Ran the stove, Harmann XXV, last winter without an OAK. Finally got around to installing it before firing it up. Even though the final pipe is 4" you can really feel it sucking in the air. Check it out and let me know what you think. http://home.comcast.net/~habsfan/oak/oak.htm After a few days of burning already notice that the glass is hardly dirty where last year it would get dirty within a day or two.