All unvented fuel fired heaters are prohibited for all occupancies except one- and two-family dwellings; provided, that a single unvented fuel fired heater is permitted for demonstration purposes in authorized mercantile applications when installed in accordance with manufacturers recommendations. The single heater shall be connected to a permanent source of fuel and shall not be used as a permanent or alternate source of heating. The unvented heater shall be shut off at the end of each business day.