Hmm, yeah, this would confuse anyone. The truth be told in a bisque firing it’s really good to ventilate your kiln to help remove or oxidize the organics being burned out. The problem is how much? As the more you ventilate, the more energy it takes to heat. Sort of like leaving a window open in your house and heating it. Anyway the truth is a very small amount of ventilation is a great compromise.
For bisque, at some point, all the organics are gone so there is no need. In glaze firings a little bit of oxidation brightens up the glaze colors for similar reasons.
Your kiln manual provides the most consistent advice and some ventilation (a tiny amount) will also improve the life of your elements by removing harsh corrosive gasses, especially during the bisque. After 600c though most everything is burned out so closing the bottom vent is appropriate for sure.
closing the top vent will likely save energy as well as kilns are really not super air tight so there will always be some infiltration. That kiln should be piped to exhaust the fumes outdoors. The height of that pipe will affect the draw as well. The bottom port of that exhaust allows air in the room to move up the pipe and a very small amount of kiln air to be mixed in and the room air helps keep the exhaust pipe relatively cool, say 100-200c degrees
I personally would make sure it’s piped outside so folks would not breath in the fumes and not plug the bottom port of the vent after 600c just to ensure there was some ventilation. My experience - the kiln manufacture seems to have the most reasonable approach.
