Heat transfer problems

Hi, is there a reason why a project gone well without heat transfer when I turn on heat transfer often fails? Do I need to improve mesh accuracy? (usually I already have a good mesh)
Another question: When I change volume's substance ( I only turn on heat transfer) I lose the inlet informations (volume flow rate, velocity..).
Are these my mistakes?

Assuming Incompressible Heat Transfer

I'll assume you want to simulate incompressible heat transfer.

Are there large temperature gradients and large height differences that mean you want to include buoyancy effects (natural convection)? If not try turning buoyancy off by setting Substance->Reference->g (gravity) = [0 0 0] for just forced convection.

Changing the Substance->State settings will cause the boundary condition types to reset to defaults - you are changing solvers and some BCs only apply to certain solvers.

Thanks. I understand your

Thanks. I understand your second answer.
For the first answer, I forgot to say that I have already set buoyancy off, and I can't run the simulation again. I don't understand if this could be a problem of mesh or my mistake.
(you are right, incompressible heat transfer)

Try K-Omega SST Turbulence Model

I doubt it's a mesh problem if you previously ran a successful simulation without heat transfer. Try setting Substance->Solver->Turbulence Model = K-Omega SST and then set your turbulent reference values.