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TRNSYS Frequently Asked Questions

General TRNSYS Questions

Purchasing TRNSYS

TRNSYS Simulation Studio / Online Registration

TRNBuild / Building model (Type 56) / TRNSYS3D

Troubleshooting TRNSYS Simulations

General questions

Q: What is TRNSYS?

A: TRNSYS (pronounced: 'tran-sis') is a software package that has been commercially available since 1975. The software package consisits of a graphical front-end (TRNSYS Simulation Studio) to intuitively create a simulation, an interface for the detailed TRNSYS multi-zone building (TRNBuild/Type56), a SketchUp plugin for creating the multi-zone building envelope (TRNSYS3d), and a tool for manually editing the TRNSYS input files and creating stand-alone TRNSYS-based applications (TRNEdit/TRNSED). TRNSYS takes a modular, "black box" component approach to developing and solving simulations: the outputs of one component are sent to the inputs of another component (transient successive subsitution). TRNSYS has been used extensively but is definitely not limited to simulate solar processes and other renewable energy, high performance buildings, and electric power generation.

Q: Where may I find more information about TRNSYS Training Courses or TRNSYS related events?

A: Announcements and events will be communicated via the the TRNSYS Users mailing list and the TRNSYS facebook page. Some of the TRNSYS distributors offer TRNSYS Training Courses throughout the year and host TRNSYS Workshops at the International Building Performance Simulation Association (IBPSA) conferences.

Q: What standards does TRNSYS comply to?

A: TRNSYS has become reference software throughout the world. It is one of the listed simulation programs in the recent European Standards on solar thermal systems (ENV-12977-2). The level of detail of TRNSYS' building model, known as "Type 56", is compliant with the requirements of ANSI/ASHRAE Standard 140-2001. The level of detail of Type 56 also meets the general technical requirements of the European Directive on the Energy Performance of Buildings, which makes TRNSYS a potential candidate for compliance with the directive's implementations in various EU countries. TRNSYS was not only used for the IEA Task34/43, to increase the accuracy of the ground-coupled heat transfer, but it was also the only whole building energy simulation program that was deemed as a BESTEST reference standard. TRNSYS has also been used for building simulation for numerous LEED projects.

Q: Is there an evaluation or demo version available for TRNSYS?

A: Yes, the demo version may be obtained from the Demo Version page.

Q: How do I reference TRNSYS for a publication?

A: Klein, S.A. et al, 2017, TRNSYS 18: A Transient System Simulation Program, Solar Energy Laboratory, University of Wisconsin, Madison, USA,


Purchasing TRNSYS

Q: How do I purchase TRNSYS?

A: TRNSYS may be purchased from one of the official TRNSYS distributors. Please contact the respective distributor, and please note that there are exclusive distributors for the United States(TESS), France(CSTB), Germany(Transsolar), and Portugal/Spain(Aiguasol). If your company or institution is not located in one of those countries, you may purchase from any of the official TRNSYS distributors.

Q: How much does TRNSYS cost?

A: Please contact your official TRNSYS distributor for a price list or more information.

Q: I am a software reseller. Do I get a discount?

A: No discounts are applied to software resellers as the official TRNSYS distributor is responsible for the technical support of the software.


  TRNSYS Simulation Studio / Online Registration

Q: What is the difference between the node-locked and floating licenses?

A: A node-locked license is assigned to a specific computer. A floating license is checked out only when TRNSYS is actually used on a computer. Utilizing the floating license requires an active internet connections whenever the software is running.

Q: What is the Online Studio Registration, and how do I implement it?

A: The TRNSYS Simulation Studio requires an online registration to generate an activation key for TRNSYS. When you open the Simulation Studio for the first time, a small window will prompt with your specific computer machine code identification number. Highlight and copy this machine ID number. Go to your respective distributor's activation website, enter the appropriate information, and generate an activation key. Highlight and copy the activation key, and paste it back in the small window of the Simulation Studio.

Q: The Online Studio Registration did not work, what should I do now?

A: When you paste the activation key back in the small window of the Simulation Studio, make sure that you do not have any spaces before or after the activation key. Make sure that you are pasting the correct activation key and not any machine ID number. You may also need administrator privleges to implement this. If it still does not work, please contact your TRNSYS Distributor.

Q: What constraints should the simulation time step comply to ?

A: The timestep should be 1/n of an hour, where n is an integer. The reason is to make sure that an integer number of time steps will fit into one hour (many things happen at the hour: integration, data reading, etc.).
the rule is n/m where n and m both are integers. m is forced to be smaller than 1 / (minimum time step), which is currently set to 0.1 sec, i.e. 0.00002777777778 hours (so the denominator must be smaller than 36000). The reason is here again to make sure that we can count on "reaching round numbers at some point inthe simulation". Data readers, Printers and integrators etc. will warn you if their time step is not an integer multiple of the time step (and it's NOT recommended to keep it).


TRNBuild / Building model (Type 56) / TRNSYS3D

Q: I have a building description using lots of windows. I would like to try a variant of the system, replacing all windows of one type by another one. Do I have to change the WINDOW type everywhere in the project or is there a trick ?

A: You can redefine the window - type in the Window manager: open the windows manager; select the window type; click on the "W4-lib" - button; choose the new window type.

Q: What does the default value of the solar absorbtance factors used in TRNBuild mean ?

A: These factors for the inside (ABS-FRONT) describe the distribution of solar gains that a zone gets through its windows.

