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TOPIC: Rust

"Rust" Stains caused by flue gas

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The instant I capped my furnace flue, rust like deposits began to condense on the cap, drip to the chimney crown and then down to stain my metal roof. My chimney company say it's sulphur deposits. My furnace guys say the stains are very dark for sulphur but they concede that they can't suggest anything else that it could be. I have an old furnace but my maintenance company says it's working fine. They also say they do not recommend capping an oil furnace flue. If I don't cap the flue I'm going to have sulphur in the flue mixing with rain, forming sulphuric acid and eating away at my flue liner. If I do cap the flue, the gases condense and I have ugly and probably corrosive stains on my metal roof. I have two questions: 1) Is there a way to stop rain getting in the flue but preventing condensate dripping on the roof (other that turning the furnace up so hot that it vaporizes the house and wastes tons of oil)? 2)Is there a gentle way to remove these stains? They rub off the concrete crown reasonably easily but I don't want to abrade the finish of my metal roof. Please help, this is driving me nuts. Thanks a lot.
John Jennings

9/12/2008
http://www.sleepyhollowchimneysupply.com/pr_drip_cap.htm
Guest User

9/17/2009
The link provided by a reader below may be helpful. As far as cleaning, you could try Soft Scrub or perhaps a mild solvent like rubbing alcohol.
Todd Miller
Isaiah Industries, Inc.

9/19/2009
John, Just wondered if you found a fix for your shingle stains. I completely identified with your last comment "this is driving me nuts". I had the chimney liner installed, metal flue cap installed, then had the roof replaced. Now I've got a 10 foot wide stain from the chimney to the gutters. Hopefully you found your "Eureka".
Guest User

5/11/2010
Chimney/Flue gases contain a multitude of different byproducts that will form an acidic environment on the spark arrestor/cap. If that is the case, you will have some reaction and resultant deterioration of a galvanized metal. A stainless cap would help mitigate this effect, however, you may still have some staining from the condensate.
Eric Novotny
An informed customer is the Best Customer!

5/11/2010