I have seen the answer and I’m looking for the link now to provide.
In short, what I am being told is if it’s cracked at all, it is supposed to be replaced...hairline crack or bigger...doesn’t matter. Apparently they are afraid it will expand and possibly break off falling into and blocking the flue path...according to the sweeps.
I don’t buy it. Of course, I can see creosote from wood burning getting outside the liner and catching fire between the liner and the block if the creosote is bad enough. With anthracite I can’t see a problem at all...fly ash is non-combustible and the vent burns cooler.