Good question. For the aircond to keep performing, it needs the maintenance of gas refill right? Like someone said earlier I believe its responsibility of the owners, since you're renting it out as fully furnished and the facilities are just 'used' by the tenants.

If the washing machine spoils or microwave spoil, its the owners responsibility to replace/repair. Same thing I guess. My case, aircond in my tenants had leaking problem but this was quite clearcut its my problem since they had just moved in. They're paying you for staying and for maintaining the place they stay afterall.