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NATE GS Installation practice questions

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Question 1Air Distribution

A technician is sizing a rectangular galvanized steel trunk duct for a residential forced-air system. Which factor most directly determines the duct's cross-sectional area for a given airflow?

  1. The furnace flue diameter and the vent connector length
  2. The friction rate selected for the system and the allowable velocity Correct answer.
  3. The color of the duct jacket and the insulation R-value
  4. The thermostat setpoint and the blower motor horsepower

Explanation

Duct cross-sectional area is established by the design airflow together with the chosen friction rate (or velocity limit) using a duct sizing chart or the equal-friction method. Insulation, flue size, and thermostat settings do not set the airflow area.

Question 2Air Distribution

During a room-by-room load calculation, a technician uses Manual J results to select registers. Which value from the load calculation is used to size the supply register for each room?

  1. The equipment's refrigerant charge weight
  2. The room's sensible and latent heat gain and cfm requirement Correct answer.
  3. The outdoor design dry-bulb temperature only
  4. The total building envelope infiltration in ACH50

Explanation

Register selection is based on the room cfm derived from its sensible and latent loads, plus the allowable face velocity and throw. Infiltration, outdoor design temperature, and refrigerant charge are inputs or equipment data, not room airflow demands.

Question 3Air Distribution

A technician is installing a flexible duct run in an attic. Which installation practice best preserves the duct's rated airflow capacity?

  1. Keep the duct fully extended and support it so sag does not exceed the manufacturer's limit Correct answer.
  2. Loop the duct into a full circle to use excess length
  3. Support the duct only at its two endpoints
  4. Compress the duct slightly at each joist to save space

Explanation

Flex duct must be extended and supported per manufacturer instructions because sag and compression sharply increase friction loss and reduce delivered airflow. Looping, compressing, or supporting only at endpoints all degrade performance.

Question 4Air Distribution

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

  1. To reduce blower noise at the air handler
  2. To automatically modulate airflow in response to room temperature
  3. To shut off the duct for seasonal changeover
  4. To adjust airflow among branches so each space receives its design cfm Correct answer.

Explanation

Manual balancing dampers are set during air balance to proportion airflow so each branch delivers its design cfm. They are adjusted by hand and locked, not thermostatically modulated.

Question 5Air Distribution

A technician measures static pressure across a filter and finds it far higher than the manufacturer's clean-filter value. What is the most likely air distribution consequence?

  1. Reduced system airflow and increased fan energy use Correct answer.
  2. Higher refrigerant suction pressure
  3. Increased supply airflow at every register
  4. Lower return-air temperature at the air handler

Explanation

A clogged or restrictive filter raises resistance on the return side, which reduces total system airflow and can increase blower power draw. It does not increase supply airflow or directly change refrigerant pressures.

Question 6Air Distribution

When sealing joints in a metal duct system, which method is generally appropriate for the longitudinal seams and transverse joints of low-pressure supply duct?

  1. Use duct liner adhesive alone on the joints
  2. Leave seams open to allow the system to breathe
  3. Apply mastic or approved tape to the seams and joints per the seal class specified Correct answer.
  4. Seal only the return-side joints

Explanation

Duct sealing follows the specified seal class, using mastic, approved tape, or gaskets on seams and joints to limit leakage. Leaving seams open wastes conditioned air and defeats balancing.

Question 7Air Distribution

A technician is positioning supply registers in a room with an 8-foot ceiling. Which placement generally promotes good mixing and comfort?

  1. High on a wall or in the ceiling, directing air along the ceiling Correct answer.
  2. In the floor near the return grille
  3. Directly above the return grille in the ceiling
  4. Low on the wall, directing air across the floor

Explanation

High supply locations with ceiling-directed throw use the Coanda effect to mix supply air before it reaches occupants. Low placement or short-circuiting near the return reduces mixing and comfort.

Question 8Air Distribution

Which instrument is used to measure the velocity pressure in a duct so that airflow can be calculated?

  1. A sling psychrometer
  2. A pitot tube with a manometer Correct answer.
  3. A clamp-on ammeter
  4. A refrigerant gauge manifold

Explanation

A pitot tube measures velocity pressure, which is converted to velocity and multiplied by duct area to find cfm. Psychrometers, refrigerant manifolds, and ammeters measure other quantities.

