We have a small well insulated house,
heated with an add on wood/coal furnace.
Stack temp almost never gets to 300 degrees,
unless I'm doing the almost daily 500-550 burn out.
So, we get build up.
I like to brush before the first fire (didn't this year)
And what I figure is halfway through the winter.
I can tell if it's not drawing right, frequently the horizontal
pipe going to the wall gets a bit of build up.
Amazing how little it takes to affect draw.
