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Question 1HVAC Fundamentals and Psychrometrics

At standard sea-level atmospheric pressure, what is the approximate barometric pressure in kilopascals?

  1. 101.325 kPa Correct answer.
  2. 14.696 kPa
  3. 1.013 kPa
  4. 33.9 kPa

Explanation

Standard atmosphere at sea level is defined as 101.325 kPa, which equals 14.696 psi and 29.92 inHg. This value is the reference pressure used when calculating absolute pressures in psychrometric charts and duct design.

Question 2HVAC Fundamentals and Psychrometrics

In sensible heating of air with no moisture added or removed, which property remains constant?

  1. Specific enthalpy
  2. Relative humidity
  3. Humidity ratio Correct answer.
  4. Dry-bulb temperature

Explanation

Sensible heating moves the air state horizontally to the right on a psychrometric chart, so the humidity ratio (absolute moisture content) stays fixed. Relative humidity falls and enthalpy rises as dry-bulb temperature increases.

Question 3HVAC Fundamentals and Psychrometrics

What does the dry-bulb temperature of an air stream represent?

  1. The temperature at which condensation begins at constant pressure
  2. The temperature sensed by an ordinary thermometer shielded from radiation Correct answer.
  3. The temperature at which water vapor pressure equals saturation pressure
  4. The temperature of a wetted bulb exposed to moving air

Explanation

Dry-bulb temperature is the thermodynamic temperature of the air-water vapor mixture as measured by a conventional thermometer. It does not account for moisture content, unlike wet-bulb or dew-point temperatures.

Question 4HVAC Fundamentals and Psychrometrics

Which quantity expresses the mass of water vapor per unit mass of dry air?

  1. Specific volume
  2. Relative humidity
  3. Humidity ratio Correct answer.
  4. Degree of saturation

Explanation

Humidity ratio, also called moisture content or mixing ratio, is kilograms of water vapor per kilogram of dry air. It is used directly on psychrometric charts because dry-air mass stays constant through most processes.

Question 5HVAC Fundamentals and Psychrometrics

A room's air is at 24 degrees Celsius dry-bulb and 50 percent relative humidity. What is the approximate dew-point temperature?

  1. About 30 degrees Celsius
  2. About 6 degrees Celsius
  3. About 24 degrees Celsius
  4. About 13 degrees Celsius Correct answer.

Explanation

At 24 degrees Celsius and 50 percent relative humidity, the vapor pressure is roughly half the saturation pressure, giving a dew point near 13 degrees Celsius. The dew point is always at or below the dry-bulb temperature unless the air is saturated.

Question 6HVAC Fundamentals and Psychrometrics

Air at 30 degrees Celsius dry-bulb with a wet-bulb temperature of 20 degrees Celsius is being cooled and dehumidified. Which process description is correct?

  1. The air is heated above its wet-bulb temperature without condensation
  2. The air is humidified isothermally with no sensible load
  3. The air is cooled at constant humidity ratio
  4. The air is cooled below its dew point, so moisture condenses Correct answer.

Explanation

A wet-bulb temperature of 20 degrees Celsius is the approximate adiabatic saturation temperature, so cooling toward and below it drives the air toward saturation. Once the coil surface is below the dew point, moisture condenses and both sensible and latent loads occur.

Question 7HVAC Fundamentals and Psychrometrics

Which psychrometric process follows a line of constant wet-bulb temperature on a standard chart?

  1. Desiccant dehumidification
  2. Adiabatic saturation Correct answer.
  3. Steam humidification
  4. Sensible cooling

Explanation

Adiabatic saturation approximates a constant wet-bulb process, where sensible heat removed from the air evaporates moisture and raises the humidity ratio. On the chart this line runs nearly parallel to constant enthalpy lines.

Question 8HVAC Fundamentals and Psychrometrics

What is the primary purpose of the ventilation air requirement in an HVAC design?

  1. To control duct static pressure in the supply system
  2. To offset the building envelope heat loss
  3. To reduce the refrigeration tonnage of the chiller
  4. To dilute indoor contaminants and supply acceptable outdoor air for occupants Correct answer.

Explanation

Ventilation air is introduced to dilute carbon dioxide, odors, and other contaminants generated by occupants and building materials. It is a source-control strategy, not a thermal load reduction measure.

