Condensation from gas furnace freezing around flue pipe.

Condensation collects around gas flue pipe and freezes during very cold weather. What is causing this? It seems to collect around the collar. It has dripped down the pipe onto the unit.

Condesndation collects around furnace flue pipe on roof. Freezes around the collar. What causes this?

There is not enough heat in the furnace exhaust to carry the weight of the water created as a byproduct of burning the gas. The chimney will need a liner OR somehow insulated to prevent the condensation/moisture from falling out of suspension in the flue/exhaust gas.

This problem needs to be addressed by a on site visit from a local HVAC professional that knows what he is doing.