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HVAC Air Distribution practice questions

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Question 1HVAC Air Distribution

In a low-velocity duct design, the total pressure at a point equals which combination of pressures?

  1. Velocity pressure minus static pressure
  2. Static pressure plus velocity pressure Correct answer.
  3. Static pressure minus velocity pressure
  4. Static pressure multiplied by velocity pressure

Explanation

Total pressure is defined as the sum of static pressure and velocity pressure at the same location. Static pressure acts perpendicular to the duct wall while velocity pressure reflects the kinetic energy of moving air. Subtracting or multiplying the two quantities has no physical basis in air distribution work.

Question 2HVAC Air Distribution

A supply diffuser is selected primarily for which performance characteristic when comfort and noise are both critical?

  1. Duct gauge and hanger spacing
  2. Filter MERV rating and pleat count
  3. Throw, pressure drop, and NC rating Correct answer.
  4. Fan blade pitch and motor rpm

Explanation

Diffuser selection depends on throw, total pressure drop, and the noise criteria rating, which together describe how the air jet behaves and how loud the device is. Duct gauge, filter rating, and fan details belong to other parts of the system and are not the diffuser's own performance data.

Question 3HVAC Air Distribution

The equal friction method of duct sizing produces a system in which which quantity is held constant per unit length?

  1. Air density through the trunk
  2. Friction loss rate along each run Correct answer.
  3. Velocity pressure at every fitting
  4. Total pressure at each terminal

Explanation

The equal friction method sizes each duct section so that the pressure loss per unit length is the same throughout the run. This simplifies design and tends to balance branches naturally. Velocity pressure, terminal total pressure, and air density are not held constant by this method.

Question 4HVAC Air Distribution

Which duct configuration typically yields the lowest pressure loss for the same flow rate and cross-sectional area?

  1. A rectangular duct with many mitered elbows
  2. A straight run of round duct Correct answer.
  3. A flexible duct fully compressed into a short run
  4. A straight run of rectangular duct with the same area

Explanation

Round duct has the smallest perimeter for a given area, so it produces the least friction and the most uniform velocity profile. Rectangular duct of equal area has more wetted surface, and fittings or compressed flex add further losses.

Question 5HVAC Air Distribution

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

  1. To remove particulate contaminants from the airstream
  2. To increase the static pressure of the supply fan
  3. To humidify the air before it reaches the terminal
  4. To balance airflow among branches by adjusting resistance Correct answer.

Explanation

A volume damper throttles the branch so its resistance can be adjusted until the measured airflow matches the design value. This is the essence of air balancing. Dampers do not filter, pressurize, or humidify the air.

Question 6HVAC Air Distribution

In a variable air volume system, what does a VAV terminal box primarily modulate?

  1. The chilled water temperature leaving the plant
  2. The outdoor air intake damper position
  3. The return air filter loading
  4. The volume of supply air delivered to the zone Correct answer.

Explanation

A VAV terminal box varies the quantity of conditioned air supplied to its zone in response to zone load, usually by modulating a damper. Chilled water temperature, filter loading, and outdoor air intake are controlled elsewhere in the system.

Question 7HVAC Air Distribution

Which statement correctly describes the function of a return air plenum in a commercial building?

  1. It uses the ceiling cavity as a path for air back to the air handler Correct answer.
  2. It filters outdoor air before it enters the mixing box
  3. It supplies conditioned air directly to occupied zones
  4. It preheats supply air using exhaust heat

Explanation

A return plenum uses the space above the ceiling as a return path, so air travels back to the air handler without dedicated return ductwork. It does not deliver supply air, filter outdoor air, or preheat supply air.

Question 8HVAC Air Distribution

A slot diffuser is best suited to which application?

  1. Exhaust hoods over commercial cooking equipment
  2. Large industrial warehouses with high ceilings
  3. Linear perimeter zones with limited ceiling space Correct answer.
  4. Duct transitions inside a mechanical room

Explanation

Slot diffusers produce a long, narrow air pattern and fit neatly into linear ceiling or sill locations, making them ideal for perimeter zones with tight space. Warehouses, duct transitions, and kitchen hoods call for other devices.

