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Question 1Energy Modeling Fundamentals and Theory

Which mode of heat transfer does not require a physical medium or direct contact between surfaces?

  1. Conduction
  2. Natural convection
  3. Thermal radiation Correct answer.
  4. Forced convection

Explanation

Thermal radiation travels as electromagnetic waves and needs no material medium, which is how heat reaches a surface from the sun across space. Conduction requires molecular contact, and both forced and natural convection require a moving fluid.

Question 2Energy Modeling Fundamentals and Theory

In whole-building energy simulation, conduction transfer functions (CTFs) are used to:

  1. Compute transient conduction through multilayer walls Correct answer.
  2. Interpolate weather data between hourly records
  3. Determine outside-surface convective coefficients only
  4. Convert sensible loads into radiant fractions

Explanation

Conduction transfer functions relate the current heat flux at a surface to present and past temperatures and past fluxes, capturing a multilayer construction's transient response without discretizing the solid. They are the standard substitute for solving the conduction equation inside each layer at every timestep.

Question 3Energy Modeling Fundamentals and Theory

On a psychrometric chart, the curve marking 100% relative humidity is known as the:

  1. Wet-bulb depression line
  2. Enthalpy line
  3. Sensible heat ratio line
  4. Saturation curve Correct answer.

Explanation

The saturation curve connects the states in which air holds the maximum water vapor possible at each dry-bulb temperature, i.e., 100% relative humidity. It forms the upper boundary of the chart's usable region, and no state can lie above it at a given dry-bulb temperature.

Question 4Energy Modeling Fundamentals and Theory

A typical meteorological year (TMY) weather file is assembled by:

  1. Averaging every hour of a long record into one synthetic year
  2. Recording one continuous year of measured data at the site
  3. Generating weather stochastically for a single design day
  4. Selecting representative months from a multiyear record Correct answer.

Explanation

A TMY file is a synthetic year built month by month from a multiyear record, choosing months whose solar, temperature, and wind statistics best match the long-term distributions. It therefore represents typical rather than extreme conditions.

Question 5Energy Modeling Fundamentals and Theory

Which internal gain in an occupied office zone produces both a sensible and a latent component?

  1. A plug load modeled as sensible-only
  2. An electric resistance baseboard heater
  3. Overhead lighting
  4. Occupants Correct answer.

Explanation

People release sensible heat by convection and radiation from skin and clothing, and latent heat through respiration and perspiration. Lighting, sensible-only plug loads, and electric resistance heaters add heat without adding moisture.

Question 6Energy Modeling Fundamentals and Theory

Shortening the zone timestep in a simulation generally:

  1. Has no effect on results once the model converges
  2. Reduces run time while improving accuracy
  3. Removes the need for warm-up periods
  4. Increases accuracy but also increases run time Correct answer.

Explanation

A shorter timestep lets the solver track fast-changing solar gains and control actions more closely, but it multiplies the number of zone and system solutions that must be computed, so run time grows.

Question 7Energy Modeling Fundamentals and Theory

In most building simulation conventions, solar azimuth is measured:

  1. As the angle between the sun and the zenith
  2. As the angle between the sun's rays and the horizontal plane
  3. From due north, clockwise, in degrees Correct answer.
  4. From due south, counterclockwise, in degrees

Explanation

Azimuth is the compass direction of the sun's horizontal projection, expressed in degrees clockwise from north. The angle of the sun above the horizon is the altitude, which is a separate coordinate.

Question 8Energy Modeling Fundamentals and Theory

In a variable air volume (VAV) system model, zone load matching is achieved primarily by:

  1. Varying the chilled water flow only
  2. Modulating supply air temperature at the central air handler
  3. Cycling the supply fan on and off
  4. Varying supply airflow at constant supply temperature Correct answer.

Explanation

A VAV terminal varies the volume of cool air delivered to the zone while the air handler holds a roughly constant supply air temperature. This modulates delivered capacity without cycling the fan.

Question 9Energy Modeling Fundamentals and Theory

The radiant time series (RTS) method represents radiant internal and solar gains by:

  1. Converting all gains to an equivalent outdoor temperature
  2. Assuming radiant gains are absorbed entirely by the zone air
  3. Applying gains uniformly across the 24-hour day
  4. Spreading the radiant share of each gain over later hours Correct answer.