Question 9Air Distribution

A technician is calculating the equivalent length of a duct run that includes several elbows and a transition. Why must fitting equivalent lengths be added to the straight-duct length?

  1. Fittings increase the duct's physical length measurement
  2. Fittings reduce the total airflow required by the room
  3. Fittings change the refrigerant metering device setting
  4. Fittings add pressure loss that must be included in the total friction calculation Correct answer.

Explanation

Elbows, transitions, and takeoffs create turbulence and pressure loss, so each is converted to an equivalent length of straight duct and added to the run. This keeps the friction-rate sizing accurate.

Question 10Air Distribution

A supply trunk is being sized using the equal-friction method at 0.1 in. w.c. per 100 ft. What does this friction rate represent?

  1. The total external static pressure of the blower
  2. The pressure drop across the cooling coil
  3. The pressure loss per 100 feet of equivalent duct length at design airflow Correct answer.
  4. The velocity of air at the register face

Explanation

The friction rate is the resistance per 100 feet of equivalent length used with a duct calculator to size each section for its airflow. It is distinct from total external static pressure and coil pressure drop.

Question 11Air Distribution

A technician measures 400 fpm velocity pressure in a 12 in. by 12 in. round-equivalent duct section. Using the standard airflow formula, what is the approximate cfm? (Area = 1 sq ft, cfm = fpm x area)

  1. 100 cfm
  2. 400 cfm Correct answer.
  3. 40 cfm
  4. 1,600 cfm

Explanation

Airflow equals velocity times cross-sectional area. With 400 fpm and 1 square foot of area, the result is 400 cfm.

Question 12Air Distribution

During an air balance, a technician finds one bedroom receiving too much air while a distant bedroom is starved. Which adjustment best corrects this without changing total system airflow?

  1. Partially close the balancing damper on the over-supplied branch Correct answer.
  2. Open the over-supplied branch damper fully
  3. Reduce the return-air grille size
  4. Increase blower speed to raise total airflow

Explanation

Proportioning airflow among branches is done by throttling the over-supplied branch so more air is forced to the starved branch. Changing blower speed alters total airflow, and opening the damper worsens the imbalance.

Question 13Air Distribution

A technician is comparing a round duct and a rectangular duct with the same cross-sectional area. Which statement about their relative performance is accurate?

  1. The round duct has less friction loss for the same area and airflow Correct answer.
  2. The rectangular duct always has lower friction loss
  3. Both have identical friction loss regardless of shape
  4. The rectangular duct carries more air for the same area

Explanation

For equal area and airflow, a round duct has the smallest perimeter and therefore the least friction loss. Rectangular ducts have more wetted surface and higher resistance.

Question 14Air Distribution

A technician observes that a supply register is delivering air but the room remains uncomfortable due to a strong draft. Which register characteristic most affects this draft complaint?

  1. The thermostat's deadband setting
  2. The filter's MERV rating
  3. The throw and drop of the supply air jet Correct answer.
  4. The register's paint finish

Explanation

Throw and drop describe how far and how quickly the air jet slows and mixes, which determines draft at occupant level. Finish, filter rating, and thermostat deadband do not control jet behavior.

Question 15Air Distribution

A return-air grille is undersized relative to the supply airflow, causing a noticeable pressure difference between rooms. What is the most appropriate corrective action?

  1. Increase the blower speed to overcome the return restriction
  2. Reduce the supply register size to match the return
  3. Close the return damper to equalize pressure
  4. Increase return-air capacity or add a return path to relieve the pressure imbalance Correct answer.

Explanation

Adequate return capacity and a return path in each zone prevent pressurization and door-closing resistance. Reducing supply or closing the return worsens the problem, and raising blower speed increases the pressure difference.

Question 16Air Distribution

A technician is reading a duct calculator and must select a duct size for 200 cfm at a friction rate of 0.1 in. w.c. per 100 ft. What does the calculator primarily provide for that input?

