Exploding Velux window - does this often happen?

[Flyfisher]

Edit: Oh b*gger! There I go again, repeating what someone else has said, because I didn't spot the new page! Sorry, FF.

No need to apologise; 'great minds' and all that (well, consistent ones anyway :wink: ).

 

[Nigel Watts]

I'd have thought that if thermo-osmosis was the problem then all these sealed units would eventually implode?

I suspect that smaller windows are more at risk than the larger ones. Glass has some ability to flex before breaking. A drop in pressure in the gap in a large window could probably be accommodated by this natural flexing, but with smaller windows the stresses would be higher as the gap volume to glass surface area ratio will be much higher.

 

[Flyfisher]

When the temperature then equalises, a pressure difference builds up in the other direction, but the membrane does not allow molecules back to counteract this.

Is that last bit correct? I thought that osmosis is a general term for movement of molecules across a semi-permeable membrane due to some sort of gradient. For example, water will move from a weak solution (of something) to a strong solution across a membrane permeable only to water. The gradient is the concentration of water molecules (lower in a strong solution) and osmosis acts to equalise the solution strengths on both sides of the membrane. But the membrane is permeable in both directions. Indeed, without some form of active mechanism within the membrane (e.g various transport mechanisms within living cell walls), it's difficult to envisage how a passive membrane could be uni-directional.

A quick Google found this 1951 paper: http://www.jstor.org/pss/98787. It suggests that thermo-osmosis can indeed result in a pressure change as gases migrate across a permeable membrane but that the process is eventually halted at some point when the pressure differential prevents further migration, which make sense. This pressure differential is described as being "several centimetres of mercury".

One atmosphere = 760mm mercury = 14.7 psi. If "several centimetres" means 50mm then the pressure differential is just about 1psi.
Could such a low pressure really stress a sheet of glass to breaking point?

 

[middi]

That would make anybody s**t themselves. Ideal if one suffered of constipation.

 

[Nigel Watts]

I am into hypothesis mode here:

Gradient is caused initially by difference in temperature (warm glass, cold outside air), increasing pressure in the gap between the sheets of glass compared to the outside pressure. Sustained period of cold weather and nice warm room inside to heat up window maintains this temperature difference over a long period.

Argon finds it easier to get through the membrane than normal air gasses (Nitrogen, Oxygen, Carbon Dioxide) and starts to escape.

By the time the temperature gradient has gone away the escaped Argon has diffused away into the atmosphere and the air outside has a negligible concentration of this gas.

The pressure drops in the gap relative to atmospheric, but since the Argon on the outside has gone away nothing can get back in.

Barometric pressure rises and temperature differential happens to be the other way round one day and bingo, explosion occurs.

 

[Nigel Watts]

Found this on another site where someone reported a similar problem of an imploding Velux:

"I work for VELUX and you're welcome to call me.

--------------------------------------------------------------------------------

Hi Bossfan,

I work for VELUX in Glenrothes, Scotland where we also look after customers in Ireland. Your blog came up on a Google service we buy that shows where our name appears on the internet and I thought I'd reply.

Obviously, you've already been in touch with us so some of this you may already know.

The wikipedia article is pretty close. As it suggests, a double glazed unit has to be built with some tolerance for movement for all sorts of reasons. One reason is if the seal on the unit breaks down, the gas between the panes can escape at a different rate to the rate at which air gets back in, depending on the air pressure outside. The 2 panes can then get pulled in towards each other by the vacuum created. Larger pieces of glass can generally cope with more movement and over time the pressure difference will equalise. However, if the pieces of glass are small (at a guess your window will be one of our smaller sizes) or subjected to sudden changes in air pressure (such as if the weather delivers a cold snap), then, fortunately very rarely, they can break.

Our windows use toughened glass on the outside pane - to protect them from items coming through from outside. The inside pane is usually float - or ordinary glass. In your case I think what's happened is that the seal on the unit has broken at some point; a vacuum has built up inside the unit and the weaker piece of glass - the one on the inside - has broken.

This is very unusual which is why although many people in the glazing industry would know about it, most people don't. However, if there is anything else you'd like to ask me about it, please feel free to call me; main switchboard is 0044 (0) 1592 772200. "

It will be interesting to see if their software picks up on this thread!

