Every dwelling unit shall have heating facilities properly installed and maintained in safe and good working condition and capable of safely and adequately heating all habitable rooms, bathrooms, and water closet compartments contained therein to a temperature of seventy degrees Fahrenheit (70°F) at a distance of eighteen inches (18") above floor level when the outside temperature is minus ten degrees Fahrenheit (-10°F); provided that gas appliances designed exclusively for cooking or water heating purposes shall not be considered as heating facilities within the meaning of this provision. (1975 Code § 11.04)