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flue gas condensation

todd_6todd_6 Member Posts: 13
I have a Burnham V8 oil fired boiler. What would be the lowest return water temperature or temperature gauge reading that you would want to reduce or eliminate flue gas condensation? The manual doesn't state. Where would you look at the boiler to determine if you actually had flue gas condensation taking place?


  • Fred HarwoodFred Harwood Member Posts: 261
    Low temp

    Minimum sustained returning water temp -- 140F
    Lowest sustained flue gas temp at breech -- 350 - 400F

    One sign of too low temps is yellowish stains around the flue pipe joints. Some condensing in a cold chimney may be unavoidable.
  • Jim Davis_3Jim Davis_3 Member Posts: 578

    Flue gases can condense at any return water temperature if your venting interaction is incorrect and it is firing properly. I have recommended resetting boilers to 105-120 degree water without condensation in outside tile chimneys for years. Water heaters operate with 100% street water temperatures and don't condense unless there is a venting problem or a combustion problem.
  • Fred HarwoodFred Harwood Member Posts: 261

    Another way to think about it is that each pound of #2 heating oil produces a bit more than a pound of water. If enough fresh air cannot get into your flue passages to scavenge the water, you get condensation, wherever. Of course, in order to keep your combustion efficiency high, you need to limit excess air in the boiler, which, if cold, will condense wherever the flue-side temperature is below the dew point. More heat or fresh air helps, but watch that efficiency.
  • todd_6todd_6 Member Posts: 13
    how low can I go??

    Well interesting so far. To further explain, I am using a 50 gallon storage/buffer tank to extend my run cycles on the boiler to get good efficency. Then I injection pump off the buffer tank (140-180') range to my radiant loops. Using fresh air intake kit and SS1 vent. I was trying to determine if I could go below 140' before the boiler kicks back in to recharge the buffer tank without damaging the boiler. Also, seems that with the SS1, after a few minutes off time, cold air sneeks down the flue pipe and cools the boiler a bit. So I need to see where I should be inspecting for flue condensation damage to occur. Thanks for the insight.
  • Jim Davis_3Jim Davis_3 Member Posts: 578

    Monitor your flue temperature entering the chimney. If it is above 275 degrees you should have no problem as is. If it is lower than 250 degrees some modifications may need to be made.
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