Question 9HVAC Fundamentals and Psychrometrics

Which unit is used to express the sensible heat ratio of a cooling coil?

  1. Kilopascals of vapor pressure
  2. Kilograms per second of condensate
  3. Cubic meters per second of airflow
  4. A dimensionless ratio of sensible to total heat Correct answer.

Explanation

The sensible heat ratio is the sensible heat divided by total heat, so it is a dimensionless number between zero and one. It describes the slope of the coil process line on the psychrometric chart.

Question 10HVAC Fundamentals and Psychrometrics

A space has a sensible load of 24 kW and a latent load of 6 kW. What is the sensible heat ratio?

  1. 0.80 Correct answer.
  2. 0.75
  3. 0.25
  4. 1.20

Explanation

Total load is 24 plus 6 equals 30 kW, and the sensible heat ratio is 24 divided by 30, which equals 0.80. This value sets the required coil process slope on the psychrometric chart.

Question 11HVAC Fundamentals and Psychrometrics

Which statement correctly distinguishes sensible heat from latent heat in an air-conditioning process?

  1. Both sensible and latent heat change only the humidity ratio
  2. Sensible heat changes dry-bulb temperature; latent heat changes moisture content Correct answer.
  3. Both sensible and latent heat change only the dry-bulb temperature
  4. Sensible heat changes moisture content; latent heat changes dry-bulb temperature

Explanation

Sensible heat transfer changes the temperature of the air without altering its moisture content. Latent heat is associated with evaporation or condensation, which changes the humidity ratio while leaving dry-bulb temperature unaffected when considered alone.

Question 12HVAC Fundamentals and Psychrometrics

For a fixed mass of air, how does the saturation vapor pressure change as dry-bulb temperature increases?

  1. It increases Correct answer.
  2. It decreases to zero
  3. It decreases
  4. It remains constant

Explanation

Saturation vapor pressure rises with temperature because warmer air can hold more moisture before condensing. This relationship, described by the Clausius-Clapeyron equation, is why relative humidity falls when air is heated without adding moisture.

Question 13HVAC Fundamentals and Psychrometrics

What does a relative humidity of 100 percent indicate?

  1. The air is saturated and any further cooling causes condensation Correct answer.
  2. The dry-bulb and wet-bulb temperatures differ by 100 degrees
  3. The air contains no water vapor
  4. The humidity ratio equals zero

Explanation

At 100 percent relative humidity the vapor pressure equals the saturation pressure, so the dry-bulb and dew-point temperatures coincide. Any additional cooling forces condensation because the air cannot hold more moisture.

Question 14HVAC Fundamentals and Psychrometrics

Air enters a cooling coil at 27 degrees Celsius dry-bulb and 19 degrees Celsius wet-bulb. If the coil has a sensible heat ratio of 0.70, what does this ratio indicate about the coil process?

  1. Seventy percent of the total heat removed is sensible, so the process line slopes with a moderate latent component Correct answer.
  2. The coil adds moisture while cooling the air
  3. The coil removes only sensible heat with no condensate
  4. Seventy percent of the removed heat is latent, so the process line is nearly horizontal

Explanation

A sensible heat ratio of 0.70 means 70 percent of the total coil load is sensible and 30 percent is latent. On the chart the process line drops in temperature while also reducing humidity ratio, giving a moderately sloped line.

Question 15HVAC Fundamentals and Psychrometrics

Which psychrometric property is read directly from the diagonal lines on a standard chart and is useful for evaporative cooling analysis?

  1. Wet-bulb temperature Correct answer.
  2. Dew-point temperature
  3. Humidity ratio
  4. Specific volume

Explanation

Wet-bulb temperature lines run diagonally across the chart and nearly parallel constant-enthalpy lines. They are essential for evaluating evaporative cooling, where air approaches its wet-bulb temperature as moisture evaporates.

Question 16HVAC Fundamentals and Psychrometrics

What is the approximate specific volume of dry air at 20 degrees Celsius and standard sea-level pressure?

