Simulation Setting
In the Simulation Settings sub-section, parameters that influence the simulation can be adjusted on an alternative by alternative basis. These settings are broken down into three categories; General Settings, Load Design Parameters and Energy Simulation Parameters. For novice users it is strongly recommended to utilize the defaults settings that are provided with the program as some parameters have significant implications to both accuracy and runtime that should be understood before making adjustments.
In the General Settings tab, parameters that impact both load design and energy or economic analysis are defined. In the Load Design Parameters tab, parameters that are either specific to HVAC equipment sizing or have a stronger influence on the HVAC sizing can be adjusted, even though some settings on this tab may still have a minor influence on energy and economic simulation. In the Energy Simulation Parameters tab, parameters that are either specific to energy or economic simulation or have a stronger influence on energy or economics can be adjusted.
General Settings
In the General Settings tab, the user defines the simulation time frame, reference calendar, time step, and heat balance algorithm. In addition the user can adjust advanced shadow settings or set the start/end for a daylight savings period.
Holidays Calendar
This calendar sets which days will be interpreted as holiday day-types as defined in the Schedules Library. Holidays often have significantly different operational characteristics and changing the calendar allows you to quickly account for regional holiday impacts. The icon next to the Calendar selection can take you to the Calendar Library where you can see the properties of an existing calendar or create your own.
Simulation Start/End
This field defines the starting month and day for the simulation run period in case you want to do an analysis for a specific period or to match the billing cycle. These dates need to match the selected Holidays Calendar.
Simulation Algorithm
The Simulation Algorithm provides a way to select what type of heat transfer algorithm will be used for calculating the performance of the building’s surface assemblies. There are two options:
● Conduction Transfer Function: is a sensible heat only solution and does not take into account moisture storage or diffusion in the construction elements
● Conduction Finite Difference selection is also a sensible heat only solution that does not take into account moisture storage or diffusion in the construction elements. This solution technique however uses a 1-D finite difference solution in the construction elements. The Conduction Finite Difference considers materials phase change as well as thermal conductivity.
Simulation Time Step
This field specifies the time step for the simulation. The value entered here is used in the Heat Balance Model calculation as the driving time step for heat transfer and load calculations. This value determines the number of time steps to use each hour. The choice made for this field has important implications for modeling accuracy and the overall time it takes to run a simulation. Here are some considerations when choosing a value:
● Simulation results are more accurate for shorter time steps
● Shorter time steps improve the numerical solution of the Heat Balance Model because they improve how models for surface temperature and zone air temperature are coupled together
● Longer time steps introduce more lag and lead to more a dampened dynamic response
● Simulation run time increases with shorter time steps
● Although the weather data files usually have hourly data, the program interpolates the weather data between data points for use at shorter time steps.
● For analysis of the maximum rates for electricity use or demand charges, the length of the time step needs to be consistent with the tariff’s demand window.
Minimum warm up Days
This field specifies the minimum number of “warm up” days before the program will check if it has achieved convergence and can thus start simulating the particular environment in question. Research into the minimum number of warm up days indicates that six warm up days is generally enough on the minimum end of the spectrum to avoid false predictions of convergence and thus to produce enough temperature and flux history to start simulation. It also was observed that convergence performance improved when the number of warm up days increased.
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Default: 1
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Min & Max: 1 < x < Maximum warm up Days
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Typical Range: 6
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Units: days
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Maximum warm up Days
This field specifies the maximum number of “warm up” days that might be used in the simulation before “convergence” is achieved. The default number, 25, is usually more than sufficient for this task; however, some complex buildings (with complex constructions) may require more days.
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Default: 25
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Min & Max: Minimum warm up Days < x
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Typical Range: 25
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Units: days
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Enable Daylight Savings
This field specifies whether to consider if there is a Daylight saving schedule. When the box is checked, you will need to select the Daylight Savings Start and Stop dates and the program will adjust the schedules by one hour.
Advanced Shadow Settings
Shadowing calculation is important for determining the amount of sun entering the building and therefore the amount of cooling or heating load needed for maintaining the building setpoints.
Calculation Frequency
This field will cause the shadowing calculations to be done periodically using the number in the field as the number of days in each period. Using the default of 20 days in each period is the average number of days between significant changes in solar position angles.