 

[Flyfisher]

Interesting post, thanks.

It seems to me as if the DG seal is acting more like a physical valve - a bit like those valve-stopper gizmos used in wine bottles that allow air to be sucked out but not seep back in. I can envisage the heated DG gas 'pushing' on the seal to create a slight gap through which it can escape. When things cool down, the seal is 'sucked' back against the glass creating a gas-proof seal and a partial vacuum inside the DG unit.

Also interesting is the way that Velux monitor the web for any mention of their products. Soon after the web became established in the early 1990s, I remember reading that Compaq (a computer company since bought by Hewlett Packard) had a team of 6 people specifically employed to search for any mention of their products and to correct any misinformation and to assist anyone with problems. Interesting that Google now offer such searches as a commercial service - a bit like the old press cuttings services I suppose.

 

[FamilyWiggs]

It will be interesting to see if their software picks up on this thread!

Let's see if we can get CCHQ in the game too?

Velux
Explosion
London
Solution
Active Cell
Mercury Switch
Security
Gas leaking out
Flying

All of these words appeared in this thread!

 

[Flyfisher]

CONGRATULATIONS!!!

You win first prize in the spooks competition: an orange jumpsuit and an all-expenses-paid trip to a world-renowned Cuba-based water-"sports" centre.
A car will collect you sometime in the early hours. No need to wait up, they'll let themselves in.

 

[MdB]

Let's see if we can get CCHQ in the game too?

I'm not sure if you get bonus marks are negative marks for using CCHQ rather than GCHQ though there are certainly spooks in both of them...

 

[FamilyWiggs]

Let's see if we can get CCHQ in the game too?

I'm not sure if you get bonus marks are negative marks for using CCHQ rather than GCHQ though there are certainly spooks in both of them...

Don't know what I was thinking about!

 

[Mrs.R]

Exactly the same thing happened to a window in my sisters house! It was above a bath in her upstairs bathroom, and the glass just exploded, same thing as it was inside and not outside. Scary as she has 3 young children who could have been in the bath at the time, thankfully they were not! Again a small velux window, so everyone be careful, this could have serious implications.

 

[djellworth]

Hi

First time post – been lurking for a while and just moved in to an (not that old by many people's standards I guess) 1875 Victorian terraced house so lots of questions to ask the experienced amongst you in the near future which will follow in due course.

In the meantime, the Velux window issue caught my eye. It happened to a colleague who did some research and passed on to me a couple of threads from other boards. Clearly Velux are aware of the problem – and it has caused injury in the past.

http://forums.moneysavingexpert.com/showthread.html?t=1381625
http://www.boards.ie/vbulletin/showthread.php?s=e7c7071fdd54cf070306d203b534f04a&t=2055495056&page=2

If the links dont work, search on Velux Shattered window - or something like that.

cheers

 

[Jill]

We currently have a double glazed hardwood window in our living room (not velux), which was installed last summer, for an unknown reason a crack appeared on the inside glazing running vertically about 2 inch in length, the crack has progressed further at a slight angle another 4 inches, until reading this thread i wasn't too concerned. We are due to have more of the windows fitted, and this pane of glass replaced in the next couple of months, the glass is Pilkington argon filled. Just hope it doesn't shatter like Nigels.

 

[AMc]

Jill your window or the unit should be covered by whoever supplied and/or them. It's worrying that the crack has appeared - it could potentially indicate a problem with movement in the frame. If there is enough stress in the frame to crack the glass I'd be worried that the wall is moving. If it turns out to be a failure in the glass through a manufacturing fault then it isn't a major job to replace the sealed unit and again shouldn't be your problem. I would get it checked out.

 

[Flyfisher]

My guess is that the glass won't shatter because it has clearly already suffered enough stress to crack and I'd guess that if it was going to shatter it would have already done so. I suspect you have toughend glass rather than Nigel's float glass pane that shattered.

However, all that is conjecture and, right or wrong, I entirely agree with AMc that it should really be checked by the supplier/installer and, hopefully, fixed under warranty.