[TRNSYS-users] Strange intermediate values for the inputs/outputs within an iteration timestep

David Bradley bradley at tess-inc.com
Wed Feb 25 11:20:21 PST 2009

Chun Kwong,
Good job hunting down that problem - you found the guilty party! Setting
a minimum relaxation factor less than 1 can cause pumps and fans to
output less than their rated flow rate times the control signal and that
this can cause problems in downstream components like Type51. We have
run into the same problem with the radiant floors in Type56; a smaller
than 1 relaxation factor resulted in the flow from a constant speed pump
to the radiant floor going below the minimum allowable value during an
internal iteration even though the pump was plenty large enough. We had
to set all the relaxation factors back to 1 in order to run it.
Kind regards,

a8304506 at graduate.hku.hk wrote:
> Hello, Trnsys users.
> I got an error in one simulation relating to the cooling tower (Type51).  I try
> to chase the cause of the error by adding some output statements in the source
> code to record particular parameters.  Then I found some strange results.  I
> use a thermostat to control a water pump to supply water to the cooling tower. 
> Hence, the inlet water flow for the cooling tower should be either zero or
> design flow.  However, it seems that in some iterations, the logged water flow
> differs from these values.  When it becomes a small value (but not zero), error
> occurs within the subroutine Psychromatics.  I wonder if it is because I choose
> a value for the minimum relaxation factor other than 1 (I suppose that the use
> of a smaller value can enchance the convergence).  If it is so, then the
> allowance for selecting the relaxation factor is probably a trap, as the
> intermediate values for particular input/output may fall into some dangerous
> ranges.  Indeed, I encounter similar problems occasionally.  Does anyone has
> other ideas for the cause of the problem?
> Best Regards
> LEE, Chun Kwong
> Research assistant
> Division of Building Science and Technology
> City University of Hong Kong
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Thermal Energy System Specialists (TESS), LLC 
David BRADLEY                      22 N. Carroll Street - Suite 370 
Partner                            Madison, WI 53703 
P: +1.608.274.2577 
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E-mail: bradley at tess-inc.com 
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