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  1.  
    I've got some triple glazed sliding doors - the panes are approx 2m high and 0.7m wide. The frames are aluminium.

    On one section, an odd (to me at least) patch of condensation appears on the glass in certain conditions. It's on the inside (ie facing the room) of the innermost pane of glass. A kind of elongated oval, and it's right in the middle - basically the portion of the glass that is farthest from any edge.

    It seems to appear on cold nights when there's raised humidity inside from cooking etc. That's not surprising, but I would have expected that if condensation was going to appear anywhere, it should be at the edges, where the insulating properties of the triple glazing are compromised by the edge spacers and (more significantly) the aluminium frames.

    This pattern is what I'd expect to (and do sometimes) see on the *outside* of a triple glazed window - because the middle of the glass will be colder than the edges, where some heat is being conducted from the inside via the frames.

    But to me it doesn't make any sense on the *inside*. Surely the middle of the glass should be the warmest part; the part most effectively insulated from the cold outside. So what is the explanation?
    •  
      CommentAuthorDamonHD
    • CommentTimeDec 3rd 2019
     
    Maybe you have convection setting up inside those big panes, letting the middle cool?

    We get some very odd patterns on ours, eg esp look at the bottom pic:

    http://www.earth.org.uk/triple-glazing-3G.html

    Rgds

    Damob
  2.  
    Posted By: DamonHDMaybe you have convection setting up inside those big panes, letting the middle cool?

    We get some very odd patterns on ours, eg esp look at the bottom pic:

    http://www.earth.org.uk/triple-glazing-3G.html" rel="nofollow" >http://www.earth.org.uk/triple-glazing-3G.html

    Rgds

    Damob


    Yours look much like I might expect though - condensation mainly in the middle on the outside, and a little bit near the frames on the inside.

    Interesting suggestion that it could be convection...will have to think about how that would work.
    • CommentAuthorbhommels
    • CommentTimeDec 3rd 2019
     
    Are you absolutely sure the glass panes have the right orientation? Radiative losses (from the low E coating facing the wrong way) could do exactly what you describe, and works "best" on clear, cold nights.
  3.  
    Posted By: bhommelsAre you absolutely sure the glass panes have the right orientation? Radiative losses (from the low E coating facing the wrong way) could do exactly what you describe, and works "best" on clear, cold nights.


    You've got me worried now! I'm not sure how to check though, there are no specific markings on the glass that I can see, that indicate which is the coated pane.
    • CommentAuthorRobL
    • CommentTimeDec 3rd 2019
     
    I might be understanding the coatings incorrectly - but I think that the low-e coating symmetrically stops emissions and absorption of specific IR frequencies. In 3g it just needs to be used twice - once for each air cavity, and it doesn't make a huge difference which faces of glass it is on. I think there are other secondary issues with where it should go, like maybe longevity? The calculator below shows similar results with glass either way around.

    I'd be getting an IR temperature sensor pointed at it. You can estimate the U value of a window, as a ratio with other things. So if it's indoor, in still air, you start by assuming the air has a coupling conductivity U of about 8W/degC/m^2. Use the same IR sensor to measure the window(indoor surface temp), an indoor object at air temperature, an outdoor object at air temperature. The calculation is then:

    Uwindow = 8*(Tindoor_air - Tindoor_window_surface)/(Tindoor_air-Toutdoor_air)

    I tried the above, and estimated U=0.93W/degC/m^2 for a 2G window (should be 1.1).

    For best accuracy, use similar objects in each case - eg a piece of matt sticky backed paper. This avoids different surface emissivities altering the IR measurement.

    Here's a link to a free calculator, that allows you to try different window setups:
    https://spectrum.pilkington.com/Main.aspx
    • CommentAuthorbhommels
    • CommentTimeDec 3rd 2019 edited
     
    Posted By: RobLI might be understanding the coatings incorrectly - but I think that the low-e coating symmetrically stops emissions and absorption of specific IR frequencies. In 3g it just needs to be used twice - once for each air cavity, and it doesn't make a huge difference which faces of glass it is on. I think there are other secondary issues with where it should go, like maybe longevity? The calculator below shows similar results with glass either way around.

    There is an efficiency difference for coatings for IR coming from glass or from air (probably due to the refractive index), so there is a directionality. You are right in that it should make less of a difference for 3g than for 2g, but it could just explain the condensation pattern.....
  4.  
    What would be happening - the innermost pane of glass is radiating heat outwards - but more so in the middle because the frame is somehow keeping the edges warmer? Does this mean more heat is being lost from the glass by radiation than is being lost through the frame from conduction+radiation? Not sure I understand that.
    • CommentAuthorbhommels
    • CommentTimeDec 3rd 2019
     
    I agree it is far fetched. It is the only thing I could come up with that would explain the condensation pattern though.
    For the pet theory to hold, the radiative losses in the middle of the glass would have to outdo the losses at the spacers by some amount. Common wisdom is that this is normally never the case. The fact that the mystery pattern only occurs in conditions when the radiative losses are highest (clear night sky) does point to it having something to do with that.
  5.  
    Could the panel just be faulty, perhaps no argon injection occurred at the factory, so you've got a very poor performing panel, more akin to old style double glazing. It's on ONE panel I think you said? Indication of ONE being faulty. Can you not just put it back to the suppliers, and request a replacement glazing unit?

