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vent question

zepfanzepfan Member Posts: 229
i was working on an 80% furnace that is abount 12 years old and noticed white, almost calcium type substance, forming at every joint of the flue pipe that runs from the stack to the furnace.the main stack is double wall b vent and the pipe that runs to the furnace is single wall.there is no rusting or signs of this white material on anything but the joints and seams of the 90's.i have serviced this furnace every year for the past ten years and have only noticed this build up the past two is a 110,000 btu/in. furnace vented in 5" you have any idea what could be causing this condition?any assistance you can provide would be much appricated.thank you


  • Tim McElwainTim McElwain Member Posts: 3,904
    Check the environment

    in the combustion zone. Are there any volatile organic compounds being used. Bleaches, spray cans, hair sprays etc. This type of corrosion is typically due to something in the air being drawn into the combustion process along with being drawn into the draft hood.

    Did you do a combustion analysis?

    Any measurable level of Carbon Monoxide?
  • zepfanzepfan Member Posts: 229
    co measurement (white substance on flue)

    thanks for getting back to me.yes i measured the co and did a combustion analysis.both the air free co and the co in the stack were well within range.16 ppm and 37 ppm respectively. thanks again
  • Tim McElwainTim McElwain Member Posts: 3,904
    Is this a very damp

    area? Does it tend to have temperature surges up and down in the winter time? Is it just the heating system in this flue? Does the heating unit have a combustion fan (fan assisted Category I)?

    Many times the double wall stack in combination with a single wall connector will do this. It also depends on how long a run there is on the vent connector and how many elbows.

    My solution and it has worked on many situations like this is to make the connector also double wall.
  • SlimpickinsSlimpickins Member Posts: 314
    another thing

    I would check your temperature rise across the furnace and make sure its not on the low side. Check the data tag that is inside the furnace and it will tell you the range. Other then that, if the vent goes into an attic, check and make it's together. Make sure there's a termination cap and storm collar as well.
  • billbill Member Posts: 429
    Was just pondering that...

    Ya know that white stuff. It must have something to do with the Galv. connector. But what is it? All stainless or B-vent oughta stop it.

    Speaking of galvanized, Why does zinc, used in galvanizing, stop moss on roofs?

    Lots of mysteries out there.
  • Mark EathertonMark Eatherton Member Posts: 5,584
    edited October 2011
    Sweating vents...

    I have had this experience more than once, and the resolution was to eliminate the fixed draft hood, which was allowing a significant quanity of cool relief air into the vent, causing the flue gas to cool down to the condensation point. The fixed hood is replaced with a barometric damper, along with critical spill switches, and the fixed hood blocked off to control relief air going to the venting system. Sweating stopped immediately.

    Obviously, this can't (shouldn't) be done with an appliance with a draft induction fan...

    NONE of the above should be done by unqualified people. Improperly done, it can result in death or worse...

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • zepfanzepfan Member Posts: 229
    thanks,b vent is the answer

    thank you for your help. i was thinking  bvent is the only surfire way of fixing this.yes the basement is below grade so it probably gets damp,the laundry room is above the furnace room so it is out of the way of the combustion zone.i am just going to change the pipe to th furnace to be vent and be done with it.thanks again.
  • Mark EathertonMark Eatherton Member Posts: 5,584
    And ANOTHER thing...

    You mentioned that there is a laundry room above the mechanical room. If the trap seal on the floor drain serving the mechanical room has gone dry, the appliance could be ingesting chlorine fumes from the laundry waste. Wet the trap and pour an ounce of motor oil on top to keep it from evaporating.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
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