heat exchanger rusting possible dangers?
Page 1 of 2 12 LastLast
Results 1 to 13 of 25
  1. #1
    Join Date
    Jan 2005
    Posts
    70
    I went to a customers house whos heat exchanger was rusting on a 20 year old furnace. I told her to replace due to possible carbon monoxide poisoning. She said she had detectors and asked weather there could be possible danger of a fire breaking out. I wasnt sure if it was so just wanted some info of the possible dangers if the heat exchanger cracked on a furnace

  2. #2
    Join Date
    Aug 2003
    Location
    Central Kentucky
    Posts
    6,247
    Anytime you see rust in a heat exchanger that furnace has had something impede it's ability to vent properly.

    A properly venting furnace should have no rust in it at anytime.
    Have you set up a Google alert for Carbon Monoxide yet?
    Click here to find out how.

  3. #3
    Join Date
    Jan 2005
    Posts
    70

    thanks for the info

    thanks for the info but that doesnt really answer the question of what the possible dangers are once it is rusting

  4. #4
    Join Date
    May 2004
    Location
    Rapid City, SD
    Posts
    7,414
    Once it's rusting it has weak spots. Those weak spots can be a perfect place for cracks and holes to occur. Once you have a HX that has cracks or holes (no matter how small) there's a chance for CO to leak out and into the house. Obvioulsy once you have a small crack it will only get bigger as it heats up and cools down.

    I doubt I'd have to give too much info on the down sides of CO getting into the house... nothing good can come from it.
    "If you call that hard work, a koala’s life would look heroic."

  5. #5
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914
    Originally posted by davidr
    Anytime you see rust in a heat exchanger that furnace has had something impede it's ability to vent properly.

    A properly venting furnace should have no rust in it at anytime.
    I see tons of heat exchangers with standing pilots that haveh quite a bit of rust that has nothing to do the venting in the heat mode.

    In my area furnaces are installed in the attic, or in a closet that draws its combustion air from the attic.

    Burn a pilot flame on one side a piece of steel that has 120º+ air on the side with the pilot flame and 74º-80º blowing across the other side of the piece of steel.
    The results are inevitable!

    The dedicated horizontal furnaces that were made by Ducane and Colsolodated, but sold by just about every name brand, are really bad about rusting around here if the pilot is left on.
    The ones that Lennox sold that had spark ignition, or furnaces that had the pilot turned off every year during the cooling season, rarely have any rust.

    The Ducane made ones finally rust a hole in the top of the HX chamber the pilot is in.
    The Consolodated made ones will get holes all over the place in the chamber over the pilot, assuming they didn't split open at a weld or crack at the expansion joint over the burners before they got a chance to rust through, lol.
    If more government is the answer, then it's a really stupid question.

  6. #6
    Join Date
    Jun 2000
    Location
    WYO
    Posts
    2,019
    I agree with Mark on the rust issue. A 20+ yr old furnace will invariably have rust. To what degree is another matter. As to your questions bgrich, rust alone doesnt cause CO. nor a fire danger.Unless it's so excessive the heat exchanger is blocked by it,or it has caused a breach,I would say light rusting is normal,depending on the exchanger material,humidity, over or under firing,and equipment age,and other factors.
    Is your question about rust,or cracked exchangers? Sureley a person qualified to condemn a furnace would already know these answers, I would assume,If I were the customer, and you were the tech I hired.
    The effects of rust are obvious. Severity is the issue. Is it somewhat rusted with no breaches or blockages and everything else checks OK. Or severely rusted?
    never say never

  7. #7
    Join Date
    Aug 2003
    Location
    Central Kentucky
    Posts
    6,247
    Originally posted by contactor
    I agree with Mark on the rust issue. A 20+ yr old furnace will invariably have rust. To what degree is another matter.
    I would have to disagree on this, I have seen 30+ year old belt drive furnaces in crawl spaces with no rust in them whatsoever.

    If the rust is on the inside of the HX there has been or is a venting issue.
    The flue gases are condensing on the inner walls of that heat exchanger at some point.
    Have you set up a Google alert for Carbon Monoxide yet?
    Click here to find out how.

  8. #8
    Join Date
    Aug 2003
    Location
    Central Kentucky
    Posts
    6,247
    Originally posted by mark beiser
    Originally posted by davidr
    Anytime you see rust in a heat exchanger that furnace has had something impede it's ability to vent properly.

    A properly venting furnace should have no rust in it at anytime.
    I see tons of heat exchangers with standing pilots that haveh quite a bit of rust that has nothing to do the venting in the heat mode.

