closed vessel: mass conservation failed
 

Hello!
I am trying to do an isochoric compression CFD simulation with Fluent, i.e. a closed domain (NO inlet and NO outlet) initialized with air, density as ideal gas law, 10barA & 300 K temperature (pressure based - energy on- k-epsilon).
I m trying to increase gas temperature/pressure alternatively with:
- heat flux through one wall zone (with transient analysis)
- higher temperature (i.e. 1000K) on one wall zone (both with steady and transient analysis)

After inizialization (standard), i check the initial mass (reports-volume integrals) and it's ok. As i lunch the simulation, looking the plot of the mass, it starts in decreasing!! NO sense with this!!
I also tried with a transient, starting without heat flux and put it on after a few time: as heat flux is on, mass goes down!

I think, for transient something is wrong with time step and/or iteration for time step, but for steady i have no idea
Thanks in advance