Green Building Bible, Fourth Edition |
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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
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.
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.
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="
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Posted By: GreenPaddyI suspect it's quite difficult to get surface temp readings from glass surfaces with a thermal camera(?)
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?)
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.
Posted By: TimSmallthe gas is at lower pressure, causing atmospheric pressure to bow the units inwards towards the centre of the panes.
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.
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.
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).
Posted By: Peter_in_HungaryPosted 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
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.
Posted By: djhPosted 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.
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.
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