Hydraulic head undefined when running flow and heat transport

Hello,

I'm having trouble coupling heat transport to my flow model. When running simply the flow (ss or transient) there are no issues. Whenever I include heat transport, the simulation run terminates with an 'undefined' hydraulic head for all cells.
Even when applying two temperature boundary conditions, say 40 deg West and 60 deg East and an initial temperature of 50 deg throughout the model, an undefined hydraulic state is the end result. Because of that, heat transport is not included as well. Is this a common problem? I looked at increasing numeric stability which I think is very fine and robust. Is this a common issue, am I missing something?
Also, I have set the transport equation to convective, and fluid viscosity constant.

Thanks in advance!

An undefined hydraulic head points towards some quite severe numerical problems in the model. I highly recommend to contact our user support (mike.de@dhigroup.com) and provide them with the model for checking.