Michael6268
Minister of Fire
Not a real good idea to leave the bottom cap off the T. Enough on here have learned how that sucks cool air up through the bottom end of the T effecting both draft( causing poor draft from the stove), which also allows for a crapload of creo to build up in the stack.
Just seal the cleanout door with if your cleanout is outside. If its inside you can still seal it, but there wont be any real cold air coming from a inside cleanout door.