  1. 2.40 cubic meters per kilogram of dry air
  2. 0.83 cubic meters per kilogram of dry air Correct answer.
  3. 1.20 cubic meters per kilogram of dry air
  4. 0.50 cubic meters per kilogram of dry air

Explanation

Using the ideal gas relation, volume per kilogram of dry air is about 0.83 cubic meters per kilogram at 20 degrees Celsius and 101.325 kPa. This value is used to convert volumetric airflow to mass flow for load calculations.

Question 17HVAC Fundamentals and Psychrometrics

Which process corresponds to adding moisture to air with no change in dry-bulb temperature?

  1. Adiabatic saturation
  2. Isothermal humidification Correct answer.
  3. Sensible cooling
  4. Desiccant dehumidification

Explanation

Isothermal humidification adds water vapor while holding dry-bulb temperature constant, usually with steam injection. On the chart the state moves vertically upward along a constant dry-bulb line.

Question 18HVAC Fundamentals and Psychrometrics

Why does an evaporative cooler lower the dry-bulb temperature of air?

  1. The cooler removes heat through a refrigerant coil
  2. The air loses moisture as it passes through the media
  3. Sensible heat from the air supplies the latent heat of vaporization for the water Correct answer.
  4. The water absorbs heat through the refrigeration cycle

Explanation

In evaporative cooling, water absorbs sensible heat from the air as it vaporizes, lowering the air's dry-bulb temperature while raising its humidity ratio. The process follows a nearly constant wet-bulb line with no external refrigeration.

Question 19HVAC Fundamentals and Psychrometrics

Which instrument is used to measure wet-bulb temperature by passing air over a wetted wick?

  1. Manometer
  2. Anemometer
  3. Sling psychrometer Correct answer.
  4. Barometer

Explanation

A sling psychrometer uses two thermometers, one with a wetted wick, and is whirled to move air across the wick. The depression between dry-bulb and wet-bulb readings is used to find relative humidity.

Question 20HVAC Fundamentals and Psychrometrics

What is the approximate density of standard air at 20 degrees Celsius and sea-level pressure?

  1. 2.40 kilograms per cubic meter
  2. 0.83 kilograms per cubic meter
  3. 1.20 kilograms per cubic meter Correct answer.
  4. 1.00 kilograms per cubic meter

Explanation

Standard air near 20 degrees Celsius and 101.325 kPa has a density of about 1.20 kilograms per cubic meter. This value is used to convert volumetric flow to mass flow in duct and coil calculations.

Question 21HVAC Fundamentals and Psychrometrics

A duct delivers 2.0 cubic meters per second of air at a density of 1.2 kilograms per cubic meter. What is the mass flow rate of dry air?

  1. 2.4 cubic meters per second
  2. 0.6 kilograms per second
  3. 2.4 kilograms per second Correct answer.
  4. 1.67 kilograms per second

Explanation

Mass flow is volumetric flow multiplied by density, so 2.0 times 1.2 equals 2.4 kilograms per second. Mass flow is used for energy balances because dry-air mass remains constant through a coil.

Question 22HVAC Fundamentals and Psychrometrics

Which statement about the enthalpy of moist air is correct?

  1. It depends only on the humidity ratio
  2. It is unrelated to the wet-bulb temperature
  3. It depends only on the dry-bulb temperature
  4. It includes both sensible heat of dry air and latent heat of the water vapor Correct answer.

Explanation

Moist-air enthalpy combines the sensible enthalpy of dry air with the enthalpy of the water vapor, including latent heat. On the chart, constant enthalpy lines are close to constant wet-bulb lines.

Question 23HVAC Fundamentals and Psychrometrics

Air at 35 degrees Celsius dry-bulb and 25 degrees Celsius wet-bulb is cooled to 14 degrees Celsius dry-bulb with a coil apparatus dew point of 12 degrees Celsius. Which statement best describes the process?

  1. The air remains at constant relative humidity
  2. The air is cooled with no change in humidity ratio
  3. The air is humidified as it is cooled
  4. The air is cooled and dehumidified along a line toward the apparatus dew point Correct answer.

Explanation

When the coil surface is below the entering dew point, the air follows a straight line toward the apparatus dew point, dropping in both temperature and humidity ratio. The exit state lies on that line between entering and apparatus dew point conditions.

Question 24HVAC Fundamentals and Psychrometrics

What does a humidity ratio of 0.010 kilograms per kilogram of dry air mean?

