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.
Terrain
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.
Run Special Building Block Calculations During Design
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.
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.
Include Plenum in Load Sizing
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.
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.
Sizing Safety Factor (Cooling Design)
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.”
Solar Distribution (Cooling Design)
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.
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.
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).
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.
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).
Adiabatic Ground Slab During Design (Cooling Design)
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.
Optimize Warmup Days for Design Simulation (Cooling Design)
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.
Sizing Safety Factor (Heating Design)
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).
Inside Convective Algorithm
The model specified in this field is the default algorithm for the inside face all the surfaces.
The Simple model applies constant heat transfer coefficients depending on the surface orientation.
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.
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.
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.
Loads Convergence Tolerance
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.
Outside Convection Algorithm
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.
The TARP algorithm was developed for the TARP software and combines natural and winddriven convection correlations from laboratory measurements on flat plates.
DOE-2 (default) uses a correlation from measurements by Klems and Yazdanian for rough surfaces.
MoWitt uses a correlation from measurements by Klems and Yazdanian for smooth surfaces and, therefore, is most appropriate for windows.
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.
Inside Face Surface Temperature Convergence
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.
Lower values may further increase stability at the expense of longer runtimes, while higher values may decrease runtimes but lead to possible instabilities.
Maximum Surface Convection Heat Transfer Coefficient Value
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.
Minimum Surface Convection Heat Transfer Coefficient Value
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.
Temperature Convergence Tolerance
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.
Surface Temperature Upper Limit
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”.
Zone Air Heat Balance Algorithm
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.
The Analytical Solution selection uses the integration approach to solve the zone air energy and moisture balance equations.
The Euler Method selection uses the first order finite backward difference approximation to solve the zone air energy and moisture balance equations.
Automatically increase VAV system airflow to meeting heating demand
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.
Remove Curves from Load Design
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).
Sizing System Preheat Coil for Ventilation Load
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).
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.
