Hi All
I have recently renewed DG units in timber frames as they had all failed. The property was recently bought needing renovation and I factored in this cost on purchase, all well and good. However it became apparent the units were a max of 25 years old and could be much newer. I then learnt that DG units have a usual guarantee of 10 years and that friends had replaced units within 10 years and many times not much beyond this period.
My question centres around the carbon footprint of making the units (glass being high in this aspect I understand) against their value of reducing heat loss taking into account their average lifespan.
Has a study been undertaken as to DG units actual overall carbon footprint value?
There is nothing more than my idle curiosity in raising this point and look forward to a healthy discussion (if one hasn't been already )
Kind regards]
Andy

This should be pretty simple to work out if you trust the ICE database.

Taking a worse case where you use virgin glass the embodied energy is 15 MJ/kg (toughened being 23.5)
The density of float glass is 2500 kg/m^3 (2.5 kg/(mm.m^2)

The rest is just arithmetic.

Please excuse the silly question but; How do you know when DG has failed?

I reckon the energy payback is around 4 months:

Energy to make glass is 15MJ/Kg, equiv to 4.2KWh/Kg
DG windows weigh 20Kg/m^2
So energy to make window (glass) is 4.2*20 = 84KWh/m^2

If the SG unit was U=5, the new one is U=1.2, then the difference is 3.8W/m^2/degC
Assuming a mean outdoor temp of 11C and indoor of 19C, the mean power loss is 8*3.8=30W/m^2
Thus the yearly energy saving is 30*24*365/1000 = 263KWh/m^2/year

84/263 is around a third (of a year).

I think glass isn't re-used as labour costs massively dominate over energy costs - and re-using stuff takes more manpower than starting from clean good raw materials.