    All a bit obvious, sorry :shamed:
  6.  
    Posted By: GreenPaddyCould the panel just be faulty, perhaps no argon injection occurred at the factory, so you've got a very poor performing panel, more akin to old style double glazing. It's on ONE panel I think you said? Indication of ONE being faulty. Can you not just put it back to the suppliers, and request a replacement glazing unit?

    All a bit obvious, sorryhttp:///newforum/extensions/Vanillacons/smilies/standard/shamed.gif" alt=":shamed:" title=":shamed:" >


    I don't really see what kind of fault would cause this particular pattern though. It's true that this doesn't happen on the other panel, but it could be to do with location. The sliding doors are on a landing part way up some stairs; I am wondering if the stairs create some kind of weird channeled flow of humid air or something.
  7.  
    To condense, the surface would have to be around say 12oC (check your actual air humidity and go blind staring at a psychrometric chart :shocked: )

    So that suggests to me that the 3G is not functioning, by keeping that internal face warm. As you mentioned above, normally the frame/edges are the coldest part internally, and the warmest part externally, when 3G is working, hence the external "oval" patterns in the morning till the external glass pane warms up a bit.

    My thought process says your internal glass pane is colder than the frame, hence the internal "oval" shape, as the frame/edges are warmer internally, which can only mean the 3G is not functioning. I suspect it's quite difficult to get surface temp readings from glass surfaces with a thermal camera(?), but you already have a thermal representation in the form of condensation occurring at the coldest bits, and the centre should deff not be the coldest bit.
    •  
      CommentAuthordjh
    • CommentTimeDec 7th 2019
     
    Posted By: GreenPaddyI suspect it's quite difficult to get surface temp readings from glass surfaces with a thermal camera(?)

    I haven't tried it, but I gather it's fairly easy if you put a piece of sticky tape at the points where you want to measure the temperature.
  8.  
    +1, but hold the IR thermometer right up against the tape so the tape fills its field of vision, which is wider-angle than I expected.

    The panel may have lost its inert gas fill, so the middle of the pane is being cooled by the outer panes. The edges of the pane are warmed by the frame, which is warm on the inside (do you have EWI?)
  9.  
    Posted By: WillInAberdeen+1, but hold the IR thermometer right up against the tape so the tape fills its field of vision, which is wider-angle than I expected.

    The panel may have lost its inert gas fill, so the middle of the pane is being cooled by the outer panes. The edges of the pane are warmed by the frame, which is warm on the inside (do you have EWI?)


    I could just about see how that might happen with say timber frames with insulation oveelapping them on the outside.

    But in this case the frames are aluminium. No insulation to the outside of them - this location is internally insulated and that insulation partly overlaps the frames on the inside. One edge of the panel in question is a meeting stile with a sliding panel. Although the frames are thermally broken i would be surprised to find that the inside of them is warmer than the glass. That said, my physics understanding of exactly what goes on with radiated vs conducted heat loss is a bit sketchy and there may be an explanation in there somewhere. It does seem to happen on clear nights which as someone pointed out upthread may be relevant.
  10.  
    By the way are the cheapish IR thermometers you can get on ebay etc any good or a waste of time? I'd be curious to measure what's going on with the temperatures.
    • CommentAuthorTimSmall
    • CommentTime4 days ago edited
     
    I've seen a similar pattern on a large double glazed unit which was caused by a manufacturing fault.

    The unit was assembled flat, with the bottom pane sitting on a flat workbench top, causing the top pane to sag towards the middle of the unit.

    The unit had a 14 mm gap between the panes at the edges, but only about an 8mm gap in the centre.

    Larger sealed units should be checked to ensure the panes are parallel, and I now do this at delivery if possible!

    I believe it can also happen if units are assembled in very hot conditions - when exposed to colder weather, the gas is at lower pressure, causing atmospheric pressure to bow the units inwards towards the centre of the panes.

    Difficult to fully check on a triple glazed unit, but you could get a rough idea by putting a straight edge across the inside and outside of the unit.

    Fix is to drill an access hole in the edge of the unit, inject extra argon, and then reseal.
  11.  
    Posted By: TimSmallI've seen a similar pattern on a large double glazed unit which was caused by a manufacturing fault.

    The unit was assembled flat, with the bottom pane sitting on a flat workbench top, causing the top pane to sag towards the middle of the unit.

