Condensation on Furnace flue

What do you say about condensation from the furnace, that leaks from the flue.



When a flue wall has condensation dripping down (as in your image) it’s usually from the flue being too long or it may consist of too many elbows.

What happens is…the exhaust gases are hot when they leave the combustion chamber and vent up through the vent connectors. If the vent piping is too long, the gases tend to cool down at the top portion of the piping (before they actually leave the long pipe) and this cooling down effect will condensate on the inside walls of the piping. This is when the built-up condensation simply drips down from the inner walls of the piping and seeps right out of the seams.

How long is the exhaust piping?

Furnace is located in the basement. Maybe 25 ft. ?


25 feet is quite normal.
I’m willing to bet that the vent connectors have excessive elbows…


Thanks! Here is a photo of flue in attic.

Is that step flashing installed behind the sheathing on the rake wall behind the flue? If so, I have never seen that type of installation!


This flue has a rain cap, right?

It sounds like a stupid question because they come equipped, but ya just never know with today’s homeowners.