I am researching chimney liners and have a few questions. I am looking to possibly vent an Osburn 2400 insert that is in an exisiting masonry fireplace. I have seen many different ways to vent. I plan to do a full re-line of the chimney. The problem is around my damper/smoke shelf there is only about 5 1/2 inches of space, not enough room for a 6" pipe. I was told by one salesman that I could "oval-out" the flex pipe to make it fit. Is it that easy or is he just trying to make a sale? He said its pliable enough to just step on it or bend it by hand to make it fit. Also I have seen on some websites, they recommend using solid 6" pipe to a section of insulated oval flex pipe that is 4 1/2" in diameter so it fits around the smoke shelf. This method is a bit more expensive. Now is it better to go this route or is it better to go with all flex? I've seen many sites that say all flex is fine. Does the flex need to be insulated? Also some flex is very inexpensive, I've seen some on e-bay for $300 for 25' of 6" flex, while other sites have that amount for hundreds more. I know usually you get what you pay for, but at the same time I don't want to spend money unnecesarilly. Basically I'm looking for someone with an unbiased opinion (no alterior motives) to give me a little schooling in venting 101. Thanks for the help.