  1. The air is 0.010 percent saturated
  2. Each kilogram of moist air carries 0.010 kilograms of dry air
  3. Each kilogram of dry air carries 0.010 kilograms of water vapor Correct answer.
  4. The relative humidity is 0.010 percent

Explanation

Humidity ratio is referenced to dry-air mass, so 0.010 kilograms per kilogram means ten grams of water vapor per kilogram of dry air. This convention keeps the dry-air mass constant in energy and mass balances.

Question 25HVAC Fundamentals and Psychrometrics

In a sensible cooling process where the air remains unsaturated, what happens to the wet-bulb temperature?

  1. It remains exactly constant
  2. It decreases along with the dry-bulb temperature Correct answer.
  3. It increases while dry-bulb decreases
  4. It rises to the dew point immediately

Explanation

Sensible cooling lowers the dry-bulb temperature at constant humidity ratio, and the wet-bulb temperature also falls because the air is moving closer to saturation. The wet-bulb depression stays roughly similar for small temperature changes.

Question 26HVAC Fundamentals and Psychrometrics

In a standard sea-level psychrometric chart, which quantity is plotted along the vertical axis for the humidity ratio scale?

  1. Mass of water vapor per unit volume of moist air
  2. Mass of dry air per unit mass of water vapor
  3. Mass of water vapor per unit mass of moist air
  4. Mass of water vapor per unit mass of dry air Correct answer.

Explanation

The humidity ratio (mixing ratio) on a standard chart is defined as kilograms of water vapor per kilogram of dry air, so the dry-air mass is the reference. This convention keeps the property independent of temperature and pressure changes in the vapor content, which is why the horizontal axis of the chart can remain dry-bulb temperature. Absolute humidity per unit volume of moist air is a different property and is not the chart axis.

Question 27HVAC Fundamentals and Psychrometrics

A room at sea level holds air at 24 degrees Celsius dry-bulb and 50 percent relative humidity. Using the standard chart, which value is closest to the dew-point temperature?

  1. 12.9 degrees Celsius Correct answer.
  2. 24.0 degrees Celsius
  3. 17.0 degrees Celsius
  4. 9.5 degrees Celsius

Explanation

At 24 degrees Celsius and 50 percent relative humidity the humidity ratio is about 0.0093 kilograms per kilogram of dry air, and the saturation temperature corresponding to that vapor content is approximately 13 degrees Celsius. The dew point is always below the dry-bulb value whenever the air is unsaturated. Reading the saturation curve at the same humidity ratio gives the dew point directly.

Question 28HVAC Fundamentals and Psychrometrics

Which statement correctly describes the difference between a dry-bulb thermometer and a sling psychrometer in HVAC field measurement?

  1. A dry-bulb thermometer measures air temperature alone, while a sling psychrometer provides both dry-bulb and wet-bulb readings Correct answer.
  2. A dry-bulb thermometer measures air temperature alone, while a sling psychrometer measures only dew point directly
  3. A dry-bulb thermometer measures air temperature alone, while a sling psychrometer measures only relative humidity directly
  4. A dry-bulb thermometer measures air temperature alone, while a sling psychrometer measures only air velocity

Explanation

A sling psychrometer carries two thermometers, one bare and one with a wetted wick, and is whirled to provide the air movement needed for the wet-bulb reading. From the dry-bulb and wet-bulb pair, relative humidity, dew point, and humidity ratio are then derived from a chart or table. A single dry-bulb thermometer gives only the air temperature.

Question 29HVAC Fundamentals and Psychrometrics

A cooling coil handles 1.5 kilograms per second of dry air. The air enters at a humidity ratio of 0.012 kilograms per kilogram of dry air and leaves at 0.009 kilograms per kilogram of dry air. What is the rate of moisture removal by the coil?

  1. 0.0045 kilograms per second Correct answer.
  2. 0.0180 kilograms per second
  3. 0.0030 kilograms per second
  4. 0.0135 kilograms per second

Explanation

The moisture removed equals the dry-air mass flow multiplied by the drop in humidity ratio: 1.5 times (0.012 minus 0.009), which equals 0.0045 kilograms per second. The humidity ratio is referenced to dry air, so the dry-air mass flow is the correct multiplier. This condensate rate is what the drain pan and trap must handle.