Calculation Method
This field is used to control how the solar, shading, and daylighting models are calculated with respect to the time of calculations during the simulation. The default and fastest method is Average over Days in Frequency. A more detailed and slower method is the Timestep Frequency method used for modeling dynamic fenestration and shading surfaces.
Polygon Clipping Algorithm
The internal polygon clipping method is used to treat overlapping shadows.
There are two available options:
1. Sutherland-Hodgman (default): is a simpler algorithm but it works well in cases where receiving surfaces (of shadows) are non-convex.
2. Convex Weiler- Atherton: is only accurate where both casting and receiving surfaces are convex.
Maximum Overlapping Shadows
This field will allow to increase the number of figures in shadow overlaps. Due to the shadowing algorithm, the number of shadows in a figure may grow quite large even with fairly reasonable looking structures. Of course, the inclusion of more allowed figures will increase calculation time. Likewise, too few figures may not result in as accurate calculations as desired.
Sky Diffuse Algorithm
Depending on the shadowing surfaces, there are two options:
● Simple Sky Diffuse Modeling (default): performs a one-time calculation for sky diffuse properties.
● Detailed Sky Diffuse Modeling is meant for surfaces where the shading transmittance varies. When the detailed modeling is selected, the Calculation Frequency should be greater than one. This option will increase calculation time.
Use Weather File Snow Indicators
Weather files can contain “snow” indicators. Snow changes the reflectivity of the ground and cascades changes of this reflectivity. Checking this field allows the weather file conditions to account for the snow if it is included in the weather file.
Use Weather File Rain Indicators
Weather files can contain “rain” indicators. Rain indicates wet surfaces which change the film convection coefficient for the surface. Checking this field allows the weather file conditions to account for that rain if it is included in the weather file.
Set All Radian Fractions to Zero
Shadowing calculation is importan
No = Do nothing (default).
Yes = Override all internal load radiation fractions to be zero. The override will not be observed in the user interface. The affect will be observed in the results.
Purpose is to force all internal loads to be instant sensible or latent load on the zone load by component report. With all internal radiation removed from the model, the only remaining source of radiation would be the sun. Radiant loads affect Time Delay Sensible.
Set All Lost Heat Fractions to Zero
No = Do nothing (default).
Yes = Override all miscellaneous equipment load lost heat fractions to be zero. The override will not be observed in the user interface. The affect will be observed in the results.
Lost heat is energy consumed that does not create heat load. Setting this value to zero ensures all energy consumed produces heat load.
Set All Light Return Air Path Fractions to Zero
No = Do nothing (default).
Yes = Override all light return air path heat fractions to be zero. The override will not be observed in the user interface. The affect will be observed in the results.
Purpose is to force the RA Sensible (Lights) row on the zone load by component report be zero and push the load up to the Lights category. RA Sensible (Lights) is not included for the zone airflow calculation, while the Lights category is included for the airflow calculation.
Round Up Occupancy
No = Do nothing (default).
Yes = Round each people load up to the next whole integer. The override will not be observed in the user interface. The affect will be observed in the results.
In the example below, 200 ft2/person was input. The result would have been 12.5 people in the room. The setting rounded the load to 13 people in the results.
Result can be observed on the Zone Load by Component Report.
Load Design Parameters
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In the Load Design Parameters tab, the user defines parameters related to calculation methods, iteration convergence limits, and overrides such as safety factors. Expand all sections to see all parameters.
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Terrain
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The site’s terrain affects how the wind hits the building – as does the building height. The options provide the engine with general assumptions to make about how wind impacts the building.
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Run Special Building Block Calculations During Design
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Setting this field to yes turns on an additional set of calculations to compare the conservative sizing calculations with a sizing run that more closely reflects the actual operation of the building. The comparison is reported as the over/undersizing output in the coils tables of the System Component Summary report. This will also be reported on the Psychrometric report as a secondary set of tables.
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Turning this on will increase calculation time and does not have an impact on the calculated equipment capacities used in the load and energy runs.
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Include Plenum in Load Sizing
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When this field is set to No, all loads that would go to the plenum are assumed to go direct to the space for load sizing calculations. (Plenum loads always go to the plenum during the energy simulation.) The return air and outside air mix to determine the coil inlet temperature. The return air does not pick up heat in the plenum so the coil inlet temperature will likely be lower than when plenums are included in load design.
