How to rectify the miscomputation of Pressure and Continuity in HVAC CFD Simulation U
 

I'm using OpenFOAM to run a CFD study of thermal comfort within a hall. I'm using OpenFOAM v1912 and BouyantBoussinesq(Simple&Pimple)Foam solver. I have 6 inlets and outlets each with specific flow rates. I specified the flow rates accordingly at the inlet using flowrateinletvelocity BC and at the outlet using flowrateouletvelocity BC. Based on the height, I computed (p-rgh)/rho and fixed it at one of the outlets.
My mesh quality is good (Non-Orthogonality less than 50). The dimension of my hall is 30m *30m *4.5m. It was okay so far when I was doing the simulation for small halls, but I'm facing this problem at the moment.
This happens each time I start simulation, at beginning p rgh iteration reaches 2000. The final residual is very high (greater than zero). Cumulative value (continuity error) begins to rise and becomes greater than 1. The value of Pressure ranges from 1e18 to -1e18 even after after 1 or 2 time steps, then the simulation blows off.
I have tried the following options
1)potentialFoam initialization
2)Mesh quality check
3)Defining pressure at all outlets
4)prefcell and prefpoint in fvsolutions to define pressure.
5) URF very low and also reltor for pressure to 0.0
6)First order and 2nd order fvschemes
7)nNonOrthogonal correction to 20.
8) turned off turbulence.