Question 9HVAC Air Distribution

A duct section carries 1200 cfm at a velocity of 1000 fpm. What is the required cross-sectional area in square feet?

  1. 1.2 sq ft Correct answer.
  2. 0.83 sq ft
  3. 0.12 sq ft
  4. 12 sq ft

Explanation

Area equals flow divided by velocity, so 1200 cfm divided by 1000 fpm gives 1.2 square feet. Always keep units consistent, since cfm divided by fpm yields square feet directly.

Question 10HVAC Air Distribution

Static pressure readings along a simple supply duct run decrease in the direction of flow. What does this indicate?

  1. The fan is adding energy downstream of the reading
  2. The duct is leaking air into the occupied space
  3. The air density is increasing along the run
  4. Friction and fitting losses are consuming pressure energy Correct answer.

Explanation

As air moves downstream, friction and turbulence at fittings dissipate pressure energy, so static pressure falls along the run. A downstream fan would raise pressure, and density changes or leakage are not the primary explanation for the steady gradient.

Question 11HVAC Air Distribution

A designer wants to reduce noise at a supply diffuser without changing airflow. Which change is most directly effective?

  1. Closing the branch damper partway
  2. Selecting a larger diffuser with lower face velocity Correct answer.
  3. Reducing the duct size upstream of the diffuser
  4. Increasing the fan static pressure setpoint

Explanation

Diffuser noise is driven mainly by face velocity and pressure drop, so a larger unit at the same flow lowers velocity and NC level. Raising fan pressure, shrinking duct, or throttling the damper all increase resistance and noise.

Question 12HVAC Air Distribution

In the velocity reduction method of duct sizing, how are duct sections sized?

  1. By matching duct area to the fan curve
  2. By assigning progressively lower velocities to downstream sections Correct answer.
  3. By holding friction loss per unit length constant
  4. By holding total pressure constant at every terminal

Explanation

The velocity reduction method assigns a chosen velocity to each section, decreasing it as the duct moves toward the terminals. Equal friction is a different method, and neither constant terminal pressure nor fan-curve matching describes velocity reduction sizing.

Question 13HVAC Air Distribution

A rectangular elbow with turning vanes is compared to an elbow without vanes at the same velocity. What is the expected effect of the vanes?

  1. They increase the loss coefficient by adding surface friction
  2. They reverse the direction of the pressure gradient
  3. They have no measurable effect on pressure loss
  4. They reduce the loss coefficient by guiding the airflow around the turn Correct answer.

Explanation

Turning vanes guide the airstream through the bend and suppress separation, which lowers the elbow's loss coefficient compared with an unvaned elbow. The added surface area is small compared with the savings from reduced turbulence.

Question 14HVAC Air Distribution

A fan delivers 10,000 cfm at 2.0 in. w.c. total static pressure. If the system resistance is increased so flow drops to 8,000 cfm, what static pressure does the fan develop at the new point according to the fan laws?

  1. 3.13 in. w.c.
  2. 2.50 in. w.c.
  3. 1.60 in. w.c.
  4. 1.28 in. w.c. Correct answer.

Explanation

Fan laws state that pressure varies with the square of the flow ratio. The ratio is 0.8, squared is 0.64, and 0.64 times 2.0 in. w.c. equals 1.28 in. w.c. Linear scaling or inverse scaling gives the wrong values.

Question 15HVAC Air Distribution

What is the main consequence of excessive duct leakage in a supply system?

  1. Delivered airflow falls short of design and energy use rises Correct answer.
  2. Return air temperature drops below the setpoint
  3. Fan power decreases because less air is moved
  4. Static pressure at the terminals increases above design

Explanation

Leakage diverts conditioned air before it reaches the terminals, so the zones receive less than design flow and the fan runs longer or harder, raising energy use. Terminal pressure does not rise, and fan power does not fall as a benefit.

Question 16HVAC Air Distribution

During proportional balancing of a supply system, what is the recommended sequence?

  1. Adjust the fan speed before measuring any airflow
  2. Set all dampers fully open and leave them
  3. Balance the branch dampers first, then the main trunk Correct answer.
  4. Balance the main trunk first, then the branches

Explanation

Proportional balancing starts at the terminals and branches so that each branch gets its proportional share, then the main damper is adjusted to bring the whole system to design flow. Doing the trunk first would disturb the branch settings.

