Do you wait until cat is in active zone to close the door?
In my experience, even from a cold start, less than 5 minutes ( usually 2-3 minutes) is enough for the fire to get going. Then I close the door. Wait some more to have the cat to be in active zone. As soon as it is on the borderline of active zone, I close the bypass. ( Can you close the bypass a bit earlier than it is in active zone?)
Then I let it run for 15-30 min for charring.
I just had a quick chimney fire because, IMO, I kept the door cracked too long during warmup. Good thing I was there watching my flue temperature suddenly start whipping up. I slammed everything shut which on the bk actually means just that little air hole in the intake flapper. The young fire snuffed and the probe meter only climbed to about 1100 before stopping.
Whoa!
I pulled the probe meter out and the probe is shiney copper. No roaring or rumbling but crackling in the pipe.
Everything is normal now.