The way I look at it, you need to maintain a certain temperature and humidity during incubation. At some point the embryo develops to a point that it needs fresh air. My Hovabator is a forced air, it has a fan.
I take the red plug out at Day 1 and never put it in. As long as you can maintain temperature and humidity I don't see why it matters if the plug is in or out relative to temperature and humidity. Fresh air does matter later in incubation and hatch so with the plug out I have that controlled. Since I stabilize mine with the plug out to start with, I don't have to go through a re-stabilization part way through the incubation. I like to keep it simple.
You can always contact the manufacturer and see what they say about it, I often do things like that. It doesn't embarrass me at all to ask them. If you do I'd appreciate it if you would post their response on here. I'd like to see what they have to say.