Explanation

RTS splits each gain into convective and radiant portions; the convective part enters the zone air immediately, while the radiant part strikes surfaces and is returned as a decaying series of hourly convective fractions.

Question 10Energy Modeling Fundamentals and Theory

In the simplified infiltration model used in many simulation tools, the zone infiltration rate increases with:

  1. Mechanical exhaust flow only
  2. Zone pressurization from outdoor air intakes
  3. Both the indoor-outdoor temperature difference and the wind speed Correct answer.
  4. Indoor-outdoor temperature difference only, independent of wind

Explanation

The simplified model combines a stack term driven by the indoor-outdoor temperature difference with a wind term driven by wind speed. Depending on the formulation, the two effects are added or the larger one is used.

Question 11Energy Modeling Fundamentals and Theory

A zone air heat balance in an iterative simulation is considered converged when:

  1. The number of iterations equals the number of timesteps
  2. Successive iterations change zone temperature by less than the tolerance Correct answer.
  3. The simulation clock reaches the end of the design day
  4. The HVAC system has delivered exactly the design capacity to every zone

Explanation

The solver repeats the zone heat balance until the updated zone air temperature no longer changes by more than the specified convergence tolerance, then advances to the next timestep.

Question 12Energy Modeling Fundamentals and Theory

Under ASHRAE Guideline 14, hourly calibration of a simulation model is generally judged acceptable when:

  1. NMBE is within ±5% and CV(RMSE) within ±15% for monthly data
  2. NMBE is within ±10% and CV(RMSE) within 30% Correct answer.
  3. R-squared exceeds 0.5 with no bias requirement
  4. NMBE is within ±20% and CV(RMSE) within ±50%

Explanation

Guideline 14 sets statistical thresholds for calibrated simulation; for hourly data the criteria are a normalized mean bias error within ±10% and a coefficient of variation of RMSE within 30% of the measured mean.

Question 13Energy Modeling Fundamentals and Theory

The isotropic diffuse sky model assumes that:

  1. Diffuse radiation is negligible for vertical surfaces
  2. Diffuse radiation is concentrated near the horizon
  3. Sky diffuse radiation is uniform over the sky dome Correct answer.
  4. Diffuse radiation comes only from the circumsolar region

Explanation

The isotropic model treats the sky as a uniform diffuse source whose radiance does not depend on direction, so diffuse irradiance on a tilted surface depends only on its view factor to the sky.

Question 14Energy Modeling Fundamentals and Theory

Adding thermal mass to the interior of a zone generally:

  1. Eliminates the need to model internal gains
  2. Has no effect on loads if the setpoint is constant
  3. Increases the peak cooling load and the daily total
  4. Lowers and delays the peak cooling load Correct answer.

Explanation

Interior mass absorbs part of the daytime radiant gains and releases them later, so the peak cooling load is reduced and shifted to a later hour as stored heat is removed.

Question 15Energy Modeling Fundamentals and Theory

Which situation most clearly justifies modeling two adjacent spaces as separate thermal zones?

  1. They share the same terminal unit and thermostat
  2. They are on the same floor and share the same carpet
  3. They have identical occupancy schedules and no partitions between them
  4. They differ in solar exposure and have separate thermostats Correct answer.

Explanation

Separate zones are warranted when spaces differ in loads, solar exposure, or temperature control. Different exposure plus independent thermostats means the two spaces respond differently and must be solved separately.

Question 16Energy Modeling Fundamentals and Theory

In a direct-expansion cooling coil model, the bypass factor represents:

  1. The portion of coil capacity lost at part load
  2. The fraction of refrigerant flow that bypasses the thermostatic expansion valve
  3. The ratio of latent to sensible capacity
  4. Fraction of air passing through the coil without contacting the fins Correct answer.

Explanation

The bypass factor is the share of air that passes through the coil without touching the finned surface and therefore leaves close to its entering condition. It governs the coil's sensible and latent effectiveness.

Question 17Energy Modeling Fundamentals and Theory

The apparatus dew point of a cooling coil is:

  1. The effective coil surface temperature for saturated leaving air Correct answer.
  2. The chilled water supply temperature setpoint
  3. The lowest outdoor dew point in the design weather file
  4. The dry-bulb temperature of the air entering the coil

Explanation

The apparatus dew point is the effective coil surface temperature; extending the leaving-air condition along the coil process line to saturation yields it. It sets the coil's moisture-removal capability.

