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  1. #1
    Join Date
    Jun 2005
    Location
    Ohio
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    1,720

    Flue Gas Condensing Temperature

    As I am now dealing with a job that has Lochinvar CHHN0990 non-condensing boilers that were condensing in the boiler as well as the flue, I've done a little search and found that natural gas flue gas condenses at 60*C, which equals 140*F. Coincidentally, that is the minimum allowable return water temperature to these boilers. Return water less than 140* is no longer a problem.
    As I have seen a lot of white crusty evidence of condensation on the flue where the flue goes thru an unconditioned/ventilated attic, would it be proper to assume the flue gas was/is dropping to 140* in this section? And to prove it, I want to test the flue gas temp at this point.
    Does this make sense?
    Anybody have experience with this?
    BTW, both boilers, and the Lochinvar domestic boiler show the same condensation evidence.
    jogas

  2. #2
    Join Date
    Jan 2007
    Location
    birmingham, al
    Posts
    29
    had the same problem with a raypak boiler, on a water source heat pump loop. My boiler was severely condensating, entering water temp was too low and boiler tubes were sooted. I had to replace 10 out of 17 burners,refactory, both igniters, flame sensors,and front plate. raypak states return water temp must be 105* or greater in 7 minutes. had to change how boiler was being controlled by ems and ajust bypass.

  3. #3
    Join Date
    Aug 2007
    Location
    WI
    Posts
    1,116
    Chances are, the boilers are underfired. Set it up with a combustion analyzer and make sure the draft and flow rate is ok. Flue gasses condense at temps much warmer than 140. You should see flue temps of around 400* to 500* for nat gas.

  4. #4
    Join Date
    Jun 2005
    Location
    Ohio
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    1,720
    Quote Originally Posted by DesMech View Post
    Chances are, the boilers are underfired. Set it up with a combustion analyzer and make sure the draft and flow rate is ok. Flue gasses condense at temps much warmer than 140. You should see flue temps of around 400* to 500* for nat gas.
    I realize that, at the boiler outlet, but 25' up, thru 4-6 45* el's, where it travels thru a ventilated attic, the flue gas temp drops way off.
    See attached attic flue pic.
    jogas
    Attached Images Attached Images  

  5. #5
    Join Date
    Mar 2001
    Posts
    1,213
    Pix don't work. If I remember right, under 350F will condensate flue gases. You may need to insulate the vent or change it to an insulated type ('A' vent) to protect the temperature. (possible power vent or assist?)

  6. #6
    Join Date
    Sep 2004
    Location
    oc
    Posts
    57
    water temp may still be an issue most new boilers are using a pid control
    which recirculates the hot water back into the boiler to help prolong the boiler .

  7. #7
    Join Date
    Mar 2001
    Posts
    1,213
    Now the pix work. You may want to drop a scope down that vent to check the integrity of the flue liner.. looks like 'B' vent... al. liner may be damaged.

  8. #8
    Join Date
    Aug 2007
    Location
    WI
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    1,116
    Quote Originally Posted by jogas View Post
    I realize that, at the boiler outlet, but 25' up, thru 4-6 45* el's, where it travels thru a ventilated attic, the flue gas temp drops way off.
    See attached attic flue pic.
    jogas
    Do you have combustion and draft readings?

  9. #9
    Join Date
    Apr 2002
    Posts
    11,808
    Quote Originally Posted by ersham View Post
    water temp may still be an issue most new boilers are using a pid control
    which recirculates the hot water back into the boiler to help prolong the boiler .
    even some condensing boilers with drains will use a mixing valve. the WM GV series used to have a mixing valve built right into them

  10. #10
    Join Date
    Jun 2005
    Location
    Ohio
    Posts
    1,720
    We are going to use a sewer drain camera to drop down each flue to determine the condition of the inner wall, which is class 1 (galvanized). No sense going any further if the flues are rotted. Been this way since '02.
    There's another 150M domestic water Lochinvar that is also condensing badly.
    I'm thinking about gathering the 2- 990M's and the 150M and using a modulating Exhausto style induced draft setup.
    Anybody had any good/bad experiences with them?
    I've got the return temp problem solved. 2 DDC loops, one looks at return temp, the other looks at each boiler's supply as a high limit. Working well so far.
    jogas

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