    The unit had a 14 mm gap between the panes at the edges, but only about an 8mm gap in the centre.

    Larger sealed units should be checked to ensure the panes are parallel, and I now do this at delivery if possible!

    I believe it can also happen if units are assembled in very hot conditions - when exposed to colder weather, the gas is at lower pressure, causing atmospheric pressure to bow the units inwards towards the centre of the panes.

    Difficult to fully check on a triple glazed unit, but you could get a rough idea by putting a straight edge across the inside and outside of the unit.

    Fix is to drill an access hole in the edge of the unit, inject extra argon, and then reseal.


    You're right! Putting a straight edge across it reveals that the middle of the pane is bowing towards the centre by over 5mm.

    This is the case on both the innermost and outermost panes.

    This is a slimline 3g unit, so the gaps between panes are already quite slim, and I think that amount most be significant. Time for an attempt to activate a guarantee, I think.

    Thanks.
    • CommentAuthorgyrogear
    • CommentTime3 days ago edited
     
    Posted By: TimSmallthe gas is at lower pressure, causing atmospheric pressure to bow the units inwards towards the centre of the panes.


    This reminds me of a Rolling Stones song :

    "Slumping flat glass ? It's the gas, gas, gas..."

    :devil:

    gg

    https://www.youtube.com/watch?v=9ffyuJPt3JE
    • CommentAuthorTimSmall
    • CommentTime3 days ago edited
     
    Posted By: lineweight
    You're right! Putting a straight edge across it reveals that the middle of the pane is bowing towards the centre by over 5mm.

    This is the case on both the innermost and outermost panes.

    This is a slimline 3g unit, so the gaps between panes are already quite slim, and I think that amount most be significant.


    If you want some numbers to shake at them, then you'd need to know the glass configuration.

    If we assume 4-8-4-8-4 Krypton fill, dropping down to 4-3-4-3-4 in the centre with that bow...

    4-8-4-8-4 Krypton fill with 2x Planitherm Total+, and 1x Planiclear gives a U value of 0.65 W/m²·K

    4-3-4-3-4 Krytpon fill with 2x Planitherm Total+, and 1x Planiclear gives a U value of 1.29 W/m²·K

    (NFRC cal method 0 degrees C outside, 20C inside).

    ... So yes, a significant loss.

    Actually, I've just remembered LBNL Window can simulate deflected panes too, so presumably it could give you an overall U value for the unit if you had all the figures (glass types and thickness, deflection, gas type and thickness, and overall unit size).

    Tim.
  12.  
    Posted By: lineweightYou're right! Putting a straight edge across it reveals that the middle of the pane is bowing towards the centre by over 5mm.

    I presume you have checked the other glass unit as well
  13.  
    Posted By: TimSmall

    If you want some numbers to shake at them, then you'd need to know the glass configuration.

    If we assume 4-8-4-8-4 Krypton fill, dropping down to 4-3-4-3-4 in the centre with that bow...

    4-8-4-8-4 Krypton fill with 2x Planitherm Total+, and 1x Planiclear gives a U value of 0.65 W/m²·K

    4-3-4-3-4 Krytpon fill with 2x Planitherm Total+, and 1x Planiclear gives a U value of 1.29 W/m²·K

    (NFRC cal method 0 degrees C outside, 20C inside).


    That's pretty much exactly what I've got. I think it probably actually goes to something like 4-2-4-2-4 in the middle.
  14.  
    Posted By: Peter_in_Hungary
    Posted By: lineweightYou're right! Putting a straight edge across it reveals that the middle of the pane is bowing towards the centre by over 5mm.

    I presume you have checked the other glass unit as well


    yes. It has a bow too, but only 2-3mm rather than 5-6mm. I don't know if that's enough out of spec to complain about too.
    •  
      CommentAuthordjh
    • CommentTime3 days ago
     
    Posted By: lineweightIt has a bow too, but only 2-3mm rather than 5-6mm. I don't know if that's enough out of spec to complain about too.

    FWIW I put a 6' straight edge over my three biggest panes yesterday out of interest, and they are all dead flat.
  15.  
    Posted By: djh
    Posted By: lineweightIt has a bow too, but only 2-3mm rather than 5-6mm. I don't know if that's enough out of spec to complain about too.

    FWIW I put a 6' straight edge over my three biggest panes yesterday out of interest, and they are all dead flat.


    yeah...I've just checked my other 3g window - different supplier - and it's dead flat.
  16.  
    Posted By: lineweightyes. It has a bow too, but only 2-3mm rather than 5-6mm. I don't know if that's enough out of spec to complain about too.

    I would complain about them both. They will have to come out to look at /replace /fix one of them so they might as well do them both and why should you have a sub-standard product. There is obviously a manufacturing problem and the suppliers may not even know about it - yet.
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