Question 18Energy Modeling Fundamentals and Theory

A chiller's part-load performance is modeled with an efficiency-versus-part-load-ratio curve because:

  1. Chillers operate at constant efficiency at all loads
  2. Efficiency falls at low part-load ratios and peaks at part load Correct answer.
  3. Rated COP applies only below 20% load
  4. Efficiency rises monotonically as the load approaches zero percent

Explanation

Compressor unloading, cycling losses, and heat exchanger behavior make chiller efficiency load-dependent. The curve captures the degradation at low loads and the typical efficiency peak in the mid-to-high load range.

Question 19Energy Modeling Fundamentals and Theory

In a simulation model, an air-side economizer is enabled when:

  1. The zone temperature falls below the heating setpoint
  2. Outdoor enthalpy or temperature is above the return air condition at that hour
  3. Outdoor air is favorable versus return air and below the high limit Correct answer.
  4. The cooling coil is operating at full capacity

Explanation

An economizer compares outdoor air with return air and permits free cooling when outdoor conditions are favorable, subject to a high-limit lockout that prevents admitting hot or humid air.

Question 20Energy Modeling Fundamentals and Theory

When a return air plenum above a suspended ceiling is modeled explicitly, it is typically represented as:

  1. An adiabatic surface with no heat transfer
  2. Part of the outdoor air boundary condition
  3. A surface property applied to the ceiling construction
  4. A separate zone coupled to the space below and the roof Correct answer.

Explanation

The plenum is a real air volume that picks up heat from lighting, ducts, and the roof, so it is modeled as its own zone exchanging heat with the occupied space below and the roof assembly above.

Question 21Energy Modeling Fundamentals and Theory

An explicit finite-difference zone heat balance can become numerically unstable because:

  1. The HVAC system capacity is exceeded at that hour
  2. The timestep exceeds a limit set by zone capacitance and conductance Correct answer.
  3. The CTF coefficients become negative at long timesteps
  4. The weather file reports data at hourly rather than subhourly intervals

Explanation

Explicit schemes are only conditionally stable: the timestep must remain small relative to the zone's thermal capacitance divided by its total conductance. Violating that limit produces oscillation or divergence.

Question 22Energy Modeling Fundamentals and Theory

The effective moisture capacitance approach is used in zone models to:

  1. Convert latent loads to equivalent sensible loads
  2. Eliminate the need for a zone moisture balance
  3. Account for the latent capacity of the cooling coil alone
  4. Model surface moisture buffering without full diffusion Correct answer.

Explanation

The approach lumps the moisture storage of furniture and hygroscopic surfaces into a few effective layers, capturing humidity buffering at far lower cost than solving moisture diffusion in every material.

Question 23Energy Modeling Fundamentals and Theory

Two simulations of the same building report different peak cooling loads because one uses a 15-minute timestep and the other an hourly timestep. The most likely reason is that:

  1. Hourly timesteps force steady-state conduction
  2. Longer timesteps always increase reported loads
  3. The coarser timestep cannot resolve brief coincident peaks Correct answer.
  4. The two runs were driven by different weather files at the site

Explanation

A 15-minute step resolves short-lived coincident events such as solar patches, equipment switching, and occupancy spikes that an hourly averaging step smears out. Instantaneous peaks can therefore differ even when daily totals agree.

Question 24Energy Modeling Fundamentals and Theory

The Perez anisotropic sky model differs from the isotropic model in that it:

  1. Ignores ground-reflected radiation entirely
  2. Adds circumsolar and horizon brightening Correct answer.
  3. Applies only to horizontal surfaces
  4. Requires measured beam radiation at the site

Explanation

The Perez model treats diffuse radiation as anisotropic, adding a circumsolar component around the sun and brightening near the horizon to the uniform background. This improves accuracy for tilted surfaces, particularly under clear skies.

Question 25Energy Modeling Fundamentals and Theory

In a model of a variable-speed pump, the affinity laws are used to:

  1. Relate chiller COP to condenser water temperature
  2. Estimate boiler efficiency at part load
  3. Convert supply airflow directly into fan power
  4. Scale flow, head, and power with speed Correct answer.

