frozen flue

My neighbors flue froze. there furnace is an 80% efficient unit it is vented through the roof with a double wall b-vent. The water heater uses the same vent and is only 3 years old. The top of the flue froze and then the water started melting down the pipe into the furnace filling the furnace with water. The flue is the appropriate height above the roof. I have never seen this before any ideas of what would cause it to freeze? The temperature has been below freezing for over a week but I did not think it vented enough moisture to build up an ice dam in the chase.

Very common in cold whether as the flue design temperature changes in the winter. To conteract the problem more of the flue needs to be Kept warm. It has nothing to do with moisture from the flue gas but the temperature drop while exiting the top of the flue or chimney.

I was going over to look at it today. I have never seen it happen before. you can’t insulate the b-vent so how do you keep it warm? The furnace is in the basement the flue runs through a chase on the main floor through the attic and out.

Attic section you can build a chase with proper clearance however I can’t give you anymore info then this. You may have to go down in size of the flue also for proper removal of flue gases. Too many varibles to give you anymore on this MB.

The furnace /water heater BTU is undersized for the size of the flue, of the flue pipe is oversized.

Refer to a licenced and qualified (and insured) HVAC guy.