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Condensation between double wall pipe layers

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by dmladenoff, Feb 13, 2012.

  1. dmladenoff

    dmladenoff New Member

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    We have a new fireplace, in a chase on an outside wall. Cold climate. Outside of chase is insulated. Nearly flat roof, no attic. Cold outside air vent is in the outer chase wall for air to the firebox. When nights are freezing, we get condensation/freezing water between the two flue pipe layers, because it melts in the day if it warms up with sun, water drips down onto the unit, and trickles out the front of the fireplace front air circulation vent above the firebox. Roof leaks have been eliminated as a possibility. It is a Desa unit, with air space between pipe layers. What can we do for this?

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  2. Wood Duck

    Wood Duck Minister of Fire

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    I am trying to figure out why water would condense between the layers of a double-wall flue pipe. Condensation in significant quantities requires moist air. Where does the moist air come from? Why is the outer layer cool enough to support condensation and freezing of the moisture? I can understand how you might have condensation between the layers IF you had a supply of moist air, but where is the air supply? If it is outside air it will have a low absolute humidity content because it is cold. When it gets in between the layers of flue pipe it should warm up because the inner layer of flue pipe should be pretty warm/hot. That should drive the relative humidity of the air down, so even if the outer layer of pipe is cold there shouldn't be a whole lot of condensation, and I definitely wouldn't expect the water to freeze. Maybe I am wrong about this, but I imagine the outer layer of a two-layer flue pipe warms up above freezing when you are using the fireplace. Even if the outer layer is at or below freezing, how is a supply of air getting into the space between the layers? This space should be protected from air movement so that it provides the insulating effect it is meant to provide. Is the sapce between the layers on double wall flue pipe hollow?

    So, I am wondering if you are sure that the water you see is coming from the space between the two layers of the double wall flue pipe.
  3. dmladenoff

    dmladenoff New Member

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    Thanks for the thoughtful comments. I went back to the store to look at the unit top and pipe (ours is closed in). Desa has their own pipe. It seems different because though sections fit together tightly like others, the pipe has a large space, about two inches, between the layers. This is open space. The connection to the top of the fireplace unit has large open spaces around the bottom of the outer layer. So it seems that it is designed to pull air up between the layers. I don't how others are, but from your comments, I am guessing they are not like this. I know there is pipe that has insulation between the layers too that exists. Given how ours is, the physics is not surprising; that even slightly warmer air gradually moves up the pipe, condenses and forms frost/ice on cold nights, and melts in the afternoon of a sunny day above freezing. Any further ideas?

    Our chase is bumped out wall, and a nearly flat roof with no enclosed chase above the roof. So two sections of pipe stick above the roof. Maybe enclosing it in a chase above the roof would help temper the pipe temperature? But it shouldn't we warm air in there either, right, so how much difference would that make?
  4. Wood Duck

    Wood Duck Minister of Fire

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    So there is an open space btween the inner and out flue layers, and this space pulls air from inside the house and presumably vents the air outside? Does this space draw air from inside the fireplace or from outside the fireplace but still inside the house? This seems unusual to me, but I really don't know much about flue pipes. The moving air would cool the outer flue pipe, but this seems to represent a large loss of heated air from inside your house to the outside. I don't think this is how most flue pipes are designed. Lets hope somebody else can comment on this - it doesn't seem right to me.
  5. dmladenoff

    dmladenoff New Member

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    Well, it is pulling air from within the enclosure around the fireplace, on top, you know where the pipe connects. So it is inside the envelope, since the outer walls of the chase are insulated.

    Do most fireplaces have other kinds of double wall pipe? Either closed airspace, starting right from the bottom, or with fiberglass in between?
  6. jtp10181

    jtp10181 Minister of Fire

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    Sounds like a typical air cooled flue to me. We put chimney air kits on all ours, so they pull outside air and not from the chase. That might help if desa makes one. This is self installed I assume? Or what does the dealer say about this?
  7. Wood Duck

    Wood Duck Minister of Fire

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    I thought an important reason to use insulated flue pipe is to keep the inner layer of pipe hot to reduce accumulations of creosote. Wouldn't an air-cooled flue negate this benefit?
  8. jtp10181

    jtp10181 Minister of Fire

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    air cooled is mostly used for open fireplaces, hot fast burns. Also if you have a hot fire with good wood, you have a hot flue, air cooled or insulated. The insulation or air chamber is also to keep the outer wall cool so it can pass testing and get listed, the benefits of the insulation is secondary.
  9. dmladenoff

    dmladenoff New Member

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    JTP, I checked and Desa doesn't have one. This was bought as Comfort Glow at Menard's. No help there; did you ever see the garage door spoof add? The chase is all closed in, I guess it has to be opened up. Can I adapt another kit?

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