Question 17HVAC Air Distribution

A room has a sensible cooling load of 36,000 Btu/h and is supplied 1,500 cfm of air. What supply-to-room temperature difference is required?

  1. About 32°F
  2. About 44°F
  3. About 22°F Correct answer.
  4. About 12°F

Explanation

The sensible heat equation is Q equals 1.08 times cfm times temperature difference. Solving gives 36,000 divided by (1.08 times 1,500), which is about 22°F. The other values come from using the wrong multiplier or arithmetic error.

Question 18HVAC Air Distribution

Which condition most often causes a VAV system to deliver inadequate ventilation at part load?

  1. Return fan speed is set too high
  2. Diffusers are oversized for design flow
  3. Zone thermostats are set too low
  4. Supply airflow drops while outdoor air fraction stays fixed Correct answer.

Explanation

As VAV boxes throttle down, total supply airflow falls, so a fixed outdoor air damper position delivers less outdoor air than needed. Ventilation must be reset or measured to maintain the required outdoor air fraction. Thermostat setpoints, return fan speed, and oversized diffusers do not cause this effect.

Question 19HVAC Air Distribution

What is the purpose of a barometric relief damper in a building pressurized by a supply fan?

  1. To modulate supply air to each zone
  2. To measure the static pressure in the duct
  3. To admit outdoor air into the return duct
  4. To relieve excess building pressure to outdoors Correct answer.

Explanation

A barometric relief damper opens when building pressure rises above a set level and discharges air outdoors. It does not admit outdoor air, modulate zone supply, or measure duct pressure.

Question 20HVAC Air Distribution

A designer must choose between a ceiling plenum return and a ducted return in a building with strict separation requirements between tenant spaces. Which choice is appropriate and why?

  1. Ducted return, because it maintains a defined path and prevents cross-contamination between spaces Correct answer.
  2. Ducted return, because it removes the need for any return air fan
  3. Ceiling plenum return, because it is always less expensive and equally separating
  4. Ceiling plenum return, because it eliminates the need for fire dampers

Explanation

Where spaces must be kept separate, a ducted return provides a controlled path that prevents air from migrating through a shared plenum. Plenum returns are economical but do not guarantee separation, and neither option eliminates fire dampers or return fans.

Question 21HVAC Air Distribution

A long supply duct run shows a large static pressure drop across a section with several transitions and elbows. Which action best reduces the total pressure loss without changing flow?

  1. Adding a volume damper downstream of the fittings
  2. Reducing the duct size to raise velocity
  3. Increasing the fan speed to overcome the loss
  4. Replacing abrupt transitions with gradual ones and adding turning vanes Correct answer.

Explanation

Most of the loss comes from fitting turbulence, so smoothing transitions and adding turning vanes lowers the loss coefficient and the total pressure drop at the same flow. Raising fan speed or velocity adds energy and loss, and a damper only adds resistance.

Question 22HVAC Air Distribution

Two identical supply branches serve zones with the same design flow, but one branch is much longer. After proportional balancing, what is expected?

  1. Both dampers will remain fully open because flows are equal
  2. The shorter branch damper will be throttled more to equalize flow Correct answer.
  3. The longer branch damper will be throttled more to equalize flow
  4. The longer branch will deliver more air than design

Explanation

The longer branch has more resistance, so at the same total pressure it naturally carries less air. To balance equal flows, the shorter branch is throttled so its resistance matches the longer path, equalizing flow.

Question 23HVAC Air Distribution

A fan curve intersects the system curve at the design point. If a parallel fan is added, how does the combined fan curve behave at the same static pressure?

  1. The combined static pressure doubles at the same flow
  2. The combined flow is approximately the sum of the individual flows Correct answer.
  3. The combined flow is approximately half of one fan's flow
  4. The combined curve is identical to a single fan curve

Explanation

Fans in parallel add flows at a given pressure, so the combined curve shows roughly the sum of the individual flow values. Pressure does not double in parallel operation, and the curve is not identical to a single fan.

Question 24HVAC Air Distribution

In a displacement ventilation design, where should supply air typically be introduced?