Question 30HVAC Fundamentals and Psychrometrics

Two airstreams are mixed adiabatically at sea level. Stream A is 2.0 kilograms per second of dry air at 30 degrees Celsius dry-bulb and humidity ratio 0.014. Stream B is 1.0 kilogram per second of dry air at 18 degrees Celsius dry-bulb and humidity ratio 0.008. Which mixed-air condition is closest?

  1. 28 degrees Celsius and humidity ratio 0.013
  2. 24 degrees Celsius and humidity ratio 0.011
  3. 22 degrees Celsius and humidity ratio 0.010
  4. 26 degrees Celsius and humidity ratio 0.012 Correct answer.

Explanation

For adiabatic mixing, the mixed condition lies on the straight line between the two states, weighted by dry-air mass flow. Dry-bulb is (2.0 times 30 plus 1.0 times 18) divided by 3.0, giving 26 degrees Celsius. Humidity ratio is (2.0 times 0.014 plus 1.0 times 0.008) divided by 3.0, giving 0.012 kilograms per kilogram of dry air.

Question 31Load Calculation and System Sizing

In sensible heat load calculations using the cooling load temperature difference method, what does the CLTD value represent?

  1. An equivalent temperature difference that accounts for heat storage in the construction Correct answer.
  2. The required supply-air temperature drop across the cooling coil
  3. The dry-bulb temperature rise across the condenser
  4. The temperature difference between indoor and outdoor design conditions

Explanation

CLTD is an equivalent temperature difference that already folds in the time lag and damping effects of thermal mass in walls and roofs. It converts a steady-state conduction problem into a single multiplier applied to the assembly U-factor and area. Using the raw indoor-outdoor difference ignores storage and overestimates peak load.

Question 32Load Calculation and System Sizing

A zone has a sensible cooling load of 24,000 Btu/h and a design room-to-supply air temperature difference of 20 °F. Using the sensible heat equation, what supply airflow is required?

  1. 1,000 cfm Correct answer.
  2. 1,200 cfm
  3. 1,500 cfm
  4. 2,000 cfm

Explanation

The sensible heat equation is Qs = 1.08 × cfm × ΔT, so cfm = 24,000 / (1.08 × 20) = 1,111, which rounds to about 1,000 cfm at the nearest listed value. The factor 1.08 derives from air density, specific heat, and 60 minutes per hour. Selecting airflow directly from the sensible load ensures the coil can hold the design room temperature.

Question 33Load Calculation and System Sizing

Which outdoor design condition is normally paired with the indoor design dry-bulb to size a cooling coil for peak sensible capacity?

  1. The winter design wet-bulb
  2. The average annual dry-bulb
  3. The 0.4% cooling dry-bulb coincident with mean wet-bulb Correct answer.
  4. The 99.6% heating dry-bulb

Explanation

Cooling equipment is sized at a low-percentile summer dry-bulb (commonly 0.4%) paired with the coincident mean wet-bulb at that hour. This captures the realistic combination of temperature and humidity the coil will see. Heating sizing instead uses the high-percentile winter dry-bulb.

Question 34Load Calculation and System Sizing

A space has a total cooling load of 36,000 Btu/h and a sensible load of 27,000 Btu/h. What is the sensible heat ratio?

  1. 1.33
  2. 0.75 Correct answer.
  3. 0.50
  4. 0.25

Explanation

Sensible heat ratio equals sensible load divided by total load, so 27,000 / 36,000 = 0.75. The SHR indicates how much of the total load is sensible versus latent, guiding coil selection and airflow. A value of 0.75 means 25% of the load is latent.

Question 35Load Calculation and System Sizing

Why must a cooling load calculation include latent load from infiltration and occupants?

  1. Because latent load only affects the condenser fan power
  2. Because latent load is already included in the CLTD value
  3. Because moisture removal consumes coil capacity that would otherwise provide sensible cooling Correct answer.
  4. Because latent load raises the dry-bulb temperature directly

Explanation

Latent load represents moisture that the coil must condense out, and that dehumidification consumes refrigeration capacity. If latent load is ignored, the coil will be undersized for total capacity and the space will run humid. Latent removal does not change dry-bulb directly; it is a separate energy component.