Explanation

The affinity laws relate rotational speed to flow (proportional to speed), head (proportional to speed squared), and power (proportional to speed cubed), providing part-load performance for variable-speed pumps and fans.

Question 26Energy Modeling Fundamentals and Theory

In the conduction transfer function (CTF) method, the heat flux at a wall surface is calculated from which set of quantities?

  1. A constant U-factor multiplied by the current temperature difference
  2. Only the current outdoor and indoor air temperatures
  3. The zone air humidity ratio and infiltration rate
  4. Current and past surface temperatures and past heat fluxes Correct answer.

Explanation

CTFs are linear time-series coefficients that link surface heat flux to current and previous temperatures and to previous heat fluxes, which captures thermal storage in the construction. A steady-state U-factor approach would ignore that storage.

Question 27Energy Modeling Fundamentals and Theory

In building energy simulation, sol-air temperature is best described as:

  1. The saturation temperature of outdoor air at the design humidity ratio
  2. The temperature of air leaving a solar thermal collector under peak sun
  3. An equivalent temperature including absorbed solar and longwave effects Correct answer.
  4. The dry-bulb temperature reduced by wind chill and evaporative cooling

Explanation

Sol-air temperature converts the combined convective and radiative outdoor boundary conditions into an equivalent air temperature for conduction calculations. It accounts for absorbed solar radiation and longwave exchange with the sky and surroundings.

Question 28Energy Modeling Fundamentals and Theory

A schedule value of 0.5 applied to a design-level internal gain means that:

  1. The equipment operates at 50 percent thermal efficiency
  2. Half of the zone floor area is conditioned at that time
  3. The gain equals half its design level at that time Correct answer.
  4. The gain is active for only half of the year

Explanation

A schedule fraction is a multiplier on the design-level value of the associated input. At a value of 0.5, the modeled gain is half of its design magnitude for that timestep.

Question 29Energy Modeling Fundamentals and Theory

On a design-day weather profile, the daily dry-bulb range specifies the:

  1. Range of humidity ratios over the day
  2. Range of wind speeds over the day
  3. Difference between the peak and minimum dry-bulb temperatures Correct answer.
  4. Difference between indoor and outdoor design temperatures

Explanation

The daily range on a design day describes the spread between the maximum and minimum dry-bulb temperatures used to shape the hourly profile. It is distinct from the indoor-outdoor design temperature difference.

Question 30Energy Modeling Fundamentals and Theory

A zone multiplier of 8 assigned to a modeled guest room means that the model:

  1. Eight separate HVAC systems for the room
  2. Eight times the outdoor air to that room
  3. Eight times the room volume for comfort calculations
  4. Eight identical rooms sharing loads and schedules Correct answer.

Explanation

A zone multiplier lets one representative zone stand in for several identical zones, scaling results in totals while simulating the representative zone once. It does not multiply airflow, system count, or volume in the simulated zone.

Question 31Energy Modeling Fundamentals and Theory

In a zone load calculation, the sensible-to-latent split of heat gain from occupants depends primarily on:

  1. The U-factor of the exterior envelope
  2. The conditioned floor area of the zone
  3. The metabolic rate of the activity Correct answer.
  4. The outdoor wind speed at the design day

Explanation

The sensible and latent portions of occupant heat gain are functions of metabolic rate and the associated activity level. Floor area, wind, and envelope U-factor do not determine the split.

Question 32Energy Modeling Fundamentals and Theory

Which weather data set is most appropriate for estimating annual energy use in a whole-building simulation?

  1. A typical meteorological year Correct answer.
  2. A single design day
  3. A peak cooling design condition
  4. Monthly utility bills only

Explanation

A typical meteorological year provides hourly records representative of long-term conditions, which is what annual energy simulations require. Design days are intended for sizing rather than annual energy estimates.

Question 33Energy Modeling Fundamentals and Theory

The ground reflectance (albedo) input in a simulation affects:

  1. Irradiance on tilted and vertical surfaces Correct answer.
  2. The zone air capacitance
  3. The rate of infiltration through the floor slab
  4. Conduction through below-grade walls

Explanation

Ground-reflected shortwave radiation contributes to the irradiance received by tilted and vertical surfaces, so albedo changes their solar gains. It does not set infiltration, below-grade conduction, or zone capacitance.

