Prelude
This is my very first attempt at CFD so you can bet I’m making very newbie mistakes. Only have some limited experience with Fusion 360 and its body stress simulations so… please bear with me 
Project
I’m trying to simulate flow in a passive ventillation pipe that has an inlet outside a house, and two outlets in two separate rooms inside the house. Air is drawn in via the inlet because of negative (relative to the outside) pressure in one or both of the rooms.
I would like to know what happens if pressure is ‘negative’ only in one of the rooms serviced by my pipe. That is, make sure how much of the air needed in the ‘negative’ room will be sucked from the outside and how much from the other room, given the shape of the pipe.
Problem
The following simulation run(s) failed:
Pressure field started diverging. Please check the mesh quality near the reported location and try refining the mesh. If the problem occured near a boundary, please check the boundary conditions. In case of doubt, please ask for assistance via our support chat. Pressure = -5.10792e+24 at position: (8.263 m, -5654 m, -234.6 m).
Attempts
I’ve tried a few boundary conditions combinations: 2x pressure inlet (at 1013 hPa) + 1x pressure outlet (at 1000 hPa), as well as 2x pressure inlet (at 1013 hPa) + 1x velocity outlet (at 0.1 m/s). Both result in a failed simulation run as seen above. At this point I am pretty much clueless as to what I should do next. Will be grateful for any suggestions!