On the other hand ... you DO have to understand and manage it properly ... your stack extension was apparently enough to overpower the other stack.
A predictable, repeatable draft is the nature of the beast and you'll figure it out in time.
Just for comparison - I run Edgar on 3 square inches of draft and a 4" diameter round stack that's 36 inches long but the flu is only half open. He stabilizes at 280 - 300 with the baffles wide open, 230 with them closed. fwiw
![Big Grin :D](./images/smilies/4.gif)