Old aluminium d/glazing?

[tracey2412]

Am new to this forum & not very technical, so please forgive the lengthy post: We were looking to replace the d/g throughout the house over the next year or so as we build extensions & do a loft conversion etc.. However, I hate to throw away anything if it's still working. The existing aluminium units seem to have a gap between the panes of 28mm, isn't this the same as new pvc units? If this is so, how do we calculate if they are as efficient and/or if they need simply re-filling? Also, the picture window in the bedroom has 2 huge circles on the inside of the panes (presumably from the glass holding device when they were fitted by the previous house owner) - does anyone know of a (local?) company (we live in Bedford) who would be able to advise us without simply persuading us to buy their new windows? If we need new, we will buy, but would like to check out this way first. Thank you.