convergence issue using HETVAL

[eklee]

I am simulating the heat transfer for an underground warehouse using 3 different target temperatures. I am using the HETVAL subroutine to account for the internal heat generation for each target temperature. It works just fine for temperatures above 0 degrees Celsius. However, for negative temperatures I receive a convergence error. The error is: "FIXED TIME INCREMENT IS TOO LARGE", "NOTE: THE SOLUTION APPEARS TO BE DIVERGING. CONVERGENCE IS JUDGED UNLIKELY".

I would like to simulate the heat flux for a target temperature of -25 degrees Celsius; however HETVAL does not seem to work for the cooling down of the warehouse.

Any suggestions on how to overcome this problem? Any ideas would be greatly appreciated.

Thanks!