  1. Through the return grille at the ceiling
  2. At low level near the floor at low velocity Correct answer.
  3. At high level near the ceiling at high velocity
  4. Directly into the occupied zone at the diffuser face

Explanation

Displacement ventilation supplies cool air at low level and low velocity so it spreads across the floor and rises as it picks up heat and contaminants. High-level high-velocity supply describes mixing ventilation, and supply through a return grille is not a displacement strategy.

Question 25HVAC Air Distribution

A duct system is being commissioned, and measured total pressure at the fan outlet is below the fan curve value at the measured flow. Which explanation is most consistent?

  1. The system resistance is lower than the fan curve assumes
  2. The fan is operating above its design speed
  3. The air density is higher than standard
  4. Duct leakage or an open test port is reducing the system pressure Correct answer.

Explanation

A lower-than-expected total pressure at the measured flow suggests the system is not building the intended resistance, often because of leakage or an open test port. Higher fan speed or higher density would raise pressure, and lower system resistance would shift the operating point rather than directly explain a shortfall against the curve.

Question 26HVAC Air Distribution

Which duct system component is intended to convert a portion of the air stream's velocity pressure into static pressure before the air enters the occupied space?

  1. A pressure-relief damper
  2. A volume damper
  3. A static pressure regain fitting Correct answer.
  4. A fire damper

Explanation

A static pressure regain fitting, such as a gradual diverging transition, slows the air so velocity pressure is recovered as static pressure. This recovery reduces fan energy needed to overcome downstream losses. Dampers and fire dampers throttle or isolate flow and do not intentionally recover pressure.

Question 27HVAC Air Distribution

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

  1. To reduce radiated noise from the fan
  2. To add adjustable resistance so branch airflow can be set to design Correct answer.
  3. To isolate the branch for maintenance
  4. To prevent reverse airflow when the fan stops

Explanation

A balancing damper is adjusted during commissioning to add resistance and trim the branch to its design airflow. It remains at a fixed position after balancing. Isolation and backdraft prevention require separate devices such as isolation dampers or backdraft dampers.

Question 28HVAC Air Distribution

In a ducted return air system serving multiple rooms, what is the main function of a return air grille sized with a large free area?

  1. To filter contaminants from the return air
  2. To balance the supply airflow to each room
  3. To reduce the pressure loss and noise at the return inlet Correct answer.
  4. To increase the return air velocity for better mixing

Explanation

A large free area lowers the face velocity at the return grille, which reduces pressure loss and generated noise. Return grilles do not balance supply air or filter the airstream unless a filter is specifically installed. Higher return velocity would increase noise and loss, not improve performance.

Question 29HVAC Air Distribution

Which device is used to maintain a constant airflow through a duct branch as the static pressure in the main varies?

  1. A barometric relief damper
  2. A constant-air-volume regulator Correct answer.
  3. A motorized fire damper
  4. A turning-vane assembly

Explanation

A constant-air-volume regulator modulates internally to hold a set airflow despite changes in upstream static pressure. It is common in branches where supply must stay fixed as the main pressure shifts. Relief dampers, fire dampers, and turning vanes serve unrelated functions.

Question 30HVAC Air Distribution

What does the term 'throw' describe for an air diffuser?

  1. The angle at which the diffuser vanes are set
  2. The pressure drop measured across the diffuser face
  3. The distance from the diffuser face at which the airstream velocity decays to a specified terminal value Correct answer.
  4. The total airflow discharged by the diffuser at design conditions

Explanation

Throw is the horizontal or vertical distance from the diffuser to the point where the jet velocity falls to a stated terminal velocity, often 50 or 100 fpm. It characterizes how far the supply jet carries before mixing with room air. Airflow, pressure drop, and vane angle are separate diffuser characteristics.

Question 31HVAC Air Distribution

Which duct material is most commonly specified for a general commercial low-pressure supply system where corrosion resistance and moderate cost are both required?

  1. Stainless steel
  2. Cast iron
  3. Polyvinyl chloride
  4. Galvanized steel Correct answer.

Explanation

Galvanized steel is the standard choice for low-pressure commercial supply ductwork because it resists corrosion at moderate cost and is easily fabricated. Cast iron is used in drainage, not air distribution. Stainless steel and PVC are reserved for special corrosive or sanitary applications.

