If the vent kit you have is also from Mr. Heater, the double walled B-vent should be a 3 foot section. This is designed to go through the wall (though the thimble) and stick out a minimum of 12 inches on the outside.
Double wall vent is used when going though combustibles and when the vent...
Just a quick note...after reading the many posts on this board. The longer the pipe run (from the gas meter) the larger the diameter needed. Also, care must be taken when tying into the existing house piping.
Usually an appliance (like a hot water tank) is installed with the minimum diameter...