I have a Weather King 80 Plus Upflow Gas furnaces Just found out that I need a chimney liner all I can fit in the chimney is a 4 flex liner the space is 4.25x19 that I have to shove it through and at the lower part of the chimney its 4 exactly for about 3 feet my question is on the paperwork for the unit is has vent 6 (152) may be 1 (25mm) with type b vent. Can I use the 4 flex liner for a happy medium. The unit has a power vent on it. There is a 3 vent pipe that comes off the unit and into the chimney. On the paperwork it has May require a 3 to 4 or 3 to 5 adapter Im going to be using 4 double wall aluminum liner. I will like to order the liner but I dont want to get it if it will not work cant tack it back after its used Thanks for the help. sorry I also posted this in general