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Rusted furnace flue


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I just tell people what's up, and to keep an eye on the flue pipe. It's likely 18 years old, as well, and the zinc coating has eroded away from the galvanized metal, so it's rusting. It could be fine for several more years . . . or not.

Painting, clearly, is not an acceptable repair method. : )

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There's an exhaust gas bouyancy issue.

Single-walled connector in cold space, too many bends in that flue, wrong-sized flue pipe, etc. - something that's causing those gases to cool too quickly, lose bouyancy and stall as they leave the vent. The acidic steam settles around that vent and strips the galvanizing. It also degrades the vent, causes so much mineral salt to form and fall out of the base of the vent that it looks like it's snowing inside the appliance and it can trash the exchanger. If there's an elbow directly below the roof plane, as there often is, there's probably condensate dripping off the elbow onto the insulation.

Or,.......it's just old as hell and wearing out.

When I see those I'll normally walk over to the vent and feel the pipe to see if it's still got a substantial amount of steel left or if it's already paper thin and cracked or about to crack. Often find cracks. Sometimes holes. Sometimes the entire outside of the vent will be eaten away and what you can see from the ground is the liner.

Drive down a city street and look at the condition of those vents and you can identify from the street those houses where furnaces or boilers have an issue with weak exhaust gas bouyancy and could have vent or H.E. issues.

Got a phone call from an HVAC firm a couple of months ago after I'd called a vent after seeing this and checking the vent. Thought he was calling me to argue with me about something. Nah, he was calling to tell me that he was taking the unusual step of calling a home inspector to compliment him for being the first inspector he's ever known that's savvy enough about furnace vent issues to recognize this as an issue.

Told him he should get out more; this is inspector 101 stuff. He disagreed, said it can't be or he'd get called out for it more. Says it's HVAC 101 stuff but the majority of folks in his business don't get it or they ignore it. That explains why I find so many new furnaces with vents that look like that and when I get up on the roof to check them out I find them cracked, holed or ready to fail.



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  • 4 months later...

I am not an inspector but was impressed by the information I found on this site with just a simple google search.

I had a chimney expert come to my home today because I have a "rusting effect" on my shingles and was concerned about resale issues should I choose to move.

The explanation I got was about the condensation of gases and a detailed account of how when you replace a boiler with a more efficient one (I did), your gases aren't as hot and so it is more prone to this issue. When I asked about replacing the flue with another cap, and cleaning up the stain, I was told this would not be possible because the connection to the top would not be the same manufacturing fit, and to clean the shingles would void the warrantee. So, the solution (might) be to relace the entire piping, put a new flue on top.

Is there any other possible solution? I'm looking at a significant expenditure (and construction within my home to remove the old piping) and am out of work.

Please excuse any improper terms - I'm clearly a novice, but could use help and advice.


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  • 4 years later...

I am selling a home and the inspection called for a repair or replacement of the roof section of flue pipe. This was due to a less severe but similar instance of the original picture in this thread.

Most of the pipe had a light coating of rust that brushed right off. While I brushed it I was holding onto the pipe for leverage and didn't notice any weakness in its composition or stability.

At a joint directly in the middle of the pipe, there was a small crack

(about 3/8" X 1/64") right on the end of the female section

I used rust remover and a steel brush and sealed the damaged section of the joint with JB Weld.

Does someone with knowledge in this field know if this might qualify as the optioned "repair" that was listed in the inspection.

Secondly, will this hold up for about 5 more years when the new owner will most likely be looking at reroofing the home?

Thanks for reading this.


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Repair of gas vent is done by replacement.

The inspector may have called for replacement not because of a little rust but because the pipe was single-wall pipe. Single wall is a lousy choice in most places now. Just about all new installations use B-vent (double-wall) pipe.

No one knows for sure how long it will last. We don't carry crystal balls in our tool pouch.


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