[TRNSYS-users] error in Type 556 ?

[Andreas Heinz]

Dear TRNSYS-Users !

I just found out about a very strange behaviour of Type 556 "Horizontal Ground-Coupled Heat Exchanger".

Im using a very simple configuration, where I enter the heat exchanger with a constant mass flow and an inlet temperature, which is always 3 Kelvin lower than the outlet temperature. 
The problem is, that the resulting outlet temperature out of Type556 is strongly depending on the TRNSYS-timestep. For example, if I use a time step of 6 min I get -3.8°C, if I use 1 min I get -11.6°C. 
I attached a Studio-File with an example.
Does anyone have an explanation or has anyone observed a similar behaviour ?

Are there alternatives to this type for simulating a ground coupled horizontal pipe ?

I would be grateful for any support.

regards,
Andreas

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Dr. Andreas Heinz                          
Institut für Wärmetechnik / Institute of Thermal Engineering                                    
TU Graz / Graz University of Technology                         
Inffeldgasse 25b, A-8010 Graz                           
Tel.: +43-316-873-7313
Fax.: +43-316-873-7305
andreas.heinz at tugraz.at
www.iwt.tugraz.at