Question 32HVAC Air Distribution

What is the purpose of sealing duct joints and seams in a supply air system?

  1. To lower the sound transmission through the duct wall
  2. To increase the structural strength of the duct so hangers can be spaced farther apart
  3. To allow the duct to expand and contract with temperature changes
  4. To reduce air leakage and preserve design airflow at the terminals Correct answer.

Explanation

Sealing joints and seams limits leakage so that the air delivered by the fan reaches the terminals as designed. Leakage wastes fan energy and can cause unbalanced zones. Strength, sound transmission, and thermal movement are addressed by other means such as reinforcement, insulation, and expansion joints.

Question 33HVAC Air Distribution

Which fan arrangement draws air axially through the impeller and then discharges it radially?

  1. Centrifugal fan Correct answer.
  2. Propeller fan
  3. Tube axial fan
  4. Axial fan

Explanation

A centrifugal fan takes air in along the axis and turns it to discharge radially through the casing, which builds pressure efficiently. Propeller, axial, and tube axial fans move air along the axis of rotation. This distinction matters when selecting fans for higher-pressure duct systems.

Question 34HVAC Air Distribution

A duct section carries 800 cfm at a velocity of 1200 fpm. What is the required cross-sectional area in square feet?

  1. 0.96 sq ft
  2. 0.67 sq ft Correct answer.
  3. 2.40 sq ft
  4. 1.50 sq ft

Explanation

Area equals airflow divided by velocity, so 800 cfm divided by 1200 fpm gives 0.667 sq ft. This is the continuity relation A = Q/V used throughout duct sizing. A designer would then select a duct size with at least this area.

Question 35HVAC Air Distribution

A supply duct transitions from a smaller to a larger cross section with a gradual taper. What happens to the velocity pressure and static pressure across this transition?

  1. Velocity pressure decreases and static pressure increases Correct answer.
  2. Both velocity pressure and static pressure decrease
  3. Velocity pressure increases and static pressure decreases
  4. Both velocity pressure and static pressure remain unchanged

Explanation

Because the same airflow passes through a larger area, velocity falls, so velocity pressure decreases. In a gradual expansion much of that energy is recovered as static pressure, which is why the transition is tapered rather than abrupt. An abrupt expansion would recover less and lose more.

Question 36HVAC Air Distribution

A designer must select a diffuser for a room with a 9 ft ceiling and a cooling load that requires a high supply-to-room temperature difference. Which selection concern is most important?

  1. Choosing the smallest diffuser that meets the airflow
  2. Ensuring the supply jet does not drop into the occupied zone before mixing Correct answer.
  3. Maximizing the diffuser face velocity to increase throw
  4. Reducing the diffuser pressure drop below 0.05 in. w.c.

Explanation

A cold supply jet with a high temperature difference is denser than room air and tends to drop. The designer must verify that the jet mixes before entering the occupied zone, often by selecting a diffuser with adequate throw and a suitable pattern. High face velocity and very low pressure drop are secondary to avoiding draft.

Question 37HVAC Air Distribution

In a low-pressure duct system, which fitting generally causes the largest local pressure loss for the same nominal size?

  1. A 90-degree elbow with no turning vanes Correct answer.
  2. A gradual contraction
  3. A 45-degree elbow
  4. A straight duct section of 10 ft

Explanation

A sharp 90-degree elbow without vanes separates flow on the inner radius and produces a high loss coefficient. A 45-degree elbow turns the air less and loses less, and a gradual contraction recovers some pressure. A straight duct loses only friction over its length.

Question 38HVAC Air Distribution

A VAV terminal box with a reheat coil is serving an interior zone. What is the main reason the box is limited to a minimum airflow setpoint?

  1. To maintain ventilation and avoid stratification or coil freeze at low flow Correct answer.
  2. To ensure the box always delivers design airflow
  3. To keep the fan operating at its maximum speed
  4. To prevent the reheat coil from ever operating

Explanation

Minimum airflow ensures adequate outdoor air delivery and stable operation of the terminal and reheat coil. Without a minimum, ventilation can fall short and the coil may operate with too little airflow. The minimum does not force design flow or prevent reheat from operating.