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When this field is set to Yes, all the loads that would go to the plenum are assumed to go to the plenum for load sizing calculations. (Plenum loads always go to the plenum during the energy simulation.) The return air and outside air mix to determine the coil inlet temperature. The return air does pick up heat in the plenum so the coil inlet temperature will likely be higher than when plenums are not included in load design.
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Sizing Safety Factor (Cooling Design)
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The user may specify a sizing factor on the cooling capacity of equipment. It is recommended to leave this value as 1 and, instead, input the safety factors where desired (internal loads, airflows, ventilation, etc). The 2021 ASHRAE Handbook of Fundamentals, Section 1.2 Cooling Load Calculation Methods, page 18.2 states “All calculation inputs should be as accurate as reasonable, without using safety factors. Introducing compounding safety factors at multiple levels in the load calculation results in an unrealistic and oversized load.”
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Solar Distribution (Cooling Design)
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Minimal Shadowing: In this case, there is no exterior shadowing except from window and door reveals. All beam solar radiation entering the zone is assumed to fall on the floor, where it is absorbed according to the floor’s solar absorptance. Any reflected by the floor is added to the transmitted diffuse radiation, which is assumed to be uniformly distributed on all interior surfaces. If no floor is present in the zone, the incident beam solar radiation is absorbed on all interior surfaces according to their absorptances. The zone heat balance is then applied at each surface and on the zone’s air with the absorbed radiation being treated as a flux on the surface.
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Full Exterior: In this case, shadow patterns on exterior surfaces caused by detached shading, wings, overhangs, and exterior surfaces of all zones are computed. As for Minimal Shadowing, shadowing by window and door reveals is also calculated. Beam solar radiation entering the zone is treated as for Minimal Shadowing.
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Full Exterior With Reflections: This case is the same interior distribution as the preceding option but uses exterior reflections as well (see the section Solar Radiation Reflected from Exterior Surfaces for further explanation).
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Full Interior And Exterior: This is the same as Full Exterior except that instead of assuming all transmitted beam solar falls on the floor the program calculates the amount of beam radiation falling on each surface in the zone, including floor, walls and windows, by projecting the sun’s rays through the exterior windows, taking into account the effect of exterior shadowing surfaces and window shading devices. If this option is used, you should be sure that the surfaces of the zone totally enclose a space.
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Full Interior And Exterior With Reflections: This case is the same interior distribution as the preceding option but uses exterior reflections as well (see Solar Radiation Reflected from Exterior Surfaces for further explanation).
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Adiabatic Ground Slab During Design (Cooling Design)
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This option will prevent heat transfer from the slab to the earth during cooling design calculations. This option will not remove the heat transfer from the room to the mass of the slab itself.
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Optimize Warmup Days for Design Simulation (Cooling Design)
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This option will assume the ending conditions of the previous design day for the next design day. If this is disabled, the engine will recalculate the steady state initial condition before starting each design day scenario.
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Sizing Safety Factor (Heating Design)
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The user may specify a sizing factor on the cooling capacity of equipment. It is recommended to leave this value as 1 and, instead, input the safety factors where desired (internal loads, airflows, ventilation, etc).
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Inside Convective Algorithm
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The model specified in this field is the default algorithm for the inside face all the surfaces.
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The Simple model applies constant heat transfer coefficients depending on the surface orientation.
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The TARP (default) model correlates the heat transfer coefficient to the temperature difference for various orientations. This model is based on flat plate experiments. The Ceiling Diffuser model is a mixed and forced convection model for ceiling diffuser configurations. The model correlates the heat transfer coefficient to the air change rate for ceilings, walls and floors. These correlations are based on experiments performed in an isothermal room with a cold ceiling jet. To avoid discontinuities in surface heat transfer rate calculations, all of correlations have been extrapolated beyond the lower limit of the data set (3 ACH) to a natural convection limit that is applied during the hours when the system is off.
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The Adaptive Convection Algorithm model is an dynamic algorithm that organizes a large number of different convection models and automatically selects the one that best applies.
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The ASTMC1340 model correlates mixed convection coefficients to the surface-to-air temperature difference, heat flow direction, surface tilt angle, surface characteristic length, and air speed past the surface. These correlations are based on ASTM C1340 standard.
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Loads Convergence Tolerance
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This value represents the number at which the loads values must agree before “convergence” is reached. Loads tolerance is the change in peak zone heating and cooling loads that are predicted from previous warmup day to the current day.
