Solar water heating - Negative Tcold

Hello there. My simulations of a solar water heating system are having a very strange results on Tcold, some temperatures
are extremely low, even near -40°C. I figured out that the error occurs on
operation mode 1, with the tank somewhat stratified, and thus modifying the
tank volume affect on this negative temperature values, and my problem resolves
around a very big tank.

I don’t know who much these results are influencing the rest of the simulations, as it seems Sam is ignoring some of these results.