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Re: (Fwd) [EnergyPlus_Support] Simulation

EnergyPlus picks values to initialize the inside and outside surfaces, the 
zone air temperature, etc. and then starts repeating the first day of that 
environment over and over again until it reaches a steady periodic 
solution.  The first day may be the design day or the first day of the 
specified run period from the weather file.  Then the program compares each 
timestep with the previous days time step value for both temperature and 
load and then uses a convergence tolerance to know when it has 
converged.  This is where the  ?Temperature Convergence Tolerance 
Value?  and the "Loads Convergence Tolerance Value" come into play.

   N3 , \field Loads Convergence Tolerance Value
        \units W
        \type real
        \minimum> 0.0
        \default .04
   N4 , \field Temperature Convergence Tolerance Value
        \units C
        \type real
        \minimum> 0.0
        \default .4

Your first selection allows the program to converge when the temperature 
difference is less than 100C before it starts the actual simulation.  Very 
loose and my guess is that this happens after either the first or second 
warm up day.  When the value is set to 0.4C it will take more iterations to 
converge to a temperature difference less than 0.4C before it starts the 
actual simulation.  I believe that the tighter value (0.4C or less) will 
give you a more realistic starting value.

Your other issue is of course how much mass in the zone has to be warmed 
up.  If the zone is quite massive then tightening up the tolerances even 
further will allow more warm up time before the simulation starts.

Dr. Richard J. Liesen
Associate Director, Building Systems Laboratory
Department of Mechanical and Industrial Engineering
University of Illinois at Urbana-Champaign

------- Forwarded message follows -------
>To:                 "'Energy Plus Support'" 
>From:               "Alexander Beisteiner" 
>Date sent:          Tue, 15 Oct 2002 14:23:17 +0200
>Subject:            [EnergyPlus_Support] Simulation
>Send reply to:      EnergyPlus_Support@xxxxxxxxxxxxxxx
>[ Double-click this line for list subscription options ]
>Dear Sir/Madam
>I am modelling a zone with an internal load of 1000W and a constant
>external temperature. Setting the field ?Temperature Convergence
>Tolerance Value? in the building object to 100 °C produces a start
>temperature which almost equals the external temperature: this sounds
>quite reasonable. On the other hand, if the mentioned field is set to
>0.4 °C the start temperature is roughly 40 °C: in my opinion this far
>too high. In both cases temperature values converge in the middle of
>the simulation period is the same (i.e. about 1 month). I wonder what
>effects the ?temperature convergence tolerance value? has on the
>simulation. What value is recommended? Why does my simulation behave
>like this? I would appreciate your comments. Regards
>Alexander Beisteiner
>Forschungs- und Technologietransfer
>Pinkafeld GmbH
>Steinamangerstrasse 21
>7423 Pinkafeld
>http://www.fh-pinkafeld.ac.at <http://www.fh-pinkafeld.ac.at/>
>Tel.: 0043/3357/45370
>Fax: 0043/3357/45370/1010
>------- End of forwarded message -------

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