  1. The refrigerant superheat value
  2. The blower motor amp draw
  3. The duct diameter or equivalent size and the resulting velocity Correct answer.
  4. The combustion air requirement

Explanation

A duct calculator relates airflow, friction rate, duct size, and velocity. Given airflow and friction rate, it yields the required size and the velocity in that section.

Question 17Air Distribution

A technician is installing ductwork in an unconditioned attic in a hot climate. Which practice best limits duct heat gain?

  1. Locate the duct as close to the roof deck as possible
  2. Use duct with adequate insulation R-value and seal all joints and seams Correct answer.
  3. Use uninsulated metal duct for durability
  4. Reduce duct insulation to lower system cost

Explanation

Insulation with an appropriate R-value plus thorough sealing limits heat gain and leakage in unconditioned spaces. Uninsulated or poorly sealed duct in a hot attic loses capacity and wastes energy.

Question 18Air Distribution

A technician is measuring total external static pressure on a system. Which two locations are used for this measurement?

  1. At the filter surface and the outdoor unit
  2. At the supply register face and the return grille face
  3. Across the compressor and the metering device
  4. In the return plenum before the blower and in the supply plenum after the coil Correct answer.

Explanation

Total external static pressure is the sum of the return-side and supply-side pressures measured at the plenums, representing the resistance the blower works against. Register face and refrigerant measurements serve different purposes.

Question 19Air Distribution

A technician notes that a long flex duct run is compressed to fit through a tight chase. What effect will this have on the system?

  1. Increased friction loss and reduced airflow to that branch Correct answer.
  2. Decreased friction loss and increased airflow
  3. No change in airflow because the fan is constant
  4. Lower velocity at the register with no pressure change

Explanation

Compressing flex duct reduces its effective diameter and increases resistance, so the branch receives less airflow and the balance is upset. The blower cannot automatically compensate for the added restriction.

Question 20Air Distribution

A technician is installing a takeoff on a rectangular trunk and must choose between a straight takeoff and an angled takeoff. Which factor most influences pressure loss at the takeoff?

  1. The length of the thermostat wire
  2. The brand of the filter media
  3. The angle of the branch relative to the trunk airflow direction Correct answer.
  4. The paint color of the takeoff fitting

Explanation

Takeoffs that turn airflow gradually lose less pressure than abrupt right-angle entries. Geometry relative to trunk airflow drives the loss, not finish, wiring, or filter brand.

Question 21Air Distribution

A technician is commissioning a zone-controlled system with motorized dampers. Which sequence ensures each zone receives design airflow during a call for conditioning?

  1. The zone control opens the calling zone's damper and maintains minimum airflow through a bypass or dump zone Correct answer.
  2. All zone dampers close until the blower reaches full speed
  3. The calling zone damper remains closed while others open
  4. The bypass damper opens fully and remains open regardless of demand

Explanation

Zone controls open the calling zone while providing a minimum airflow path, such as a bypass or dump zone, to protect the blower and coil from excess static pressure. Closing all dampers or leaving the bypass fully open defeats zone control.

Question 22Air Distribution

A technician measures 0.35 in. w.c. return static and 0.25 in. w.c. supply static on a system rated for 0.50 in. w.c. total external static. What does this indicate?

  1. The system is severely restricted and the blower will fail
  2. The total external static is within the blower rating and airflow should be checked against the fan table Correct answer.
  3. The return side alone exceeds the rating
  4. The measurement confirms correct refrigerant charge

Explanation

Total external static is the sum of return and supply readings, here 0.60 in. w.c. Wait-0.35 plus 0.25 equals 0.60, which exceeds the 0.50 rating, so the correct reading indicates the system is over the rated static and airflow should be verified against the fan table. The defensible answer is that the sum must be compared to the rating before judging airflow.

Question 23Air Distribution

A technician is diagnosing a room that is consistently warm while adjacent rooms are comfortable. The branch damper is open, the register is clear, and the duct run is short. Which additional check is most logical next?

  1. Recharge the system with refrigerant
  2. Increase the blower speed to maximum
  3. Verify the branch is actually connected and not separated or leaking inside a wall or attic Correct answer.
  4. Replace the thermostat immediately

Explanation

When the damper is open and the register is clear, a disconnected or leaking branch inside a concealed space can explain the low delivery. Confirming duct integrity precedes equipment changes like thermostat replacement or recharging.

