Closed rooms have no outlets or openings so there is no boundary for the solver to use as a pressure reference region.
In addition, if there is heat transfer into the room it will heat up and the pressure will increase.
The following advice should improve convergence:
Compute an expression for the average room pressure from P = <rho>*R*<T>/W, where <> implies volumed averaged.
AverageDensity = volumeAve(density )@Domain 1
AverageTemperature = volumeAve(T)@Domain 1
AveragePressure = AverageDensity*R*AverageTemperature/W
The above expression for the average pressure assumes that W is the molecular weight of the fluid and that an ideal gas is being used.
Set the reference pressure equal to the average room pressure.
Run in double precision.