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Outside Convection Algorithm
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The Simple convection model applies heat transfer coefficients depending on the roughness and windspeed. This is a combined heat transfer coefficient that includes radiation to sky, ground, and air. The correlation is based on Figure 1.143, Page 25.1 (Thermal and Water Vapor Transmission Data), 2001 ASHRAE Handbook of Fundamentals. Note that if Simple is chosen here or in the Zone field and a SurfaceProperty:ConvectionCoefficients object attempts to override the calculation with a different choice, the action will still be one of combined calculation. To change this, you must select one of the other methods for the global default. All other convection models apply heat transfer coefficients depending on the roughness, windspeed, and terrain of the building’s location. These are convection only heat transfer coefficients; radiation heat transfer coefficients are calculated automatically by the program.
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The TARP algorithm was developed for the TARP software and combines natural and winddriven convection correlations from laboratory measurements on flat plates.
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DOE-2 (default) uses a correlation from measurements by Klems and Yazdanian for rough surfaces.
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MoWitt uses a correlation from measurements by Klems and Yazdanian for smooth surfaces and, therefore, is most appropriate for windows.
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The Adaptive Convection Algorithm model is an dynamic algorithm that organizes a large number of different convection models and automatically selects the one that best applies. Note that when the surface is wet (i.e. it is raining and the surface is exposed to wind) then the convection coefficient appears as a very large number (1000) and the surface is exposed to the Outdoor Wet-bulb Temperature rather than the Outdoor Dry-bulb Temperature.
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Inside Face Surface Temperature Convergence
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The surface heat balance model at the inside face has a numerical solver that uses a convergence parameter for a maximum allowable differences in surface temperature. This field can optionally be used to modify this convergence criteria. The default value is 0.002°C and was selected for stability.
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Lower values may further increase stability at the expense of longer runtimes, while higher values may decrease runtimes but lead to possible instabilities.
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Maximum Surface Convection Heat Transfer Coefficient Value
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This optional field is used to set an overall maximum for the value of the coefficient for surface convection heat transfer (Hc) in W/m2-K. High Hc values are used in EnergyPlus to approximate fixed surface temperature boundary conditions. This field can be used to alter the accepted range of user-defined Hc values.
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Minimum Surface Convection Heat Transfer Coefficient Value
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This optional field is used to set an overall minimum for the value of the coefficient for surface convection heat transfer (Hc). A minimum is necessary for numerical robustness because some correlations for Hc can result in zero values and create numerical problems. This field can be used to support specialized validation testing to suppress convection heat transfer and to investigate the implications of different minimum Hc values.
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Temperature Convergence Tolerance
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This value represents the number at which the zone temperatures must agree (from previous iteration) before “convergence” is reached. Convergence of the simultaneous heat balance/HVAC solution is reached when either the loads or temperature criterion is satisfied.
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Surface Temperature Upper Limit
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This field specifies the upper limit for surface temperature. If surfaces reach temperatures above this limit, the calculation will halt with “temperature out of bounds”.
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Zone Air Heat Balance Algorithm
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The Third Order Backward Difference selection is the default selection and uses the third order finite difference approximation to solve the zone air energy and moisture balance equations.
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The Analytical Solution selection uses the integration approach to solve the zone air energy and moisture balance equations.
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The Euler Method selection uses the first order finite backward difference approximation to solve the zone air energy and moisture balance equations.
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Automatically increase VAV system airflow to meeting heating demand
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This field applies to the vav minimum position. With this checked, the minimum stop position on the VAV box will be equal to the greater between ventilation airflow and heating airflow. The cooling airflow may be oversized to force the turndown ratio to be true.
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Remove Curves from Load Design
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This field applies to heat pumps of any kind. When checked, none of the Energy Plus adjustments to load design based on the background equipment unloading curves will occur. If unchecked, the design loads will be normalized to the values in the background equipment unloading curves. Default on new files is to remove curves from load design (checked).
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Sizing System Preheat Coil for Ventilation Load
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This field applies to the heating coil in main handlers. When checked, a comparison between design day ventilation load and simulation loads will occur, and the coil will be sized for the larger of the two. If unchecked, the peak simulation heating load will be used, without a comparison to the design day ventilation load. Default on new files to compare heating loads to ventilation load (checked).
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Override Cooling Design Schedules
No = Do nothing (default).
Yes = Override all internal load cooling design schedules to be 100 % for all hours of the year. The override will not be observed in the user interface. The affect will be observed in the results.