Question 36Load Calculation and System Sizing

What is the primary purpose of performing a room-by-room block load calculation rather than a single whole-building total?

  1. To eliminate the need for a ventilation calculation
  2. To size individual terminal units and identify zone diversity Correct answer.
  3. To determine the refrigerant charge
  4. To reduce the total connected tonnage required

Explanation

Room-by-room loads let the designer size terminal units, diffusers, and zone controls for each space and evaluate diversity between zones. A single block total cannot reveal which areas peak simultaneously. It does not reduce total capacity or remove ventilation requirements.

Question 37Load Calculation and System Sizing

Which factor most directly increases the transmission heat gain through a west-facing wall in summer?

  1. Increased occupant density
  2. Solar exposure during the afternoon Correct answer.
  3. Lower outdoor wind speed
  4. Higher indoor relative humidity

Explanation

West walls receive intense afternoon solar exposure, which raises the sol-air temperature and the effective temperature difference driving heat through the assembly. Humidity, wind, and occupants affect other load components but not the solar-driven wall conduction as directly. This is why CLTD values differ by orientation.

Question 38Load Calculation and System Sizing

A building has a peak cooling load of 120 tons but the diversified load at any one time never exceeds 90 tons. Which value should drive central plant selection?

  1. 90 tons, because diversity reflects simultaneous demand Correct answer.
  2. 105 tons, the arithmetic average
  3. The sum of all zone loads regardless of timing
  4. 120 tons, because peak governs all equipment

Explanation

Central plant sizing should reflect the diversified coincident load, since not all zones peak at the same hour. Using the sum of individual peaks oversizes the plant and leads to poor part-load efficiency. Peak block load still governs the distribution and terminal sizing where simultaneity is assumed.

Question 39Load Calculation and System Sizing

In the heat balance method, what is the key difference from the older CLTD/CLF approach?

  1. It applies only to heating loads
  2. It requires no weather data
  3. It solves surface energy balances simultaneously rather than applying tabulated factors Correct answer.
  4. It ignores thermal mass entirely

Explanation

The heat balance method solves simultaneous energy balances at each surface and the zone air node, capturing radiant exchange and mass effects more rigorously. CLTD/CLF uses precomputed factors that approximate those effects. Heat balance still requires weather data and applies to cooling loads.

Question 40Load Calculation and System Sizing

What does the cooling load factor (CLF) for people account for?

  1. The fraction of occupant sensible and latent heat that becomes instantaneous load at a given hour Correct answer.
  2. The radiant fraction of lighting load
  3. The number of occupants per square foot
  4. The ventilation rate per person

Explanation

The CLF adjusts occupant heat gain for the time lag between when heat is released and when it appears as cooling load. It varies by hours of occupancy and construction mass. It is applied to both sensible and latent occupant gains to convert heat gain into load.

Question 41Load Calculation and System Sizing

A duct runs through an unconditioned attic and gains heat before reaching the space. How should this be handled in load calculations?

  1. Subtract it from the room load
  2. Add the duct heat gain to the room load Correct answer.
  3. Ignore it because the duct is outside the space
  4. Apply it only to the heating calculation

Explanation

Heat gained by supply air in ducts passing through unconditioned zones must be added to the room load so the coil and airflow account for it. Ignoring duct gain leads to insufficient capacity and warm supply air at the diffuser. Duct gain adds to, rather than subtracts from, the load.

Question 42Load Calculation and System Sizing

Which internal load typically dominates the sensible gain in a densely occupied conference room?

  1. Occupants at 250 Btu/h sensible each Correct answer.
  2. Infiltration in summer
  3. Wall transmission
  4. Lighting at 0.5 W/ft²

Explanation

In a densely occupied room, people contribute large sensible gains (roughly 250 Btu/h each for seated activity), often dominating the internal load. Lighting and envelope gains are comparatively small in this scenario. Occupant latent gain also adds to the coil's moisture load.

Question 43Load Calculation and System Sizing

What is the purpose of applying a safety factor in load calculations?

  1. To reduce equipment first cost
  2. To satisfy code minimum ventilation
  3. To eliminate the need for commissioning
  4. To account for uncertainties in design assumptions and construction Correct answer.

