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  1. #1
    Join Date
    Dec 2006
    Posts
    30

    Heat Exchanger sweating

    Went to a house today that complained of water dripping from the furnace. The setup is an older horizontal gas furnace with a evap coil on the return side. The water is dripping from the heat exchanger chambers. There are 6 vertical chambers in the furnace and water is running down the insides of all the chambers. I haven't ran across this before, but my first guess is that warm outside air (90-100 degrees for the past three weeks) is coming down the flue pipe and condensing as the heat exchanger is in the cold air stream and around 55 degrees.

    Does this sound reasonable to you guys? Would it help to add a damper to the flue to stop the warm outside air from traveling into the heat exchanger? Or do you think I have something else causing the condensation problem?

    Thanks
    Josh

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,189
    Better check that HX carefully. It's likely ruined by now.

  3. #3
    Join Date
    Sep 2003
    Posts
    1,675
    Having the evaporator before the heat exchanger in the air stream is what is causing your problem.

    And most likely your furnace/heat exchanger is rusted out and will need to be replaced.

    Like Baldloonie said Check it carefully because there are most likely issues with it

  4. #4
    Join Date
    Feb 2010
    Location
    Calgary, Alberta
    Posts
    1,112
    That's against code up here.... Bad news!
    Where is the filter in this set up? I can't imagine the air flow being very good

  5. #5
    Join Date
    Apr 2012
    Location
    Greenwood Indiana
    Posts
    16
    Evaporator before heat exchanger is not right.

  6. #6
    Join Date
    Feb 2004
    Location
    Midwest
    Posts
    9,932
    That's truly a "condensing furnace", and is about to become 90% efficient - at gassing people. Good fodder for the Wall of Shame forum.

  7. #7
    Join Date
    Aug 2010
    Posts
    2,837
    Quote Originally Posted by Rickeyc View Post
    Evaporator before heat exchanger is not right.
    Some gas-electric RTUs are setup that way from factory.

  8. #8
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,189
    Most are, and rot out in under 10 years in a lot of cases!

  9. #9
    Join Date
    Jul 2007
    Location
    Delaware
    Posts
    3,824
    Where is the furnace located? Do they ventilation running? What type of ignition system?
    Always here

  10. #10
    Join Date
    Feb 2004
    Location
    Midwest
    Posts
    9,932
    The last time I saw that on a residential furnace, the heat exchangers were gone when the furnace was 7 years old. Amazingly, the guy who installed it was actually a Lennox dealer who I knew. It was already configured that way, so I guess he thought it was ok to just throw in a new furnace without correcting the problem. Not hard to guess why the one before it conked out.

  11. #11
    Join Date
    Dec 2006
    Posts
    30
    This is an old natural draft furnace. Is it possible that the hot outdoor air would travel back down the flue and condense on the walls of the cold heat exchanger?
    I did notice the pilot was on so I shut it off. Not sure how much moisture would come from just the pilot flame.

  12. #12
    Join Date
    Feb 2010
    Location
    Calgary, Alberta
    Posts
    1,112
    It's more likely the humidity from where the furnace is sitting is condensing. If your flue is back drafting you have even bigger problems!
    Once that HX gets cold it's gonna act like a 2nd evap coil condensing whatever moisture is in it, since it's an older natural draft furnace it will probably take longer to heat back up to ambient, compounding the problem.
    What make is the furnace?

  13. #13
    Join Date
    Nov 2006
    Location
    Athens, Ohio
    Posts
    1,809
    Quote Originally Posted by nehvacman View Post
    This is an old natural draft furnace. Is it possible that the hot outdoor air would travel back down the flue and condense on the walls of the cold heat exchanger?
    I did notice the pilot was on so I shut it off. Not sure how much moisture would come from just the pilot flame.
    If hot outdoor air travels back down the flue - that goes against the laws of physics! What next? Will the law of gravity be repealed?

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