No heating device shall be located, installed, maintained or used in any spray room. For comfort of persons working within the spray room or spray booth, heating appliances of indirect type may be installed on the outside of such booths or rooms for the purpose of introducing heat inside thereof, provided the temperature does not exceed 120°F at the warm air inlet to the booth or tunnel and that location and arrangement of the heating appliance and the method of heat introduction into said room is first approved.