If the owner of the upstairs unit is aware of the possible damage that can be caused to your unit as the result of his or her tenants no shutting off the humidifier and does nothing to warn his tenants to be careful as to its use and damage occurs, then both the owner of the upstairs unit and his or her tenants could possibly be responsible to you for their negligence and all resulting damages to your unit.Your damages could consist of the costs of repairs, accrued interest on the repairs and lost rent.You need to call and write both the owner of the other unit and the tenants about the continued problems resulting from the use of the humidifier and that any future problems will result in them being held liable for your damages.Good luck.