Preparing Model Inputs

The process of preparing model inputs is similar across the different model types: a description of the building, its constructions, and systems are entered into the simulation software. However the level of detail, scope of analysis, and source of input data will vary depending on the phase of design. Early-stage analysis may use a simpler representation of the building geometry and rely more on assumptions, defaults, or "typical" inputs, whereas later phases aim to provide inputs that are as representative as possible of actual information on the design documents.

Model Input Categories

Building Geometry

The model's building geometry is a representation of the building's physical design and its surfaces such as walls, floors, roofs, windows and doors. The geometry definition will also specify which surfaces are exposed to the exterior, surfaces that are in contact with the earth (floor slabs, basements), and any features that cast shadows on the building. The level of geometry detail will vary depending on the stage of design. Read the following tutorial for guidance on building geometry input approaches for different model types.

• Tutorial: Define the building geometry

Project and Site Information

These inputs include general information about the building, its location, and information about site features that may impact the building's performance.

• Tutorial: Evaluate site conditions
• Tutorial: Weather data selection
• Tutorial: Set the building orientation

Envelope

These inputs define the properties of the building enclosure (walls, windows, roofs, floors).

• Tutorial: Define opaque envelope constructions
• Tutorial: Define fenestration (glazed constructions)
• Tutorial: Define shading features

Room/Zone Information

These inputs define how rooms and zones are occupied and operation of energy consuming equipment, and sources of internal loads within a space.

• Tutorial: Define internal loads (occupants, lighting, equipment)
• Tutorial: Define infiltration

HVAC System Type

HVAC system input approaches can vary significantly depending on the software program being used. System inputs may be either component-based (highly flexible, but more effort to define the system) or menu-based (predefined system types with the ability to modify some aspects of the system via menus or other options). Read more about HVAC system input approaches.

• Tutorial: Selecting appropriate HVAC systems

Zone HVAC

Important zone-level HVAC inputs include outdoor air ventilation rate, thermostat setpoints and terminal unit controls. Some single zone HVAC system types are also assigned directly at the zone-level inputs or assigned directly to individual thermal zones in the model.

• Tutorial: Ventilation rates - calculations and inputs for mechanical ventilation
• Tutorial: Define thermostat setpoints
• Tutorial: Define terminal unit control settings

Airside HVAC

• Tutorial: Airside HVAC assumptions for early-stage models
• Tutorial: Gathering HVAC input data for detailed design models

Waterside HVAC

• Tutorial: Waterside HVAC assumptions for early-stage models

Domestic Hot Water (DHW) Systems

• Tutorial: DHW system assumptions for early stage-models

Other Energy End Uses

• Tutorial: Other end-uses overview

Specifying Output Details and Metrics

• Tutorial: Utility rate inputs and calculating energy costs

Determining Appropriate Inputs

Sources of input data in different stages of design. (Source: IBPSA-USA BEM Workshop)

The source of information needed to provide inputs for a model will vary depending on the stage of design and analysis objective.

Simple Box Models

In most cases, limited project-specific design information is available at the time when a simple box model is created. By necessity, many model inputs will be based on typical or default values. Where project-specific information is not yet available, the goal should be to develop inputs that represent typical performance and operation.  

Also consider prioritizing effort on inputs that are most relevant to the design questions being evaluated. There is almost always a limited amount of time available to perform a simple box modeling exercise. If, for example, one goal of the analysis is an accurate EUI estimate, then it will be important to gather information about likely operating schedules, plug loads and any likely unusual energy end uses, because they can have a significant impact on total energy consumption. On the other hand, if the goal is to understand the relative impact of alternatives such as varying building form or window area, then generic operating schedules and internal loads may be fine.  

As much as possible, the model should include project-specific inputs. Typical sources of project-specific information include documentation about the building program, owner’s project requirements, and conceptual design sketches. Interviews of the owner and design team can also provide useful information on the following items.

• Energy performance goals
• Expected number of occupants
• Expected operating schedule
• Building geometry constraints
• Likely presence of end uses such as exterior lighting, commercial cooking, refrigeration, computer servers or unusual plug loads.
• HVAC system preferences
• Operating data from other similar buildings, perhaps buildings occupied by the same owner
• Examples of previous projects that are being used to inform the design of the current project (if any).

Where project-specific information is not yet available, information on typical performance inputs can come from a number of sources.

• Modeler experience
• Default values from software
• Energy codes
• Other references (described later below)

Regardless of the source of input information, getting the design team to review the list of assumptions is a valuable step to take before developing the model. Create a table listing important assumptions, such as envelope component performance, lighting power, plug loads, HVAC system type and efficiency, and operating schedules and setpoints. See the section Presenting an Input Summary for an example. Consider also highlighting uncertain assumptions, especially those that might have a big impact on the analysis, so the design team can prioritize their review. The review helps in a few ways. It can help the model to be a better representation of actual expected performance. It can also help get the team to start thinking about factors that affect performance.

Detailed Models

As the design progresses into the design development and construction documents phases, much more detailed information about the building's actual design will be available on the project drawings and specifications. Detailed design models will be created from information including:

• Plans—architectural, mechanical, electrical, plumbing
• Specifications
• Basis of design document
• Sequence of operations
• Discussions with the owner and design team

The architectural drawings can provide information on building geometry and space/zone configurations, wall assembly sections, elevation drawings that indicate where different wall assemblies are used and window geometry; HVAC drawings show how the systems serve the zones, call out flow rates and equipment sizes; lighting drawings show amount of lighting installed in each space/zone and indicate power of each lighting fixture. The project specifications can provide additional information on design setpoints, efficiency requirements, operational parameters, and sequences of control/operation for systems.

The tutorials above provide more guidance on how to gather design information and use it for the model inputs.