    In my area furnaces are installed in the attic, or in a closet that draws its combustion air from the attic.

    Burn a pilot flame on one side a piece of steel that has 120º+ air on the side with the pilot flame and 74º-80º blowing across the other side of the piece of steel.
    The results are inevitable!
    So you are seeing rust on the outside of the heat exchanger then?
    Have you set up a Google alert for Carbon Monoxide yet?
    Click here to find out how.

  9. #9
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914
    Originally posted by davidr
    Originally posted by mark beiser
    Originally posted by davidr
    Anytime you see rust in a heat exchanger that furnace has had something impede it's ability to vent properly.

    A properly venting furnace should have no rust in it at anytime.
    I see tons of heat exchangers with standing pilots that haveh quite a bit of rust that has nothing to do the venting in the heat mode.

    In my area furnaces are installed in the attic, or in a closet that draws its combustion air from the attic.

    Burn a pilot flame on one side a piece of steel that has 120º+ air on the side with the pilot flame and 74º-80º blowing across the other side of the piece of steel.
    The results are inevitable!
    So you are seeing rust on the outside of the heat exchanger then?
    No, rust on the inside of the HX due to condensation of the byproducts from the pilot flame usually, sometimes just from the attic air if the people keep it particularly cool in the house.

    Imagin the conditions inside a horizontal furnace in a Texas attic on a sunny, hot and humid day.
    Lets say it is 95º and 65% RH outside and 130ºF and 30% RH in the attic.
    The homeowner runs the indoor temp down to 72º for whatever reason(really common), so the return air temperature entering the furnace is around 75º.

    Now you have 75º air on the outside of the HX and 130º 30% RH air on the inside of the HX.
    Even without a pilot flame adding water vapor to the equation, you will get some condensation because the air outside the HX is below the dew point of the air inside the HX.

    Toss in a pilot flame adding water vapor and it doesn't even take much in the way of ambient humidity to cause a problem.

    There is only ever a little bit of condensation, rarely enough to form water droplets, and it drys out between cycles, but it is enough to cause some serious corrosion over time.

    Some evidence of this is that the rust usually only forms on the walls of the HX, and is fairly uniform on the whole surface of it. Any baffles at the top of the chambers, and parts of the burner and collector boxes that inside air doesn't make contact with, rarely have any rust on them.

    [Edited by mark beiser on 11-04-2006 at 04:38 PM]
    If more government is the answer, then it's a really stupid question.

  10. #10
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914
    Originally posted by davidr
    Originally posted by contactor
    I agree with Mark on the rust issue. A 20+ yr old furnace will invariably have rust. To what degree is another matter.
    I would have to disagree on this, I have seen 30+ year old belt drive furnaces in crawl spaces with no rust in them whatsoever.
    How often is the temperature of the return air in the cooling season lower than the dew point of the air in those crawl spaces?

    It is relativly common during parts of the year around here that the return air temperature with the AC on is lower than the dew point of the attic air that the furnace is located in.

    [Edited by mark beiser on 11-04-2006 at 04:33 PM]
    If more government is the answer, then it's a really stupid question.

  11. #11
    Join Date
    Jun 2000
    Location
    WYO
    Posts
    2,019
    [i]posted by davidr[i]
    [b]If the rust is on the inside of the HX there has been or is a venting issue.
    The flue gases are condensing on the inner walls of that heat exchanger at some point.[b]

    Flue gas condenses on the cool walls of the heat exchanger everytime the unit initially fires up. A venting problem would no doubt cause more rusting,but furnaces with no vent problem still condense slightly upon firing up untill exchanger warms up.
    I still disagree with no rust on 20+ units. Cant say Iv'e ever seen a rust free steel exchanger ever over that age. I have seen minimal rust on these units. Stainless and aluminized exchangers of course dont rust.
    never say never

  12. #12
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914
    BTW, I'm not arguing that venting issues don't cause heat exchangers to rust, I'm just saying that venting issues are not the only thing that causes them to rust, and the rust can be totally independant of any venting issue.
    If more government is the answer, then it's a really stupid question.

  13. #13
    Join Date
    Aug 2003
    Location
    Central Kentucky
    Posts
    6,247
    I just read the last part of one of your post Mark & it appears our definitions of what degree of rusting we were talking about varied.


    What is odd is that on any furnaces that we had previous rusting issues with once fitted with a barometric damper pretty much disappeared.
    Have you set up a Google alert for Carbon Monoxide yet?
    Click here to find out how.

Page 1 of 2 12 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event