Calculate HVAC loads with BIM

3-D building information modeling tools are improving the process of calculating peak cooling and heating loads for buildings.

By Stephen Roth, PE, LEED AP, Carmel Software Corp., San Rafael, Calif. August 1, 2009

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HVAC cooling and heating load analysis for buildings currently is performed by using widely available software tools. For the past 25 years, this method has become the status quo for HVAC design engineers, and it has proven to be an effective way to accurately calculate building cooling and heating losses.

However, this method has some disadvantages, including tediousness of inputting data for large buildings and difficulty in reusing the inputted building data for other types of analysis such as lighting and energy calculations.

With improvements in 3-D modeling software tools and more powerful computers, a different method of performing HVAC cooling loads has emerged. This method allows designers to create smart 3-D building models that can export all of the building information to analysis software tools to perform peak HVAC cooling and heating load analysis. This article discusses the evolution of this technology and its advantages over older methods.

The traditional method

Calculating HVAC cooling and heating loads for buildings is essential to properly sizing HVAC equipment such as rooftop units, chillers, and boilers. The information required to properly calculate loads includes areas and properties of the building envelope components, quantities and types of internal loads such as building occupancy, lighting density and equipment types, weather and solar data, and properties of the HVAC system itself. The process of modeling a large building often requires a tremendous amount of information.

HVAC engineers have been performing load calculations in various forms for many years. Initially, these calculations were done on paper using ASHRAE-based calculations. Another calculation method was a rule-of-thumb process of taking the total square footage of a building and multiplying it by a Btuh/sq ft factor based upon the building type and location. While this provided a quick and inexpensive way to size equipment, it often resulted in oversized equipment that caused short-cycling and much wasted energy.

The energy crisis in the 1970s was a wake-up call for HVAC engineers to more accurately calculate the cooling and heating loads for a building. The 1980s saw the advent of software that could be used by both small and large engineering firms to accurately calculate the building cooling and heating loads. This was a big change in the way HVAC engineers performed their building analysis, and it provided more accurate results than previous methods.

The format of this type of software has remained fairly consistent for 25 years. Most load calculation analysis software requires inputting data in a tabular format. This may include manually inputting the areas and properties of building envelopes based upon a takeoff from a 2-D drawing. Unfortunately, this method is often tedious for large buildings and also prone to errors. A number of software tools do include 2-D drawing functionality where values automatically transfer from the drawing to the analysis portion. However, these drawing programs do not accurately represent buildings in a 3-D world.

This conventional way of performing cooling and heating loads is flawed as demonstrated by the following items:

Manually inputting the properties of building envelope constructions and internal characteristics in a typical 10-story high-rise office complex involves specifying up to 17,000 pieces of discrete data, any of which would need to be updated based upon building modifications.

Inputting 2-D data into analysis software that really requires 3-D data properties may produce less accurate results. 2-D models are good for construction documentation. However, they are not able to store all building information and then convey that information to other analysis applications.

By manually inputting so much data, it is difficult to perform a "what-if" analysis. This type of analysis allows the user to vary, say, the R-value of a wall to see how much it will affect the total heating load, or vary the orientation of a building to minimize the cooling load. "What-if" analysis is becoming more important as engineers and architects perform sustainable design.

HVAC analysis software is quite complex, and its use often requires a solid engineering background and extensive training. This leaves architects at the mercy of HVAC engineers when doing this type of analysis at early stages of design.

The new method

While the current way of calculating HVAC cooling and heating loads has been around for 25 years, a new way has emerged that promises to revolutionize the way engineers perform their heating and cooling load analysis. This new method involves 3-D modeling authoring tools and built-in or third-party analysis tools. We will talk about each of these components in more detail.

3-D modeling authoring tools: 3-D modeling authoring tools are CAD software that allow the user to draw a realistic representation of a new or existing building. 3-D has been around for a while, but until recently, 3-D modeling tools were just a collection of lines, arcs, and circles that visually conveyed the geometry of the building and nothing else.

With the advent of building information modeling (BIM) authoring tools, these 3-D models have become much more intelligent. Not only do they present the 3-D geometry of a building, but they also contain detailed information about each of the components of the building including wall types, window types, number of people, equipment characteristics, and any other information important for analysis purposes. We now have the capability to create an information-rich 3-D model that can be used not only for construction documentation, but also for consumption by analysis tools for calculating cooling and heating loads, and performing building energy and lighting analysis.

Some examples of 3-D BIM-enabled authoring tools are Autodesk Revit , Bentley Building Mechanical Systems , and Graphisoft ArchiCAD .

Built-in or third-party analysis tools: A number of 3-D modeling tools also include built-in analysis capabilities for calculating heating and cooling loads and performing lighting analysis. This type of functionality is very convenient for the end user because all authoring and analysis capabilities are located within one software tool.

However, many engineers are comfortable with the stand-alone third-party analysis software that they have been using for many years. Therefore, a new method of interoperability has emerged that allows the transfer of information from 3-D models to existing stand-alone analysis applications.

Comparing methods

The use of BIM provides advantages over more traditional methods of performing general building analysis, including:

It eliminates the need to manually input building data into tabular-input software programs.

Since it speeds up the entire authoring/analysis process, it allows for much more what-if analysis. Architects can easily rotate a building and determine which orientation is the optimal one for reducing energy usage. Using conceptual analysis tools is appropriate at the early stage of building design because architects are looking for the energy usage delta comparing one scenario to another.

BIM adheres to the "write once, run anywhere" paradigm where just one source of geometric and building data exists that can be reused by multiple analysis tools.

BIM can be used at all stages of the building construction and design lifecycle process, allowing building owners and facility managers to refer to it for maintenance and operations purposes.