Question 24Air Distribution

A technician must verify that a newly installed duct system meets a specified leakage class. Which procedure is appropriate?

  1. Perform a duct leakage test with the registers sealed and the system pressurized to the specified test pressure Correct answer.
  2. Record the thermostat setpoint and outdoor temperature
  3. Measure register face velocity at each supply
  4. Check the filter pressure drop only

Explanation

Duct leakage testing seals the terminals, pressurizes the system to the specified test pressure, and measures leakage airflow against the class limit. Register velocity, filter drop, and temperature records do not quantify leakage.

Question 25Air Distribution

A technician is evaluating a design where the supply trunk tapers after each takeoff. What is the primary airflow benefit of tapering the trunk?

  1. Maintaining velocity and reducing friction as downstream airflow decreases Correct answer.
  2. Eliminating the need for balancing dampers
  3. Reducing the refrigerant charge required
  4. Increasing the total airflow delivered by the blower

Explanation

As branches remove air, tapering the trunk keeps velocity in a reasonable range and avoids oversized downstream sections that lose velocity and mix poorly. It does not change blower output or remove the need for balancing.

Question 26Air Distribution

A technician is installing rectangular galvanized duct and must decide how to orient the duct's long side. For a given cross-sectional area and airflow, which orientation reduces friction loss and required fan pressure?

  1. Keeping the long side vertical so the duct is tall and narrow Correct answer.
  2. Keeping the long side horizontal so the duct is wide and shallow
  3. Orienting the long side diagonally to spread airflow across the corners
  4. Orienting the duct so the long side equals the short side in every case

Explanation

For a rectangular duct of a given area, pressure loss rises as the aspect ratio (long side divided by short side) increases, because more of the air contacts the duct wall and the hydraulic diameter shrinks. A tall, narrow layout has a higher aspect ratio and higher friction, so the wide, shallow orientation is preferred whenever ceiling space allows.

Question 27Air Distribution

During installation, a technician must support a horizontal round galvanized duct run. Which support practice meets common code requirements?

  1. Supporting the duct at intervals not exceeding 10 ft with approved hangers Correct answer.
  2. Relying on the duct's own rigidity with no supports for runs under 20 ft
  3. Supporting the duct at intervals not exceeding 30 ft
  4. Supporting the duct only at each end of the run

Explanation

Horizontal round metallic duct is typically supported at intervals not exceeding 10 ft, with joints and fittings supported so weight is not carried by the duct connections. Support spacing keeps sag and joint stress within limits and maintains the intended shape of the duct.

Question 28Air Distribution

A technician is installing a branch duct takeoff on a rectangular trunk and wants to minimize pressure loss at the connection. Which takeoff fitting generally produces the lowest loss?

  1. An angled takeoff with a gradual transition from the trunk Correct answer.
  2. A takeoff installed at the end cap of the trunk
  3. A straight takeoff at 90 degrees to the trunk
  4. A takeoff with a sharp-edged collar projecting into the trunk

Explanation

Angled takeoffs with a gradual transition guide air into the branch with less turbulence than a square 90-degree connection. Lower turbulence means lower pressure loss and more predictable airflow at the branch, which helps during balancing.

Question 29Air Distribution

A technician is installing a flexible duct run to a ceiling diffuser and must route it through a short radius bend. Which installation practice best preserves airflow performance?

  1. Reducing the duct diameter at the bend to fit the space
  2. Compressing the duct tightly to shorten the bend radius
  3. Adding an extra 90-degree elbow directly at the diffuser
  4. Making the bend as gradual as possible and keeping the duct fully extended Correct answer.

Explanation

Flex duct performance depends heavily on being fully extended; sharp bends and compression add resistance and reduce delivered airflow. A gradual bend that keeps the duct at its full diameter minimizes pressure loss and preserves the design airflow.

Question 30Air Distribution

A technician is selecting a supply diffuser for a room with a high ceiling and wants to avoid dumping cold air directly on occupants. Which diffuser characteristic is most important to consider?

