Ill be buying a Wood Stove to sit partially in the fireplace . The Fireplace has an 8" heavy wall metal Vent going straight up about 20' . I have alot of 6" B-Vent (double wall) Vent which id like to use since its a straight shot all the way up. Can you see any reason why I couldnt use B-Vent ? The instructions say to use a flexible stainless steel liner ... but it doesnt say B-Vent cant be used. Any advice ? Thanks.