In article ,
"Reentrant" writes:
Our double-glazing is about 16 years old, and in the last 6 months three
units have failed with condensation inside. All the failed units are
south-facing - presumably the more extreme heating cycle weakens the seal
eventually.

1) What's a reasonable lifetime for a DG unit?

16 years isn't too bad for the first failures to show up.
Good quality units correctly fitted would last longer.

2) Would K glass help to extend the life of a south-facing window?

No. If anything, it would make it worse because the differential
expansion would be larger.

3) Can I replace the units myself like for like, or does a FENSA bod have to
do do (or approve) the work?

You can do it yourself. Some glaziers may remake the units using
your glass for you, providing the safety aspects of the glass
meet current regs, and the glass can be cleaned up.

When the units are replaced, check and clean the drainage channels
under the units, so that water that inevitably gets into the frames
can drain out without collecting. A common reason for failure is
the units end up sitting in a pool of water in the frame which
isn't draining away, which damages the seals over time, and pretty
instantly if it freezes. They should be lifted clear of the water
drainage channel.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]