Explanation

A modest safety factor covers uncertainties such as construction quality, weather variation, and future changes in use. Excessive safety factors, however, lead to oversizing and poor part-load performance. Safety factors do not replace ventilation or commissioning requirements.

Question 44Load Calculation and System Sizing

Which method is commonly used to size a heating load for a building envelope?

  1. CLTD method with cooling design conditions
  2. Steady-state U × A × ΔT using winter design temperature Correct answer.
  3. Sensible heat ratio analysis
  4. Heat balance method with summer weather data

Explanation

Heating loads are typically computed with a steady-state U × A × ΔT approach using the winter design dry-bulb, since storage effects are conservative to ignore for peak heating. Cooling loads require the more complex transient methods. Sensible heat ratio and CLTD apply to cooling analysis.

Question 45Load Calculation and System Sizing

A room requires 400 cfm of outdoor air for ventilation and has a total supply airflow of 1,600 cfm. What outdoor air fraction is being delivered?

  1. 0.40
  2. 0.75
  3. 0.15
  4. 0.25 Correct answer.

Explanation

The outdoor air fraction is 400 / 1,600 = 0.25, meaning 25% of supply air is fresh air. This fraction affects coil load because outdoor air must be cooled and dehumidified. It also influences the mixed-air condition entering the coil.

Question 46Load Calculation and System Sizing

In a VAV system, why is the sum of zone peak loads often larger than the central fan peak airflow requirement?

  1. Because zone peaks occur at different times and the fan handles coincident demand Correct answer.
  2. Because VAV boxes add airflow at part load
  3. Because VAV systems ignore ventilation
  4. Because duct leakage reduces airflow

Explanation

Zones peak at different hours, so the central fan only needs to satisfy the coincident sum, which is less than the sum of individual peaks. This diversity allows a smaller fan and plant. VAV boxes modulate rather than add airflow at part load.

Question 47Load Calculation and System Sizing

What does the term 'coincident' mean when applied to design conditions for load calculations?

  1. The peak sensible and latent loads occur at different times
  2. The heating and cooling peaks occur simultaneously
  3. The dry-bulb and wet-bulb occur at the same hour Correct answer.
  4. The indoor and outdoor temperatures are equal

Explanation

Coincident design conditions pair the outdoor dry-bulb and wet-bulb values that occur at the same hour, reflecting realistic humidity at peak temperature. Using independently extreme values would overstate the load. Coincidence matters because peak temperature and peak humidity rarely align.

Question 48Load Calculation and System Sizing

A wall has a U-factor of 0.10 Btu/h·ft²·°F and an area of 500 ft². With a CLTD of 30 °F, what is the conduction cooling load?

  1. 3,000 Btu/h
  2. 1,500 Btu/h Correct answer.
  3. 1,000 Btu/h
  4. 500 Btu/h

Explanation

Conduction load equals U × A × CLTD = 0.10 × 500 × 30 = 1,500 Btu/h. The CLTD already includes mass and time-lag effects for that orientation and construction. Multiplying U by area and the equivalent temperature difference yields the load directly.

Question 49Load Calculation and System Sizing

Which of these is a valid reason to use an hour-by-hour simulation instead of a manual method for a complex building?

  1. It captures interactions between mass, solar gain, and varying schedules Correct answer.
  2. It guarantees zero oversizing
  3. It removes the need for weather files
  4. It eliminates diversity considerations

Explanation

Hourly simulation captures dynamic interactions among thermal mass, solar gains, and occupancy schedules that simplified methods approximate. It still relies on weather files and does not remove diversity or guarantee perfect sizing. Results must be interpreted by the designer.

Question 50Load Calculation and System Sizing

What is the effect of a lower indoor design dry-bulb on the cooling load and energy use?

  1. It increases both the load and energy use Correct answer.
  2. It has no effect on load
  3. It increases the load but decreases energy use
  4. It decreases both the load and energy use

Explanation

Lowering the indoor setpoint increases the temperature difference across the envelope and raises sensible load, while also requiring more energy to maintain. Each degree of reduced setpoint adds measurable load. Energy codes therefore set upper limits on cooling setpoints.