http://www.iqglass.co.uk/p/h/Products/Control_of_IQ_Glass/219/

Describes these windows as consuming up to 250 Watt/m² and how it is regulated. How many m² of glazing does this house have?

Pretty odd 'peace of mind' if you are actually *expecting* someone to shoot at you (or just at your windows - maybe someone who *particularly* dislikes the idea of heated glass) . On the other hand, if they are bullet-proof but explode randomly, I suppose you still get the adrenaline rush. :)

<blockquote><cite>Posted By: Nick Parsons</cite>Pretty odd 'peace of mind' if you are actually *expecting* someone to shoot at you (or just at your windows - maybe someone who *particularly* dislikes the idea of heated glass) . On the other hand, if they are bullet-proof but explode randomly, I suppose you still get the adrenaline rush. :)</blockquote>

Depends where you live I suppose. A while ago I was chatting with a friend who lives near Los Angeles. There had just been a street shooting (pretty much a daily occurrence there) where a police officer killed a vagrant and we decided to compare just a subset of shooting statistics. It turned out that the chance of being shot by a police officer in the UK was about 1 in 23 million and the chance of getting shot by a police officer in Los Angeles was about 1 in 140,000. My guess is that bullet proof glass might well be a good selling point in a place like that..........

And AS POSTED EARLIER!!!!

"The inside pane is the heating pane. An invisible film heats the glass and radiates the heat throughout the room. The glass on the inside of the room gets warm…The outer pane has an insulating coating which reflects the heat back in and prevents it escaping."

And continues with the plain-English version of qeiple's quote... "the glass remains completely cold on the outside."

And no one has yet come up with an answer to qeiple's earlier challenge on his last set of calcs.

Hands up how many people actually bothered to look carefully at that data sheet from Southwall Technologies?

I actually wasn't asking you. because you've referred to the information and, further, explained your position with regard to the laws of physics and the various standards laid down as ideal, but have done so without considering that other factors are in play which are equally important considerations in the design of a house - unless you're intent on passivhaus, which you obviously are, in which case your options are limited.

I suppose I'm less prescriptive and more inclined to look at a bigger picture.

agreed -- ie it uses more energy than any other form of heating to get the same result.

<blockquote><cite>Posted By: qeipl</cite>

First let's consider heat flow through the glass:

My understanding is that heat flow through a material (conduction) requires a temperature differential between one face of the material and the other.

If the outer (heated) face of the glass is kept at the same temperature as the inner face then there is no temperature differential so there will be no heat flow (conduction) through the glass.
The room cannot lose any heat through the glass.

Please point out any errors that I have made above.</blockquote>

Looks OK to me so far.



<blockquote><cite>Posted By: qeipl</cite>

Now let's consider the energy input (electrical) into the glass:

The energy input does two things:

1. It increases the temperature of the glass to 20C.
This means we have a temperature differential between the inner surface of the argon gas (which is in contact with the outer surface of the heated glass) and the outer surface of the argon gas, so there will be a flow of heat through the gas (to the next pane of glass in the unit), but this will be relatively small because gas is a poor conductor and the sealed unit will limit the effects of convection.
</blockquote>

As I understand it the heated glazing heats the inner face to a temperature above the room temperature, to allow it to heat the room.

The inner face will be close to the room temperature anyway without the electric heating element turned on. I've just measured my living room big window (which is two fixed panes about 4ft square) in the centre of a pane to see what the thermal gradient is for relatively poor glazing. These windows are ordinary 2G, 28mm overall, 4 - 20 - 4 I think, with low E glass. They have a Ug of around 1.8 W/m².K, so around double that of the IQ electrically heated triple glazing. The thermal gradient is:

Room temperature 19.1 deg C
Glass inner surface 18.1 deg C
Glass outer surface 10.2 deg C
Outside temperature 7.6 deg C

The heat flow through the glass is quite high compared to the IR loss (which the reflective coating largely prevents) and is given by the U value. In the case of the "mirror glass" it's 0.6 W/m².K. The heat transfer mechanism causing the heat loss is primarily conduction through the glass and gas filling, plus some convection within the gas filling.

<blockquote><cite>Posted By: qeipl</cite>

2. It radiates electromagnetic energy inwards and outwards.
The inward radiation passes through the glass, and is absorbed in the room.
80% (sic) of the outward radiation is reflected back into the room and absorbed.
The rest of the outward radiation passes through the outer panes of the unit and is absorbed outside.

Again, please point out any errors that I have made.</blockquote>

This sounds right for glass with a good IR reflectivity. Ordinary low E glass (not optimised for high solar gain) has an IR reflectivity of around 65 to 70%, so it seems reasonable for this "mirror glass" to be a bit better.

Oh, here's an easy-to-use version of that link
http://www.glasceyssens.com/afbeeldingen/fiches/Cylite%20Eco%20Passive%20-%2006%20Wm%2028mm%20Krypton%20HM88.pdf

And here's the PHI's explanation about windows
http://www.passivhaustagung.de/Passive_House_E/windows_passive_houses_06.html

Posted By: djh

Posted By: qeiplOne of us has got the physics wrong and if it is me, I want somebody to enlighten me

It's you that has the physics wrong.

