hoar frost plugging exhaust pipe

[jtrammel]

OK I'll ask the question. What the H@!! is "hoar frost"?

Frost that accumulates on a surface according to wiki. So basically your standard frost.

[pacnw]

there is something special about hoar frost, we had a front page newspaper story about it the other day.

To OP, I would think nothing would need to be done about this. No location posted, but in our area we have had 5 issues of frozen exhaust in the last 5 years that I can recall. most of these were due to something the homeowner did to cause exhaust to be blocked.