Corrosive action on gas flues in an attic

[Michael Bronner]

Question. Can anyone enlighten me on what might be causing the corrosive action on two gas furnace flues (class b) in an attic space on a home that is less than one year old. The two flues had this substance on it and when I scratched some off, it was beginning to pit the vent pipe some. Also, the condensate over flow pan was beginning to do the same thing. The attic was empty of any stored items and no corrosive agents in the attic was visible. I am a bit dumbfounded as to what might be causing this.

[John Kogel]

It looks like drops of condensation, in fact you can see where the moisture ran down the underside of the pipe. What made that moisture corrosive would be NG byproducts, no? Any sign of backdrafting?

The pipe had to be cold when the condensation occurred, so it is a mystery, for sure. Not likely the furnaces are taking turns damaging each others vent pipes. There must be a moisture source when the furnaces are off.

[Ian Page]

There appears to be some kind of dark 'spotting' on what looks like insulation behind the vent. I wonder if someone was spraying something, maybe during construction as the house is only a year old. Perhaps some laborer was screwing around and spraying another with coke, which can be extremely corrosive.