To prevent an explosive atmosphere in the case of a leak, units with charges above a certain limit can only be used in rooms exceeding a minimum floor size.
This room size is calculated based on the refrigerant charge, and the lower flammability limit (LFL) of the refrigerant. The manufacturer needs to display the restriction prominently, on a standard label complying with IEC 60417-6415.
The minimum room size (in m2) is equal to:
the refrigerant charge (in kg), divided by
one quarter of the LFL (in kg/m3) multiplied by 2.2 (the assumed height of the room)
So, for example, a system with 153g of R290 would require a minimum floor area of:
0.153kg / 2.2m x (0.25 x 0.038kg/m3) = 7.3m2
But a system with 494g of R290, the required area is much greater:
0.494kg / 2.2m x (0.25 x 0.038kg/m3) = 23.6m2