Question 34Energy Modeling Fundamentals and Theory

In a zone heat balance, the interior convective heat transfer coefficient for a wall surface is typically estimated from:

  1. The zone volume and the air changes per hour
  2. Orientation and the surface-to-air temperature difference Correct answer.
  3. The outdoor humidity ratio at the design condition
  4. The wall's thermal conductivity alone, independent of air motion

Explanation

Interior convection correlations depend mainly on surface orientation and the temperature difference driving natural convection, with air speed added for forced convection. Conductivity, zone volume, and outdoor humidity are not the primary variables.

Question 35Energy Modeling Fundamentals and Theory

When a supply fan is modeled downstream of the cooling coil, the fan energy:

  1. Is rejected entirely to the outdoors
  2. Adds sensible heat to the supply airstream Correct answer.
  3. Adds only latent heat to the airstream
  4. Lowers the supply air temperature

Explanation

Fan power is converted to heat in the airstream, and a fan downstream of the coil adds that sensible heat after cooling. This raises the supply air temperature and can increase the required coil capacity.

Question 36Energy Modeling Fundamentals and Theory

Part-load ratio (PLR) for a cooling coil is defined as the:

  1. Ratio of runtime to available time
  2. Actual output divided by full-load output Correct answer.
  3. Ratio of energy input to energy output
  4. Ratio of full-load capacity to actual capacity

Explanation

PLR compares the capacity actually delivered with the full-load capacity available at the same operating conditions. Runtime fraction is related but distinct, and efficiency is an output-input relationship.

Question 37Energy Modeling Fundamentals and Theory

For a supply duct routed through an unconditioned attic, the model should account for heat gain that:

  1. Increases the fan static pressure only
  2. Adds latent load to the attic
  3. Cools the supply air before it reaches the zone
  4. Raises supply air temperature and reduces delivered cooling Correct answer.

Explanation

Duct heat gain from a hot attic warms the supply air before it reaches the zone, reducing the sensible cooling delivered. This effect is separate from fan pressure and attic latent load.

Question 38Energy Modeling Fundamentals and Theory

The approach of a cooling tower is defined as the:

  1. Wet-bulb temperature minus dry-bulb temperature
  2. Leaving water temperature minus entering air wet-bulb Correct answer.
  3. Tower range plus the ambient dry-bulb temperature
  4. Entering water temperature minus leaving water temperature

Explanation

Cooling tower approach is how closely the leaving water temperature comes to the entering air wet-bulb temperature, calculated as leaving water temperature minus wet-bulb. The range is the water temperature drop across the tower.

Question 39Energy Modeling Fundamentals and Theory

A boiler part-load curve whose efficiency falls sharply at low part-load ratios mainly represents:

  1. The effect of outdoor reset on supply temperature
  2. Increased latent load from the boiler
  3. Cycling and standby losses at low loads Correct answer.
  4. Improved combustion at low firing rates

Explanation

At low part-load ratios, a boiler may cycle frequently and incur jacket and standby losses, reducing average efficiency. Combustion generally does not improve at low firing rates.

Question 40Energy Modeling Fundamentals and Theory

Raising the supply air temperature setpoint at low zone loads in a VAV model generally:

  1. Reduces coil capacity and increases reheat energy only
  2. Increases fan energy and increases dehumidification
  3. Reduces fan energy but may cut dehumidification Correct answer.
  4. Has no effect on fan energy or dehumidification

Explanation

Higher supply air temperature reduces the airflow needed at a given load, saving fan energy, but warmer air removes less moisture and may reduce dehumidification. It does not simply increase fan energy.

Question 41Energy Modeling Fundamentals and Theory

The solar heat gain coefficient (SHGC) of a window is the:

  1. Heat admitted, including the inward-flowing absorbed portion Correct answer.
  2. U-factor multiplied by the window area
  3. Ratio of visible light transmitted to incident visible light
  4. Fraction of incident solar absorbed by the glass only

Explanation

SHGC is the fraction of incident solar radiation that enters the space as heat, including directly transmitted radiation and the inward-flowing share of radiation absorbed by the glazing. It is broader than visible transmittance or absorptance alone.

