Hi, i have a harman pellet stove im currently make my chimney set up . my question is for the wall thimble do i need to insulate inside the wall thimble where the chimney pass i have maybe 3 inch each side?
I just got done installing a thimble for a straight-out vent through my wall. My pipe was rated for 1" clearance. The thimble was built such that there was a 1" air gap between the vent pipe and the thimble wall. Insulation can come in contact with the *outside* thimble wall, because I still meet the requirement for a 1" air gap.
In your case it sounds like you have a 3" requirement and the thimble provides a 3" gap between your vent pipe and the outside thimble wall. Always consult the manual, but it sounds like you would be safe insulating outside of the thimble wall (NOT the 3" area between the pipe and thimble wall).
My "thimble" is in a 12"x12" hole cut (with a chain saw in 1992) through two levels logs, then blocked around with 2x4 for a ledge, then there is a 12x12 sheet steel in house and a matching 12x12 outside, they are 3 1/2" apart and there is a hole that came in the sheets dead center that tightly hugs the pell vent, there is also a slightly larger 3 1/2" tall tube that surrounds the pell vent between the two sheets.
A number of years ago, I used a hole saw to cut a 2" hole in top corner through both sheets, had local exhaust bender bend me a piece to fit the holes and put a 90 degree bend in it, clamped screen over inlet end and secured to a log end that was handy, that is my cold air intake. connects to stove with suitable size short piece radiator hose.
No insulation .... just air space and that extra larger tube around pell vent. In spring, I'll repaint the "thimble" and air inlet as I replace last of pell vent. That inner extra tube is same diameter as that recess around pell vent, one side was a hair smaller that other and when installed, they form one extra tube.