TRNSYS logo.

An energy simulation program whose modular system approach makes it one of the most flexible tools available. TRNSYS (TRaNsient SYstem Simulation Program) includes a graphical interface, a simulation engine, and a library of components that range from various building models to standard HVAC equipment to renewable energy and emerging technologies. TRNSYS also includes a method for creating new components that do not exist in the standard package. This simulation package has been used for more than 30 years for HVAC analysis and sizing, multizone airflow analyses, electric power simulation, solar design, building thermal performance, analysis of control schemes, etc. Screen Shots


energy simulation, load calculation, building performance, simulation, research, energy performance, renewable energy, emerging technology


TRNSYS is primarily an equation solving program based on standard numerical techniques. However, validation does become important for the individual components in TRNSYS. See the TRNSYS web site for validation articles related to TRNSYS or its components.

Expertise Required

None to use standard package; FORTRAN knowledge helpful for developing new components.


Over 500.


Engineers, researchers, consulting firms, architects.


The TRNSYS input file, including building input description, characteristics of system components, manner in which components are interconnected, and separate weather data (supplied with program) are all ASCII files. All input files can be generated with a graphical user interface known as Simulation Studio. The envelope for the TRNSYS building can now be three-dimensionally created in the TRNSYS3D plugin for Google SketchUp™ and edited in the TRNBuild interface.


Basic output format is ASCII. The data included in those files can be life cycle costs; monthly summaries; annual results; histograms; plotting of desired variables (by time unit). It is also possible toplot variables online (as the simulation progresses).

Computer Platform

Windows 98 or higher (NT, 2000, ME, XP, Vista, Windows 7, etc.) for TRNSYS interface programs. (Distributed source code will compile and run on any Fortran platform.)

Programming Language

FORTRAN (although unnecessary for standard or add-on components). Components can also be written in C++.


Due to its modular approach, TRNSYS is extremely flexible for modeling a variety of energy systems in differing levels of complexity. Supplied source code and documentation provide an easy method for users to modify or add components not in the standard library; extensive documentation on component routines, including explanation, background, typical uses and governing equations; supplied time step, starting and stopping times allowing choice of modeling periods. The TRNSYS program includes a graphical interface to drag-and-drop components for creating input files (Simulation Studio), a utility for easily creating a building input file (TRNBuild), and a program for building TRNSYS-based applications for distribution to non-users (TRNEdit). Web-based library of additional components and frequent downloadable updates are also available to users. Extensive libraries of non standard components for TRNSYS are available commercially from TRNSYS distributors. TRNSYS also interfaces with various other simulation packages such as COMIS, CONTAM, EES, Excel, FLUENT, GenOpt and MATLAB.


No assumptions about the building or system are made (although default information is provided) so the user must have detailed information about the building and system and enter this information into the TRNSYS interface.



Thermal Energy System Specialists


22 North Carroll Street
Madison, Wisconsin 53703
United States


(608) 274-2577


(608) 278-1475




Version 17, Commercial: $4500, Educational: $2250. Free demonstration version and information are available from technical contact. International distributors are located in Belgium, China, France, Germany, India, Japan, the Republic of Korea, Spain, Sweden, Tunisia, and the United States.

Related Web Sites:
TRNSYS - Official Web site
TRNSYS Facebook Page
Solar Energy Laboratory at the University of Wisconsin
Thermal Energy Systems Specialists – U.S. TRNSYS distributor and content developers
CSTB - French distributor and developer of Simulation Studio (graphical front end to TRNSYS)
Transsolar - German distributor and source of TRNBUILD graphical building description processor
AIGUASOL - Spanish distributor
SERC - Swedish distributor
University of Liege - Belgian Distributor
Quattro Design - Japanese distributor
Solugen- Korean distributor
SESI- Indian distributor
AES- Tunisian distributor
CABR- Chinese distributor