Yes it should be atleast the size of the flue. And regardless you arent going to keep a house warm with a fireplace. Most of them do the oposite.My son's seems to be the size of the flue, maybe a little smaller, but it was part of the construction. I have no clue about the one they did after in the 70's. Just remember it was a lot of noise, and thinking it was funny they sealed their house so well to keep it warm they could not build a fire anymore to keep it warm.