The system on the 166s is, as I keep saying, a lash-up, created as an after-thought and fitted in as best as could be managed on to the 165 design.
Unlike the 158, which was designed with air con from the word go and which Salisbury depot somehow seems to keep working pretty well on its 158s and 159s, with grey-painted roofs, while other places have more of a struggle, whatever colour the paint on the roof.
It is high time the 166 system was ditched and air cooling, which we know from many years of use on Chiltern's 165s works pretty well, is provided instead.