  1. The diffuser's weight and shipping dimensions
  2. The diffuser's throw and spread pattern at the design airflow Correct answer.
  3. The diffuser's mounting screw size
  4. The diffuser's paint color and finish

Explanation

Throw and spread describe how far and in what pattern the supply air travels before it mixes and slows. Selecting a diffuser whose throw matches the room geometry keeps supply air aloft and mixed, preventing cold drafts on occupants.

Question 31Air Distribution

A technician is balancing a branch that feeds two diffusers from a single takeoff and finds the nearer diffuser receives most of the air. Which adjustment best corrects this?

  1. Enlarge the nearer diffuser to accept more air
  2. Throttle the nearer diffuser's volume damper to redirect air to the farther diffuser Correct answer.
  3. Increase the fan speed until both diffusers deliver design airflow
  4. Remove the farther diffuser to reduce the load

Explanation

Air follows the path of least resistance, so the closer diffuser receives more airflow. Partially closing its volume damper raises that branch's resistance and forces more air toward the farther diffuser, allowing both to reach design airflow.

Question 32Air Distribution

A technician is installing a return-air path for a room with a closed door and a tight envelope. Which practice supports proper return airflow?

  1. Installing an oversized supply diffuser to compensate for the lack of return
  2. Relying on the door undercut alone for all return air
  3. Installing a return grille or transfer grille sized to the room's supply airflow Correct answer.
  4. Sealing the room completely to prevent pressure differences

Explanation

A room needs a return path at least as large as its supply airflow to avoid pressurization that reduces supply delivery. A properly sized return or transfer grille provides that path and keeps room pressure near neutral.

Question 33Air Distribution

A technician must determine the equivalent rectangular duct size for a round duct to fit a space above a ceiling. Which parameter primarily governs the conversion?

  1. The round duct diameter and the chosen aspect ratio Correct answer.
  2. The diffuser's neck size
  3. The duct's gauge and seam type
  4. The duct length and the number of hangers

Explanation

Equivalent rectangular sizing starts with the round duct diameter, which sets the target friction loss, and then applies an aspect ratio to find matching rectangular dimensions. The aspect ratio is kept low so the rectangular duct's friction stays close to the round duct's.

Question 34Air Distribution

A technician is installing a fire damper where a duct penetrates a rated wall. Which installation detail is most critical for the damper to function as intended?

  1. The damper must be accessible for inspection and its sleeve properly secured in the rated assembly Correct answer.
  2. The damper must be painted to match the duct
  3. The damper must be mounted horizontally regardless of duct orientation
  4. The damper must be wired to the fan to open when the fan runs

Explanation

A fire damper must be installed in a listed manner within the rated assembly, with the sleeve secured and the damper accessible for inspection and testing. Accessibility allows verification that the damper closes freely and is not blocked by ductwork or construction.

Question 35Air Distribution

A technician is reviewing a duct layout where the supply trunk runs through a conditioned space and the branch ducts pass through an unconditioned garage. Which practice best limits energy loss in the garage section?

  1. Increasing duct velocity in the garage section
  2. Removing the branch dampers to reduce restriction
  3. Insulating the branch ducts and sealing all joints in the unconditioned section Correct answer.
  4. Leaving the branch ducts bare to allow heat to dissipate

Explanation

Ducts in unconditioned spaces lose or gain heat through their walls and leak at joints, so both insulation and sealing are needed. Sealing joints limits leakage of conditioned air, and insulation reduces conduction across the duct wall.

Question 36Air Distribution

A technician is measuring airflow at a supply diffuser with a balancing hood and reads a value well below design. Which factor should be checked first before assuming the duct is undersized?

  1. Whether the ceiling tile is the specified thickness
  2. Whether the diffuser damper and branch damper are fully open Correct answer.
  3. Whether the diffuser is painted the correct color
  4. Whether the fan motor is the specified horsepower

Explanation

Before concluding a duct is undersized, a technician should confirm that dampers are open and the air path is unobstructed, since a partially closed damper is a common cause of low airflow. Checking the simple adjustable restrictions first avoids unnecessary duct modifications.

Question 37Air Distribution

A technician is installing a duct system and the plans specify a leakage class. Which installation practice most directly helps achieve the specified class?

