Cat. 1 venting is not approved for positive flue gas pressure. CO can blow out of those seams or chimney! I would Red Tag this and shut it down. Better make a call in the morning. Condensation and corrosion aren't the only reasons for PVC.

HTH and welcome!
Bob Harper
Certified Fireplace Inspector
Certified Master Hearth Professional

__________________
disgusted with some people on this forum. Out of here!