I live in a townhouse that has a 20 year old cheap condo sandpan type gas fireplace. I am planning renovations and I have been looking at my options for replacing the fireplace. I am researching the B vent. I have not opened up the wall yet, but I have removed the louvers and measured a 4" bvent coupling. So I am assuming a 4" Bvent or possibly a 5" Bvent pipe to the roof. The bvent pipe length is about 30 feet to the roof straight up with no horizontal runs. 4" bvent is limiting for what I can choose for a fireplace that looks nice. I am still researching. I found a useful bvent application guide from Duravent that has educated me quite a bit. Would it be possible to couple a 5" collar to a 4" bvent using a reducer? That would give me at least some options for upgrading the fireplace. The new fireplace will be no more that 24000 BTU. So looking at the Bvent tables even a 4" Bvent would be adequate for a 30' vertical rise. I realize that the size of the vent is also a function of maintaining a proper draft. Unfortunalty most of the nicer Bvent fireplaces are 6", but FMI seems to make a nice one from the Vantage line that is 5" Bvent. Thanks for your advice in advance. Jan.