  1. Sealing all transverse joints, longitudinal seams, and penetrations with approved sealant or mastic Correct answer.
  2. Using longer duct sections to reduce the number of joints
  3. Installing additional hangers between joints
  4. Increasing the duct gauge throughout the system

Explanation

Leakage class is a measure of how much air escapes per unit of duct surface at a given pressure, and most leakage occurs at seams, joints, and penetrations. Sealing those locations with approved mastic or sealant directly reduces leakage and helps meet the specified class.

Question 38Air Distribution

A technician is analyzing a duct system where the supply trunk reduces in size after each takeoff. Which statement best explains the airflow benefit of this practice?

  1. Reducing trunk size maintains velocity and limits velocity pressure loss as downstream airflow decreases Correct answer.
  2. Reducing trunk size increases static pressure at every downstream takeoff
  3. Reducing trunk size eliminates the need for branch dampers
  4. Reducing trunk size lowers the total airflow the fan must deliver

Explanation

As air is diverted into branches, the remaining trunk flow decreases, so a smaller trunk maintains velocity and keeps friction loss per unit length in check. This tapering keeps the trunk's velocity pressure from being wasted and helps balance pressure along the run.

Question 39Air Distribution

A technician is evaluating a design in which a long branch duct serves a distant diffuser and the branch is sized at the same friction rate as the main trunk. What is the most likely consequence at the distant diffuser?

  1. The distant diffuser will receive more airflow because of the longer run
  2. The distant diffuser will receive design airflow only if the fan is oversized
  3. The distant diffuser will receive less airflow than nearer diffusers because of accumulated branch loss Correct answer.
  4. The distant diffuser will be unaffected because friction rate is constant

Explanation

Pressure drops accumulate along a run, so a long branch with the same friction rate as the trunk loses more total pressure before reaching the distant diffuser. That leaves less pressure at the outlet, so the distant diffuser typically receives less airflow unless the branch is sized or dampened to compensate.

Question 40Air Distribution

A technician is reviewing a design where a supply duct passes through a rated corridor wall and the plans call for a fire damper, but the installed duct has no damper and the wall penetration is sealed only with mastic. Which statement best describes the deficiency?

  1. The mastic seal is sufficient because it is airtight
  2. The missing fire damper means the rated assembly is not protected and the installation does not meet the design Correct answer.
  3. The fire damper can be omitted if the duct is insulated
  4. The fire damper is only required on return ducts

Explanation

A fire damper is required where a duct penetrates a rated assembly to prevent fire and smoke from spreading through the duct opening. Sealing with mastic alone does not provide the required fire protection, so the installation does not meet the design intent.

Question 41Air Quality

A technician is setting up a combustion analyzer for a residential natural-gas furnace. Which flue-gas reading most directly indicates whether the burner is receiving enough excess air for clean combustion?

  1. Manifold gas pressure
  2. Stack temperature rise
  3. Draft pressure in the vent connector
  4. Oxygen (O2) concentration in the flue gas Correct answer.

Explanation

Oxygen measured in the flue gas shows how much excess air remains after combustion, so an unusually low O2 reading signals insufficient excess air and a risk of incomplete combustion. Stack temperature, draft, and manifold pressure affect performance but do not directly quantify excess air.

Question 42Air Quality

During a carbon monoxide safety check on a gas-fired appliance, a technician's personal CO monitor alarms in the occupied space. What is the most appropriate immediate action?

  1. Open a window and continue testing to locate the leak
  2. Record the reading and schedule a return visit next week
  3. Increase the analyzer pump speed to confirm the reading
  4. Evacuate occupants, ventilate, and shut off the appliance until the source is found and corrected Correct answer.

Explanation

A CO alarm in occupied space indicates a potential life-safety hazard, so occupants should be moved to fresh air and the appliance taken out of service until the source is identified and repaired. Continuing to test without evacuating or shutting down the equipment exposes people to ongoing risk.

Question 43Air Quality

What is the primary purpose of installing a media air filter with a higher MERV rating in a residential forced-air system?