Question 42Energy Modeling Fundamentals and Theory

In a simplified wall model, the effect of steel studs on conduction is commonly represented by:

  1. A convective coefficient added to the interior surface
  2. An area-weighted effective U-factor Correct answer.
  3. A latent heat source at the studs
  4. An increased wall thermal mass

Explanation

Framing creates parallel conduction paths, so an area-weighted effective U-factor or parallel-path calculation captures the added heat flow. The studs do not act as a latent source or change the interior convection correlation.

Question 43Energy Modeling Fundamentals and Theory

For a slab-on-grade model, the deep ground temperature is often approximated by:

  1. The peak summer wet-bulb temperature
  2. The winter design dry-bulb temperature
  3. The annual average outdoor air temperature Correct answer.
  4. The indoor setpoint temperature

Explanation

Deep ground temperature is relatively stable and is commonly approximated by the local annual average outdoor air temperature. Design extremes and indoor setpoints do not represent that stable boundary.

Question 44Energy Modeling Fundamentals and Theory

Unmet load hours in an annual simulation are used to:

  1. Measure fan energy
  2. Calculate the building's water use
  3. Estimate infiltration
  4. Check whether setpoints are maintained Correct answer.

Explanation

Unmet load hours count time when the HVAC system cannot maintain zone setpoints, which is a sizing and comfort check. They are not an energy, infiltration, or water-use metric.

Question 45Energy Modeling Fundamentals and Theory

A one-at-a-time sensitivity analysis performed on an energy model is used to:

  1. Identify the most expensive energy conservation measure
  2. Generate a local weather file
  3. Rank input effects on simulation outputs Correct answer.
  4. Calibrate the model to utility bills

Explanation

One-at-a-time sensitivity varies inputs individually to see how outputs respond, which helps rank influential parameters. It is not a calibration method and does not create weather data.

Question 46Energy Modeling Fundamentals and Theory

In a zone heat balance, the radiant fraction of internal gains mainly affects:

  1. The wall conduction transfer function coefficients
  2. The total magnitude of the internal gain
  3. The radiant-convective split of the gains Correct answer.
  4. The infiltration rate through the envelope

Explanation

The radiant fraction determines how much of a gain heats surfaces for later release and how much enters the zone air directly. It changes the transient temperature response, not the total gain magnitude.

Question 47Energy Modeling Fundamentals and Theory

A zone model that omits moisture adsorption and desorption on interior surfaces will tend to:

  1. Overpredict the amplitude of zone humidity swings Correct answer.
  2. Predict no latent load from occupants
  3. Underpredict peak humidity ratios
  4. Eliminate the need for a cooling coil

Explanation

Surface moisture buffering absorbs and releases water vapor, damping zone humidity swings. Omitting it leaves the modeled humidity more responsive, so the swings are exaggerated.

Question 48Energy Modeling Fundamentals and Theory

For a tilted surface, the cosine of the solar angle of incidence is calculated using:

  1. Zenith angle, tilt, and azimuth difference Correct answer.
  2. Only the solar altitude
  3. Dry-bulb temperature and wind speed
  4. Only the surface azimuth

Explanation

The incidence cosine depends on the sun's zenith angle, the surface tilt, and the difference between the solar and surface azimuths. Altitude alone or surface azimuth alone is insufficient.

Question 49Energy Modeling Fundamentals and Theory

A single-speed DX unit operating below its minimum stable capacity is modeled as:

  1. Cycling on and off with a degradation coefficient Correct answer.
  2. Operating at full capacity without cycling losses
  3. Modulating continuously to match the load
  4. Turning off until the load reaches full capacity

Explanation

A single-speed unit cannot modulate below its minimum, so it cycles to match average load. A degradation coefficient represents the efficiency penalty from cycling.

Question 50Energy Modeling Fundamentals and Theory

Increasing the zone air capacitance multiplier above 1 in a heat balance model has which effect?

  1. It reduces the required HVAC capacity
  2. It increases the zone's physical air volume
  3. It damps numerical instability and distorts peaks Correct answer.
  4. It changes the wall transfer function coefficients

Explanation

A capacitance multiplier is a numerical device that slows the modeled air-temperature response, which can stabilize iterations but distorts fast transients and peak temperatures. It does not alter physical volume or construction coefficients.