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  1. #1
    Join Date
    Jul 2011
    Location
    Tucson, AZ
    Posts
    106

    Rusty Heat Exchanger and Flue Gas ?'s

    Is Flue gas spillage dangerous even when there is no Carbon Monoxide present?
    Also some one told me that our heat exchangers here in Tucson, AZ are rusted out because the furnaces are over sized (winters are pretty mild here).
    Any way.... What is the relationship between a heat exchanger being rusted out and the furnace being overs sized?
    Also, is the rust on the out side of the heat exchanger from condensation during the cooling cycle?

  2. #2
    Join Date
    Jul 2009
    Location
    Columbia, MD
    Posts
    3,836
    Flue gas spillage is dangerous. You have 5 minutes to establish draft under ANSI standards. If you cant draft the appliance should be shutdown and tagged.

    Heat exchangers rust out because of underfiring which has lower flue temps. This causes the flue gases to condense and rust of the h/x. They also rust out if they are not venting.

    An oversized furnace may not run long enough which could cause the above.

    Also, furnaces do not have a long enough post purge(if they have any). That means at shutdown flue gases stay in the h/x, condense, and rust of the h/x.

    the rust on the outside is probably from condensation.

  3. #3
    Join Date
    Jul 2011
    Location
    Tucson, AZ
    Posts
    106
    Thanks for the feed back. I was just thinking about what you said about lack of purging and I think I made the connection (Light bulb). The furnaces here are coming on and going off more often, thus leaving behind flue gas/water vapor every time they cycle, as opposed to running for longer periods of time and having less on/off cycles.

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