Mattax
Just the facts, ma'am
Comes out the 'thimble' after the stove heats up. Like it's cooking out of the block. But nothing is open up top. So almost has to be getting in down the flues.
And it only shows when the fire is burning?
Any chance its condensation that's dripping down the inside of the flue and then coming out the joint at the thimble?

















