a friction chart, which input pair is required before the duct dimensions can be read?

  1. Airflow in cfm and design friction rate in inches of water per 100 ft Correct answer.
  2. Register face velocity and throw distance
  3. Duct length and number of fittings
  4. Static pressure at the fan outlet and motor horsepower

Explanation

A friction chart plots airflow against pressure loss per unit length, so the designer must supply the cfm the duct carries and the target friction rate (commonly 0.1 in. w.c. per 100 ft for residential work). Duct length and fitting counts enter later when adding losses, not when reading size off the chart.

Question 42Duct Design, Sizing, and Static Pressure

A designer wants the total equivalent length method to account for fittings. What does the method do?

  1. Increases the design friction rate by a fixed percentage for each elbow
  2. Adds fitting pressure losses as extra feet of straight duct of the same size Correct answer.
  3. Sizes each fitting as a separate duct section with its own friction rate
  4. Reduces the required cfm to compensate for turbulence at fittings

Explanation

The equivalent length approach converts each fitting's loss into an additional length of straight duct, which is added to the measured run length. The total equivalent length is then used with the friction rate to find the pressure drop for the whole run.

Question 43Duct Design, Sizing, and Static Pressure

What is the primary reason supply duct velocity is limited in a branch serving a bedroom?

  1. To prevent condensation from forming inside the duct
  2. To keep the generated noise below an acceptable level at the diffuser Correct answer.
  3. To keep the duct's external static pressure below the fan curve
  4. To ensure the duct fits within the joist space

Explanation

Branch velocity limits are set mainly by noise criteria, because high velocity at the takeoff and diffuser produces objectionable air noise in occupied rooms. Duct sizing charts pair each velocity limit with a corresponding friction rate for this reason.

Question 44Duct Design, Sizing, and Static Pressure

A return duct run and a supply duct run carry identical airflow and use the same friction rate. Which statement about their sizes is generally true?

  1. The return is typically sized equal to or larger than the supply to keep return noise and pressure low Correct answer.
  2. The return size is governed only by the filter rack dimensions
  3. The return can usually be sized slightly smaller because return grilles tolerate higher velocity
  4. The return must always be exactly one nominal size smaller than the supply

Explanation

Return paths are commonly sized at least as large as the supply because return grilles and filters add resistance and because low return velocity keeps noise down. Undersized returns raise the system's external static pressure and reduce delivered airflow.

Question 45Duct Design, Sizing, and Static Pressure

What does external static pressure (ESP) represent when checking a blower's available airflow?

  1. The difference between supply and return air temperatures
  2. The pressure measured inside the blower housing only
  3. The pressure the blower must overcome in the duct system, coil, and filter outside the cabinet Correct answer.
  4. The velocity pressure at the supply plenum

Explanation

ESP is the resistance the blower works against across the entire connected system, including ductwork, coils, filters, and grilles. The manufacturer's blower table is entered with ESP to find the delivered cfm.

Question 46Duct Design, Sizing, and Static Pressure

A duct is described as having 0.25 in. w.c. of total pressure loss over a 100 ft run. What does this value express?

  1. The air velocity inside the duct in feet per minute
  2. The resistance of that run at the design airflow Correct answer.
  3. The duct's leakage class at design pressure
  4. The fan's brake horsepower per 100 ft

Explanation

Total pressure loss across a run is the resistance the air encounters at the design flow, and it is what the blower must overcome. Velocity and leakage class are separate quantities, though they can influence the loss.

Question 47Duct Design, Sizing, and Static Pressure

Which duct material characteristic most directly increases friction loss per foot at a given velocity?

  1. A brighter galvanized coating
  2. A longer duct section length
  3. A larger duct wall gauge number
  4. A rougher internal surface, reflected in a higher friction factor Correct answer.

Explanation

Friction loss depends on surface roughness as well as velocity and hydraulic diameter. Flexible duct with its corrugations, for example, has a higher friction factor than smooth galvanized steel and loses more pressure per foot.

Question 48Duct Design, Sizing, and Static Pressure

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

  1. To balance airflow among branches by adding adjustable resistance Correct answer.
  2. To prevent backdraft when the fan cycles off
  3. To automatically regulate airflow in response to room temperature
  4. To reduce duct leakage at joints

Explanation

A manual balancing damper is set by the technician to trim airflow in a branch so the system delivers design cfm to each zone. It adds resistance deliberately, so it must be accounted for in the pressure budget.

Question 49Duct Design, Sizing, and Static Pressure

A 400 cfm branch is sized at 0.1 in. w.c. per 100 ft. If the measured equivalent length of the run is 60 ft, what is the approximate friction loss for the run?

  1. 0.06 in. w.c. Correct answer.
  2. 0.60 in. w.c.
  3. 0.10 in. w.c.
  4. 0.016 in. w.c.

Explanation

Friction loss equals the rate times the equivalent length divided by 100: 0.1 times 60/100 equals 0.06 in. w.c. The 400 cfm figure is used to pick the size from the chart, not to scale the loss.

Question 50Duct Design, Sizing, and Static Pressure

A designer must deliver 900 cfm through a run whose available static pressure budget is 0.15 in. w.c. over 150 equivalent feet. What friction rate should be used to enter the sizing chart?

  1. 0.10 in. w.c. per 100 ft Correct answer.
  2. 0.15 in. w.c. per 100 ft
  3. 0.015 in. w.c. per 100 ft
  4. 0.225 in. w.c. per 100 ft

Explanation

The rate is the budget divided by equivalent length and multiplied by 100: 0.15 divided by 150 times 100 equals 0.10 in. w.c. per 100 ft. That rate is then used with the 900 cfm to read the duct size from the chart.