The default value of 0.6 is the case for a vertical wall. Other recomended values are

   0.1 for ceilings and
   0.8 for floors

Q: Where can I get the WINDOW program needed to introduce new windows into TrnBuild ?

A: The program is available for free at the following website.

Q: I want to define an airflow from one zone (z1) to another one (z2), without a flow in the other way (from z2 to z1) - i.e. define cross ventilation. Where does the air in z1 come from and how do I define it ?

A: The "coupling air flow" specified for a zone is always the flow into the zone - it CANNOT be negative. To compensate for air leaving a zone, you have to adapt the infiltration rate of the zone (z1, in the example). You always need to specify air coming into a zone (by specifying coupling flowrates, or adapting infiltration rates). Air going out is "removed automatically". (You have to make sure that zones do not "implode"; "exploding" zones are no problem).

Q: Does the ESHADE factor cut direct and diffuse radiation in the same way ?

A:   ESHADE reduces both direct and diffuse radiation in the same way.

Q: Heat transfer coefficients are defined by a radiative and a convective part; in TrnBuild, the convective part can be entered. How to input the radiative part ?

A: All surfaces are assumed to be black for long wave radiative exchange and radiative internal gains for surfaces inside the zone. This is a good approximation for most real world cases (exceptions would be huge mirrors or stainless steel panels at inside surfaces or windows with low-E coatings towards the zone). It is not possible to change this assumption easily in type 56. A possible workaround would be to define additional wall gains, but it is usually not worth the effort. All external surfaces are assumed to be grey (E=0.9) for longwave radiation exchange with the sky. The values of longwave emissivity for windows are taken from the “w4-lib.dat” file.        

Q: Is it possible to use different values for Absorption and Emissivity in type 56 ?

A: The longwave absorption and emissivity for opaque walls are assumed to be equal (E=A=1 inside the zone).

Q: What is alpha-calculation (in the BUI)  ?

A: These are coefficients for the automatic heat transfer in type 56  like you could do externally with TYPE 80 in previous versions of TRNSYS (15 and earlier).


Troubleshooting TRNSYS Simulations

Q: How to optimize my simulations for speed ?

A: You can optimize the 'TRNSYS side' and the 'programming' (FORTRAN, C,...) side. Common tricks for the TRNSYS side include:
- tune GLOBAL cards carfully (convergence limits, integration precission, ...) - the usual precission vs. speed tradeoff
- tune component parameters carefully (minimum number of nodes necessary, ...). This is usually most efficient.
- optimize component order (components which do not compute a lot (printers, onlline, ...) at the end, ...)

In your programming environment (e.g., FORTRAN compiler), make sure you use the 'Release' build (not the debug build) and check 'Optimize for speed' in the settings (e.g. FORTRAN/Optimisations category). Make sure to apply this to ALL modules (modifiy settings for all modules, rebuild all modules). Be careful, however: some unstable third-party types do not support optimization (e.g. Range Check error in type 140). In this case, you can 'locally' deactivate optimization for this type only.

Q: I have a TRNSYS ERROR 25 when using equations, although the unit used in the equation appears in the deck

A: This problem may be caused when copying equation models between projects, renaming models, or transfering models between projects AFTER the equations have been defined in the equation model.  Delete the equation causing the problem and redefine it.

Q: How can I run TRNSYS (perhaps from another program or batch file) and avoid having to click 'Continue' or 'Yes' at the end of each simulation?

A: From the command line, the "/N" switch is used to tell TRNExe64.exe (or TRNExe.exe) that you don't want the dialog box at the end of the simulation. For example, to run the input file called C:\Trnsys18\Examples\SDHW\SDHW.dck  such that no dialog box is produced at the end of the simulation, the following command line statement would be used:
"C:\Trnsys18\Exe\TRNExe64.exe" "C:\Trnsys18\Examples\SDHW\SDHW.dck" /n
Furthermore, the "/h" switch is also available if you need TRNSYS to run completely in the background (without even creating a Window). The /h switch implies /n and is only applicable to simulations that do not use the online plotter (Type 65):
"C:\Trnsys18\Exe\TRNExe64.exe" "C:\Trnsys18\Examples\SDHW\SDHW.dck" /h

Q: I want to distribute my TRNSYS simulation in TRNSED format to others. What do I have to send to the client?

A: The "TRNSED/Create Distributable" command in the TRNEdit environment will place all the TRNSYS files needed in the destination folder. You then need to add all the input files, pictures, etc. required by your application. We recommend that you test your application on a different machine or after renaming your TRNSYS directory to make sure you are not using some files from the TRNSYS installation.

Q: When using a type 53, my project crashes at execution time.

A: Check that the external data file for chiller performances does not contain any zero data points; type 53 doesn't accept them. This is a workaround which should be fixed in future versions.

Q: How to connect type 34 to type 56 ?

A:   To distinguish between the reduction of direct and diffuse radiation by wingwalls or overhangs ESHADE should not be used ; instead use Type 34.


-          define a new orientation for the shaded window in TrnBuild (e.g. SWINDOW, for a window facing south); this will create 3 new inputs to type 56:

a) ITSWINDOW - Incident Radiation for orientation SWINDOW
b) IBSWINDOW – Incident Beam Radiation for orientation SWINDOW
c) AISWINDOW – Angle of Incidence for orientation SWINDOW

-          use type 34 output 1 for a) and 2 for b)

use type 15 or type 16 output to compute c)

-          make sure type 34 precedes type 56 in the simulation order


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