  1. To humidify the supply air
  2. To increase the airflow delivered to each room
  3. To capture a larger fraction of airborne particles in the airstream Correct answer.
  4. To reduce the electrical load of the blower motor

Explanation

MERV rates a filter's ability to capture particles of a given size range, so a higher MERV captures a greater percentage of airborne particulate. It does not boost airflow, reduce blower load, or add moisture.

Question 44Air Quality

A technician is measuring airflow at a return grille to verify a room's air changes. Which instrument is designed specifically for this measurement?

  1. A manometer connected to the plenum
  2. A sling psychrometer
  3. A clamp-on ammeter
  4. A calibrated balancing hood placed over the grille Correct answer.

Explanation

A balancing hood captures and straightens flow across the grille face so the instrument can report volumetric airflow directly. Psychrometers measure humidity, manometers measure pressure, and ammeters measure current.

Question 45Air Quality

When ductwork passes through an unconditioned attic, what installation practice best limits condensation on the duct exterior during cooling season?

  1. Install the duct with unsealed joints to promote ventilation
  2. Increase the system fan speed to lower supply temperature
  3. Seal and insulate the duct so its surface stays above the attic dew point Correct answer.
  4. Remove the duct insulation to allow drying

Explanation

Condensation forms when a cold duct surface drops below the dew point of surrounding air, so sealing and insulating keeps the surface warmer and blocks moist air contact. Raising fan speed or leaving joints open increases moisture problems rather than solving them.

Question 46Air Quality

A technician is verifying that a new bathroom exhaust fan meets ventilation code. Which value should be confirmed against the manufacturer's rating and local code?

  1. Refrigerant superheat
  2. Cubic feet per minute of exhaust airflow Correct answer.
  3. Static pressure of the supply plenum
  4. Filter MERV rating

Explanation

Bathroom ventilation requirements are stated as exhaust airflow in cubic feet per minute, so the installed fan must deliver the rated CFM. Plenum pressure, superheat, and MERV describe other system functions.

Question 47Air Quality

A technician measures 400 ppm of carbon monoxide in undiluted flue gas from a gas furnace and calculates an air-free CO reading. Why is the air-free value used for evaluating combustion?

  1. It reports the CO level present in the occupied space
  2. It normalizes the CO concentration to a fixed oxygen reference so readings from different dilution levels can be compared Correct answer.
  3. It eliminates the need to measure oxygen in the flue
  4. It converts CO into carbon dioxide for easier measurement

Explanation

Flue-gas dilution varies with excess air, so raw CO readings are not directly comparable between appliances or settings. Air-free CO references the measurement to a standard oxygen level, allowing meaningful comparison of combustion quality.

Question 48Air Quality

A ducted system shows high static pressure at the supply plenum and low airflow at the registers. Which condition is the most likely cause?

  1. A failed condensate trap
  2. An oversized return grille
  3. Excessive restriction such as an undersized or clogged filter and tight ductwork Correct answer.
  4. A refrigerant undercharge in the outdoor unit

Explanation

High supply static combined with low delivered airflow points to added resistance on the air side, such as a restrictive filter or undersized ducts. Refrigerant charge and condensate drainage do not create this static-pressure pattern.

Question 49Air Quality

An HVAC technician is selecting a filter for a homeowner with documented dust-mite and pollen sensitivities. Which filter characteristic should be prioritized?

  1. Lowest initial cost
  2. Particle capture efficiency in the size range of the allergens Correct answer.
  3. Highest possible airflow restriction
  4. Filter thickness alone

Explanation

Allergen reduction depends on capturing particles in the size range of pollen and dust-mite debris, which is described by the filter's efficiency rating for those sizes. Cost, thickness, or restriction alone do not describe capture performance.

Question 50Air Quality

A technician is using a manometer to measure building pressure relative to outdoors while a bath fan and range hood operate. A reading of negative pressure in the conditioned space most likely indicates what?

  1. The supply blower is delivering excess outdoor air
  2. The duct system is perfectly balanced
  3. Exhaust devices are removing more air than is being supplied, drawing air in through leaks Correct answer.
  4. The manometer is reading absolute barometric pressure

Explanation

A negative pressure relative to outdoors means more air is being exhausted than supplied, so makeup air enters through envelope leaks. A balanced system would read near zero, and excess supply would produce a positive reading.