Additional advantages to using a 3-D BIM model specific for heating and cooling load analysis include the following:

It provides the ability to take into account adjacent buildings that shade the building being analyzed. Conventional tabular input-type software does not accurately represent shading caused by adjacent buildings or even large trees. Usually, the engineer has to tweak the software by specifying a wall that is 100% shaded at a northern-facing orientation (in the Northern Hemisphere). With 3-D modeling software, the user can include the models of adjacent buildings. This better represents the shading effects for different times of the day, and it more accurately models the building’s daily cooling load profile.

It provides an "analytical" version of the model so that users can see which parts of the building are actually being analyzed versus which parts are being ignored. A fully formed 3-D model of a building is required for construction documentation purposes. However, not every component of the 3-D model is required for engineering analysis purposes. For example, a simple wall located between two conditioned rooms will not affect the total cooling and heating loads. Therefore, an analytical model of the full 3-D model is required that shows only those surfaces and other building components that are used by the analysis tools. This provides the user with a snapshot of what is being analyzed.

It allows the user to re-import calculated results into the 3-D model and view the results in a more visual manner. In other words, the user is able to observe the relative amounts of cooling and heating loads required for each room by displaying color codes that represent ranges of heating/cooling load values.

It provides the ability to use the results from the analytical tools to accurately size mechanical piping and duct systems.

The 3-D model could potentially be used to control and monitor BAS. By using the same 3-D model to both design and control/monitor the building could help owners and facility managers compare actual versus expected building behavior.

There are some disadvantages to using 3-D BIM versus tabular input methods for load calculation purposes including:

Learning to use 3-D modeling tools requires in-depth training and a new way of thinking about mechanical design. Many engineers simply may be too entrenched in their ways to accept such a shift in thinking. In addition, the cost of the software licenses and training may be prohibitive for many engineering firms. Complex 3-D modeling software often requires at least a week of formalized training and many months of on-the-job learning. Often, it is advised that engineers using these authoring tools start out with a simple project and progressively tackle more complex ones.

It requires a new way of thinking about plenums and how they are interpreted in heating/cooling load calculations. Plenums are no longer considered just properties of the occupied spaces themselves, but separate entities. Plenums are defined as unoccupied spaces in a building where return air is free to travel to the inlet of a fan. Plenums often are adjacent to occupied spaces so there is some heat gain/loss that affects the occupied space load and also the temperature of the air entering the fan. Traditional load calculation applications have the user input the characteristics of a plenum for each occupied area. Therefore, if a plenum spans more than one room, this information may have to be input more than once. This often results in duplicate data entry and difficulty in representing the true geometry and adjacency of the plenum. In a 3-D modeling application, these issues are resolved. A space that is going to be a plenum can be defined as such, and the model will automatically determine which occupied spaces are adjacent to the plenum. This eliminates duplication of entries and also helps the user better visualize the true characteristics of the plenum and how it relates to the other spaces.

Third-party load calculation applications need constant updating so that they can properly import BIM from the latest version of 3-D modeling tools. Luckily, this has been partially resolved by developing interoperability schemas discussed below.

Interoperability using gbXML

A number of 3-D modeling software vendors offer both modeling and analysis capabilities within the same software program. However, so many engineers are comfortable with the stand-alone analysis software tools they have used for years that it is hard for them to learn and trust the analysis results from these modeling applications. Fortunately, it is possible for engineers to have the best of both worlds: enjoy the advantages of 3-D modeling while continuing to use their beloved analysis applications. This is accomplished via interoperability, meaning that an authoring application can export its building information to a standard format (aka schema) that can then be imported into any number of analysis applications.

A commonly accepted interoperability schema that is becoming more widely used in the building analysis industry is Green Building XML (gbXML). This is an open file format, or schema, that allows for the transfer of building data from 3-D modeling applications to analysis software. It has been in existence for 10 years and was developed by Green Building Studio. Autodesk acquired Green Building Studio’s assets in 2008, including gbXML, but Autodesk does not own gbXML. In fact, a new advisory board has been formed that comprises a number of different companies that will manage the schema going forward.

gbXML has become quite popular due to its laser-sharp focus on allowing information specific to building energy analysis to be transferred from one application to another. In addition, the schema is very easy to understand and use, giving incentive to software development companies to create integration tools between their applications and 3-D modeling software.

The modeling application will then export the building information via gbXML to a third-party analysis application, and this application subsequently will import the gbXML file and extract all of the required building information. Ultimately, some manual input into the analysis application will be required, but the bulk of the geometry will be generated automatically, thereby minimizing input time and potential for errors.

Once the results have been calculated, they can be imported back into the 3-D modeling software tool so the engineer can use it to model various mechanical and electrical systems. Also, the engineer and architect can perform "what-if" analysis by altering any of the building properties and then exporting that information back into the analysis software tool to see how the results change.

Modeling HVAC with BIM

The following is an example of workflow that involves modeling a building and performing HVAC load calculation analysis upon it:

Thenext screen shot shows a floor plan view of the 3-D model with thermal spaces defined. 3-D software tools have evolved in such a way that thermal spaces can be generated automatically according to the room and space boundaries.

Next, properties of these thermal spaces can be assigned, including occupancy schedules, heat contributions from equipment, and anything else required by analysis software.

Once all relevant properties have been assigned, the user can view an analytical model of the building.

Author Information

Roth is president of Carmel Software and was most recently Senior Product Manager for Autodesk Revit MEP. He is also the administrator for the Green Building XML (gbXML) advisory board. Roth is a member of ASHRAE and active in several technical and ad-hoc committees.