1. It increases the temperature of the glass to 20C.
This means we have a temperature differential between the inner surface of the argon gas (which is in contact with the outer surface of the heated glass) and the outer surface of the argon gas, so there will be a flow of heat through the gas (to the next pane of glass in the unit), but this will be relatively small because gas is a poor conductor and the sealed unit will limit the effects of convection.

Yes. This is the bit that is unclear to me and I'll try to make sense of it in reply to David.

<blockquote><cite>Posted By: djh</cite>BTW #1, JSH, just one degree difference between your glass temp and your room air temp sounds very small. Are you sure it is accurate? The PH design limit for the difference is 3 C for comfort reasons. You want to be as close to that as possible for energy-saving reasons.</blockquote>

There could be an error of 1/2 a degree on those figures, as that's the accuracy of the sensor, plus there's a bit of uncertainty as to how good the thermal contact was. I measured the glass temperature by sticking the sensor to it with a bit of plasticine (makes a mess on the window........) behind the sensor, but this may well not have worked that well at keeping it in close contact. The sensor itself has a plastic case, and the flat bit may well not have been making perfect contact with the glass either. The same goes for the outer pane measurement.

Posted By: davidfreeboroughIt's the inner face (or pane facing the room) which is heated.

No. The IQ glass has its heating element on the outer face of the inner pane -
http://www.iqglass.co.uk/p/h/Products/How_is_IQ_Glass_made%3F/215//a" > - when they say 'on the inside' they mean inside the glazing unit. Read further down to the laminated glass bit for confirmation.

However, because the U value of the window has not been changed, exactly the same amount of heat will be lost through the window.

Because the heating element is on the outer face of the inner pane there is no temperature differential between the faces of the glass, so there can be no (conduction) heat transfer through the inner pane of glass. So the heat loss is not *through the window* but from the heating film on the outer face of the inner pane to the outer face of the outer pane.

All of the heat loss through the window (whether conduction, radiation or convection) is taken into account by the U value. All that matters is the U value of the window and the temperature at the inner face of the window.

No. The u-value is for the window as a whole - all the glass and the frame - and I strongly suspect that it's determined by the materials alone, not including the effects of the heated glass.
With the heating element turned on the temperature differential is between the heating film and the outer face of the outer pane, so it's the u-value of everything outside the inner pane that's pertinent.

The only difference is that all of that heat will be provided by the electric heater, not by the air in the room.

Yes. And this is important because the energy provided by the heater is in a different form to the energy trying to escape from the room.

As far as I can understand all electric heating is radiant - conduction and convection occur after radiant energy has been absorbed by some mass.

So the heating element on the outer face of the inner pane of glass radiates energy, a tiny proportion of which heats up the inner pane of glass (a tiny proportion because it's low emissivity glass, which means it's also a poor absorber of energy).

The rest of the electrical energy must be transferred as radiation, of which some goes directly into the room and most of the rest is reflected off the 'heat mirror' back into the room.

So the energy lost by the heated glass could be less than the energy lost via the unheated window.

Which bits of that don't make sense?

I'm beginning to think that we may not all be looking at this the same way. I've just drawn a quick sketch that crudely illustrates the heat transmission paths through triple glazing. Hopefully it may get us all thinking along similar lines.

The upper heat path (drawn angled just to distinguish it) shows radiated heat passing through the glazing. Some of the radiated heat gets reflected back by the "mirror" element and the low emissivity coating on the glass. This results in around 80% of that part of the heat loss being recovered, in effect.

The lower heat path shows the conducted (and small amount of convected in the gas/air) heat loss through the glazing. This is the part that primarily determines the window Ug value.

Posted By: JSHarrisI'm beginning to think that we may not all be looking at this the same way. I've just drawn a quick sketch that crudely illustrates the heat transmission paths through triple glazing. Hopefully it may get us all thinking along similar lines.

The upper heat path (drawn angled just to distinguish it) shows radiated heat passing through the glazing. Some of the radiated heat gets reflected back by the "mirror" element and the low emissivity coating on the glass. This results in around 80% of that part of the heat loss being recovered, in effect.

The lower heat path shows the conducted (and small amount of convected in the gas/air) heat loss through the glazing. This is the part that primarily determines the window Ug value.

So if we agree that heating the outer face of the inner pane to room temp prevents any conducted and convected losses through the inner pane, does this mean the heated glass only loses radiated heat (and only a proportion of it)?

JSHarris,

Your diagram shows heat loss through the window without any heated element.

This diagram (my first attempt using google's drawing thing) shows my understanding of the window with the heating element and makes me realise that I haven't been expressing myself clearly.

For clarity, the diagram separates the heating element from the inner pane of glass.
If the heating element is at or above 20C then it's impossible for any energy to transfer from the inner pane of glass to the outside.

So you have a temperature difference of 15c between the inside of the inner pane and the outside and no temperature gradient so the temperature suddenly drops from 20 to 5 on the outer surface of the outer window.

How does that happen?

I think there is a major flaw in your thinking.