Question 39HVAC Air Distribution

A room requires 400 cfm of outdoor air. The air handling unit supplies 2000 cfm with a return air fraction of 0.75. What outdoor air fraction must the system provide to meet the requirement?

  1. 0.20 Correct answer.
  2. 0.40
  3. 0.10
  4. 0.25

Explanation

The outdoor air fraction equals the required outdoor airflow divided by total supply airflow, so 400 divided by 2000 equals 0.20. With a return fraction of 0.75, the remaining 0.25 of return air would be exhausted and 0.20 of the supply would be outdoor air if the economizer and mixing are set accordingly. The calculation confirms the minimum outdoor air damper position.

Question 40HVAC Air Distribution

During a duct pressure test, a section is pressurized to 1.0 in. w.c. and the measured leakage is higher than allowed. Which action most directly reduces leakage?

  1. Add more hangers to support the duct
  2. Increase the fan speed to raise duct pressure
  3. Install a larger diffuser at the end of the run
  4. Seal the transverse joints and longitudinal seams Correct answer.

Explanation

Leakage occurs mainly at joints and seams, so sealing them directly reduces leakage. Raising pressure would increase leakage rather than reduce it. Hangers and diffusers address support and air distribution, not duct tightness.

Question 41HVAC Air Distribution

A supply fan delivers air to a duct system with a total pressure loss of 1.5 in. w.c. at 8000 cfm. If the airflow is reduced to 6000 cfm, what is the approximate new system total pressure loss assuming the same system curve?

  1. 1.13 in. w.c.
  2. 2.00 in. w.c.
  3. 0.84 in. w.c. Correct answer.
  4. 1.50 in. w.c.

Explanation

System pressure varies with the square of airflow, so the new loss is 1.5 times (6000/8000) squared. That equals 1.5 times 0.5625, or about 0.84 in. w.c. This fan-law relationship underlies variable-speed control and balancing calculations.

Question 42HVAC Air Distribution

A ceiling plenum is used as a return air path above a suspended ceiling. Which condition most directly limits its use?

  1. The plenum must be pressurized above the occupied space
  2. The plenum must be separated from spaces that require isolation, such as kitchens or laboratories Correct answer.
  3. The plenum must contain supply duct insulation
  4. The plenum must have a fire damper at every diffuser

Explanation

A ceiling plenum can carry return air only where the space above the ceiling is acceptable as an air path. It must be separated from areas with contaminants, odors, or code restrictions, and the ceiling must be tight enough to serve as a return boundary. Pressurization, insulation, and dampers at diffusers are not the governing limitation.

Question 43HVAC Air Distribution

A designer is comparing two duct layouts for the same zone. Layout A uses several short radius elbows and a long run; Layout B uses fewer fittings and a shorter run. Which comparison best supports the lower operating cost?

  1. Layout A, because more elbows improve air mixing
  2. Layout A, because a longer run increases static regain
  3. Layout B, because fewer fittings and a shorter run reduce total pressure loss Correct answer.
  4. Layout B, because shorter runs always require larger ducts

Explanation

Operating cost is driven by fan pressure, and both fittings and duct length add to total pressure loss. Fewer fittings and a shorter run generally lower the required fan pressure and energy use. Mixing and static regain do not offset the added losses of extra elbows and length.

Question 44HVAC Air Distribution

A room has a sensible cooling load of 24,000 Btu/h and is supplied 1000 cfm of air at 55 F. What supply-to-room temperature difference is required, using the sensible heat equation with 1.08 as the constant?

  1. About 30 F
  2. About 18 F
  3. About 22 F Correct answer.
  4. About 12 F

Explanation

The sensible heat equation is Q = 1.08 times cfm times temperature difference. Rearranging gives 24,000 divided by (1.08 times 1000), or about 22 F. This means the room would be near 77 F with 55 F supply air, which a designer would check against comfort limits.

Question 45HVAC Air Distribution

In a constant-volume system, a zone becomes too warm while the supply fan and duct static pressure remain at design. Which fault most directly explains the condition?

