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  1. #1
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    Default Condensation On Flue Pipe

    This pipe had some condensation coming back down the pipe. The flue vent pipe run was long, probably about 45 - 50 feet straight up.



    Is there something that would help the situation, such as a powered vent or something similar? Also, the room was a little light on the combustion air. There were two 6" ducts to the outside. The boiler was 160,000 BTU input, which according to my calculations should have had two 8" pipes. That wasn't even accounting for the louvers and screens at the ends, which are restricting air flow into the room. Would this also have an effect on the condensation?

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    Jim Robinson
    New Mexico, USA

  2. #2
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    Mar 2007
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    Ormond Beach, Florida
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    Default Re: Condensation On Flue Pipe

    Someplace, I forget which code it was, I had a code which gave a table that specified the Btu input, size of the vent, and maximum height allowed, the maximum height depended on a combination of Btu input and vent size. I'm out camping right now so all I have is what is on my computer, I think it was in the older Florida Building Codes, all I have with me are the newer ones.

    Bob H., do you have a table showing that information, or was it possibly Florida only?

    Jerry Peck, Construction / Litigation Consultant
    Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
    www.AskCodeMan.com

  3. #3
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    Default Re: Condensation On Flue Pipe

    Off topic

    The best thing about camping.

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  4. #4
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    Default Re: Condensation On Flue Pipe

    Assuming that the vent run is too long, is there anything you can do about it at this point? It's not like they are going to be able to move the boiler. The client was asking about some type of draft fan to help move the flue gases out into the atmosphere.

    Jim Robinson
    New Mexico, USA

  5. #5
    Join Date
    Mar 2007
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    Near Philly, Pa.
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    Cool Re: Condensation On Flue Pipe

    More info needed:
    Fuel? Oil vs. gas
    single appliance or common vent? If mutliple appliances, needs specs on each and total input BTUs
    fan assisted or draft hood equipped?
    Vertical vent connector rise?

    The gas venting tables go up to 100ft. That doesn't make it a good idea but understand venting:
    Draft is the temp. difference btw flue gases vs. outside the flue and the vent height. Draft is merely a pressure gradient--not actual movement of stuff (gases). Flow is the actual movement of stuff through this conduit though most people speak generically about a chimney having a "good draft" when its actually is flowing well versus a "backdraft", which again, is a misnomer.

    When vents are oversized, the additional mass of flue liner wicks away too much heat, which reduces stack temps so the net effect is reduced draft. Just remember-- a hot flue is a happy flue. Sounds corny but it works. Therefore, while a tall flue tends to increase draft, that is only up to a point of diminishing return. After awhile, it tends to rob heat hindering draft.

    Now, a fan assisted appliance has lower stack temps to the point it needs the fan to prime the pump to get a draft (flow) established. You'll also note these units typically have smaller diameter vents compared to draft hood equipped units. draft hoods allow dilution air in to mix with the stack gases. This lowers the relative humidty of the stack gases and lowers the dewpoint or point where you get condensation. The net effect of a draft hood is to reduce damage to the vent from corrosion. It also de-couples the vent from the combustion chamber so sudden downdrafts such as from a breeze don't extinguish the pilot light.

    Jim can you fill in some of these details?
    HTH

    Keep the fire in the fireplace.

  6. #6
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    Default Re: Condensation On Flue Pipe

    Sure thing. The boiler is the only unit on the flue. It is natural gas, with a 6" flue pipe. I don't have an easy way to measure the flue height, but it's three stories plus a decent sized attic, so let's say 35 - 38 feet in height. We're also at 7,200 feet elevation if that makes any difference. It was a Teledyne Laars JVS 160,000 BTU Min Therm with draft hood. I appreciate the help for my client.

    Jim Robinson
    New Mexico, USA

  7. #7
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    Mar 2007
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    Cool Re: Condensation On Flue Pipe

    http://www.laars.com/documents/heatsystems/1025Y_SV.pdf

    This is the install manual

    A couple of notes:
    While approved for B-vent, this unit is rated at 85.4% AFUE, which technically makes it a condensing unit even though Laars doesn't consider it one (>83%AFUE). They instruct you to use B-vent the size of the flue collar and run straight up as was done here. However, at 7,200 ft above sea level, not many things will vent well. This should have been power vented at this altitude.
    Other considerations include the piping. If the bypass piping is too small, or rigged as a mix rather than bypass, it can rob so much heat from the stack, it causes condensation. Have a boiler tech inspect it. They should look at things such as primary/secondary circulation, heat loss calculations, types of radiators, etc. Since this unit has intermittent electronic ignition instead of a standing pilot, there is no priming of the flue with heat. Also, keep in mind at 160mbtu, this unit will produce over 1.5 gallons of water for every hour of elapsed firing and that water has to go somewhere. It can escape with flue gases or condense out and dribble down.

    Because of the amt. of condensation I see here, I would also specify the heat exchanger of the boiler be scoped then tested for integrity. It could be ready to fail. This unit comes with a vent damper and blocked flue switch which also should be tested. They should perform a full combustion analysis which would indicate how the unit is operating but also give you the stack temp and draft. Next, test for spillage at the draft hood using a CO analyzer then at the base of the unit.Finally, someone needs to run the calculations for MakeUp Air(MUA). Also, any competing exhaust fans such as a clothes dryer? Have them include a Worst Case Depressurization Test.

    To recap, esp. at your altitude, I would first try for power venting. If that is somehow out of the question, I'd run B-vent sized to the appliance with a straight run. This is one case where tradition has leaned towards one size larger vent but it may cause more condensation. Tough call. Anyway, enclose the vent in an insulated chase with clearances for the height of the stack less a foot or so exposed at the termination.

    This condensation is an ominos sign that should be closely investigated.

    To answer your question, the mfr. would have to guide you in choosing a power vent that is compatible with their unit and it would have to be interlocked for safety. Hope this helps Jim!

    Keep the fire in the fireplace.

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