[TRNSYS-users] Stability criteria.

[David Bradley]

Didier,
   Type56 uses something called the conduction transfer function (CTF) 
method for determining the energy transfer through walls. The 
(admittedly oversimplified) idea of the CTF method is that a series of 
weighted averages going back in time is developed to tell how important 
the temperature difference across the wall is now, how important it was 
an hour ago, how important it was two hours ago, etc. The size of the 
step back in time is called a time base and it doesn't have to be one 
hour; you can set it in the TRNBuild "outputs" window . There are also a 
number of other factors (such as the history of the energy transfer 
through the wall) for which there are also weighted averages determined. 
Type56 and TRNBuild are limited to using the first 20 coefficients 
determined.

   If you have a wall that is too massive or not massive enough then 
TRNBuild has a hard time determining the CTF coefficients. For example, 
if you have a thick stone wall, the conditions of that wall more than 20 
hours ago may matter quite a lot to the energy transfer now (in other 
words, there isn't enough there aren't enough CTF coefficiencts 
developed). If you have a very lightweight wall (like a metal door) then 
all the conditions that matter to the energy transfer across that wall 
occur right now (in other words the time base is too big). TRNBuild 
generates the "failed to fulfill stability criteria" error. You can tell 
which wall caused the problem by looking in the *.inf file. Look first 
near the bottom and the inf file will show you the coefficients that it 
generated for each wall. Note the name of the last wall for which it 
generated coefficients then scroll up to the top of the file. The wall 
AFTER the one that successfully generated in the list of walls is the 
one that caused the problem.

  If it is a lightweight wall that caused the problem, simply treat it 
as a "resistance only" in other words, that wall should be made up only 
of a massless layer.

  If it is a very massive wall that caused the problem you can increase 
the time base.
Best,
  david




On 6/30/2010 08:48, gossard at cict.fr wrote:
> Hi all,
>
> I'm working with TRNSYS 16 with Type56. TRNBuild suddenly stops with 
> the following error message :
>
> "Error creating the wall transfer function coefficients :
>
> Stability criteria not fullfilled
>
> Check layer definition (thickness, ratio of conductivity/density)
>
> Please, check the INF-file for further information."
>
> What is this "stability criteria" ?
>
> Please find enclosed the INF and BUI-files.
>
> Best regards,
> Didier.
>
> ----------------------------------------------------------------
> This message was sent using IMP, the Internet Messaging Program.
>
>
> _______________________________________________
> TRNSYS-users mailing list
> TRNSYS-users at cae.wisc.edu
> https://www-old.cae.wisc.edu/mailman/listinfo/trnsys-users
>    

-- 
***************************
David BRADLEY
Principal
Thermal Energy Systems Specialists, LLC
22 North Carroll Street - suite 370
Madison, WI  53703 USA

P:+1.608.274.2577
F:+1.608.278.1475
bradley at tess-inc.com

http://www.tess-inc.com
http://www.trnsys.com

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.onebuilding.org/pipermail/trnsys-users-onebuilding.org/attachments/20100701/e4af5c45/attachment-0005.htm>