  1. The duct insulation is thicker than specified
  2. The return air grille is oversized for the return airflow
  3. The zone thermostat is calling for cooling but the cooling coil valve is stuck closed Correct answer.
  4. The supply diffuser has a higher throw than specified

Explanation

In a constant-volume system the airflow is fixed, so an unsatisfied cooling call points to a loss of cooling capacity or control. A stuck-closed coil valve would leave the supply air warm and the zone overheated. Return grille size, diffuser throw, and insulation thickness do not remove cooling capacity.

Question 46HVAC Air Distribution

A duct system serves two zones from a common main. Zone 1 has a short branch and Zone 2 has a long branch with several elbows. After proportional balancing, what is the expected result?

  1. The fan speed is reduced until both zones reach design flow
  2. The balancing damper in Zone 1 is closed more than the damper in Zone 2 Correct answer.
  3. Both dampers are fully open because the branches are identical in design flow
  4. The balancing damper in Zone 2 is closed more than the damper in Zone 1

Explanation

The short branch has less resistance and would receive excess air, so its damper must be closed more to match design flow. The long branch with more fittings has higher resistance and needs less throttling. Proportional balancing adjusts the lower-resistance branches first to bring all zones to the same proportion of design flow.

Question 47HVAC Air Distribution

A supply fan curve and system curve intersect at the design point. A second identical fan is added in parallel. How does the combined fan curve compare with the single-fan curve at the same static pressure?

  1. The combined curve delivers the same airflow at twice the pressure
  2. The combined curve delivers about twice the airflow at the same pressure Correct answer.
  3. The combined curve shifts downward and to the left
  4. The combined curve delivers half the airflow at the same pressure

Explanation

Fans in parallel add their airflows at a common pressure, so at any given static pressure the combined curve is roughly double the single-fan flow. The system curve then determines the actual operating point, which will be at higher flow than with one fan. Adding pressure at the same flow describes series operation, not parallel.

Question 48HVAC Air Distribution

A duct run includes a sudden expansion where the area doubles. Compared with a gradual expansion of the same area change, what is the expected effect on pressure loss?

  1. The sudden expansion recovers more static pressure because of the sharp edge
  2. The sudden expansion has a higher loss because of turbulence and poor pressure recovery Correct answer.
  3. The sudden expansion has a lower loss because it is shorter
  4. The two expansions have equal loss because the area ratio is the same

Explanation

A sudden expansion creates a separated, turbulent zone that dissipates energy and recovers little static pressure. A gradual expansion keeps the flow attached and recovers more velocity pressure as static pressure. Area ratio alone does not determine loss; the fitting geometry matters.

Question 49HVAC Air Distribution

A commissioning agent measures total pressure at the fan outlet lower than the value shown on the fan curve at the measured airflow. Which explanation is most consistent with this result?

  1. The fan curve already includes the system effect, so the readings should match exactly
  2. The fan is operating at a higher speed than the curve represents
  3. The fan curve was developed under ideal inlet and outlet conditions, and the installed duct connections add system effect losses Correct answer.
  4. The airflow measurement is incorrect because total pressure cannot be measured at a fan outlet

Explanation

Published fan curves assume uniform, unobstructed inlet and outlet conditions. Poorly designed connections, such as a tight elbow at the fan discharge, create a system effect that reduces fan performance relative to the curve. The measured total pressure therefore falls below the curve value at the same airflow. Fan speed and measurement validity are separate checks.

Question 50HVAC Air Distribution

A high-rise building uses a supply fan to pressurize the occupied space. Which control sequence best maintains acceptable building pressure as outdoor conditions and exhaust vary?

  1. Open the relief damper fully whenever the supply fan runs
  2. Run the supply fan at constant speed and disable all exhaust fans
  3. Modulate a relief damper or return fan in response to a measured building pressure setpoint Correct answer.
  4. Hold the relief damper fully closed and rely on door undercuts for relief

Explanation

Building pressure control requires a feedback loop that modulates relief capacity, either a relief damper or a return fan, in response to measured space pressure. Holding relief closed or fully open ignores the varying balance between supply and exhaust. Disabling exhaust would worsen over-pressurization and harm ventilation.