Purpose is to help the user make sense of cooling loads if a schedule is doing something different than 100 % for all hours of the year.
Override Heating Design Schedules
No = Do nothing (default).
Yes = Override all internal load heating design schedules to be 0 % for all hours of the year. The override will not be observed in the user interface. The affect will be observed in the results.
Purpose is to help the user make sense of heating loads if a schedule is doing something different than 0 % for all hours of the year.
Energy Simulation Parameters
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Energy Simulation Parameters
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This section pertains to simulation settings which primarily impact the energy analysis. Expand all sections to see all inputs.
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Air Velocity for Comfort Calculations
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This field sets the air velocity for ASHRAE 55 comfort calculations.
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Maximum Data Visualization Variables
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This field sets the maximum number of variables that may be produced in a single calculation. The purpose is to prevent memory overload. User may adjust this at their discretion.
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Minimum System Timestep
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This field sets the timestep of the system calculations. This is different from the overall Simulation Timestep in General Settings. The system calculation timestep subdivides the simulation timestep. The default is four system timesteps per simulation timestep. In new projects, the default general simulation timestep is 60 minutes and the minimum system timestep is 15 minutes, a factor of four.
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Maximum HVAC iterations
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This field sets the maximum number of system iterations the engine is allowed to perform. The purpose of this field is to reduce time if the iterations have difficulty converging.
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Minimum Plant Iterations
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This field sets the minimum number of plant iterations the engine is allowed to perform. The purpose of this field is to allow the user to force a minimum in case the plant loops are not behaving as expected.
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Maximum Plant Iterations
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This field sets the maximum number of plant iterations the engine is allowed to perform. The purpose of this field is to reduce time if the iterations have difficulty converging.
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Temperature Capacity Multiplier
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This field is used to alter the effective heat capacitance of the zone air volume. This affects the transient calculations of zone air temperature. Values greater than 1.0 have the effect of smoothing or damping the rate of change in the temperature of zone air from timestep to timestep. Note that sensible heat capacity can also be modeled using internal mass surfaces.
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Humidity Capacity Multiplier
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This field is used to alter the effective moisture capacitance of the zone air volume. This affects the transient calculations of zone air humidity ratio. Values greater than 1.0 have the effect of smoothing.
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Carbon Dioxide Capacity Multiplier
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This field is used to alter the effective carbon dioxide capacitance of the zone air volume. This affects the transient calculations of zone air carbon dioxide concentration. Values greater than 1.0 have the effect of smoothing or damping the rate of change in the carbon dioxide level of zone air from timestep to timestep.
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CBECS Benchmarking for Site Consumption
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The U.S. Energy Information Administration has taken surveys of commercial building energy consumption called the Commercial Buildings Energy Consumption Survey (CBECS). These surveys take data from different building types in different years and can be used as a benchmark for comparing individual building’s energy consumption. Select a Building Type and CBECS Survey Year in the fields in this section to compare the current model to the CBECS averages. The output comparison will appear in the Site Consumption Summary report.
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District Heating Efficiency
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This field sets the efficiency of district heating plants. An input of 0.3 means the district heating plant is 30 percent efficient. In other words, the district heating plant would consume 1/0.3 = 3.33 times the energy than the heating load.
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District Cooling COP
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This field sets the COP of district cooling plants. An input COP of 3 means the district cooling plant produces 3 units of cooling for every 1 unit of raw energy consumed. In other words, the district cooling plant would consume 1/3 = 0.333 times the energy than the cooling load.
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Steam Conversion Efficiency
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This field sets the efficiency of steam generation. An input of 0.25 means the steam heating plant is 25 percent efficient. In other words, the steam heating plant would consume 1/0.25 = 4 times the energy than the heating load.
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Equivalent Emission Factors
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These fields set the equivalent greenhouse potential of nitrous oxide, methane, and carbon dioxide. Nitrous oxide defaults to a potency 298 times greater than carbon dioxide, and methane defaults to a potency 25 times greater than carbon dioxide. See the Carbon Footprint and Renewable Energy Summary.
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Source Energy Factors
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These fields set the efficiency of delivering energy to the site from the source. For example, electricity defaults to 3.167, meaning that 3.167 units of raw energy are consumed in the power plant and transmission lines for every 1 unit delivered to the site for use. See the source energy consumption in the Site Consumption Summary report.
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