Save on the 4" x 15' DuraFlex Pro Basic Kit (4DFPRO-15K) by M&G DuraVent with PlumbersStock.com. One of the best features is that the flex line self straightens, which is a relief because straightening it manually can be tedious. They manufacture it with 20% more stainless steel per foot, compared to other liners. Its pliability facilitates easy installations when multiple offsets are needed. Applications include: fireplace inserts, furnaces, boilers, and water heaters. The difference between this and the DFPRO-KT series is the DFPRO-K models do not have a tee.