They seem to have been de-emphasised now with the advance of sealed
systems and a better understanding of pumping -over.

Thanks Andy & Derek. Curiously the heating installation had this
deaerator and a couple of those ball float designs, which seemed a bit
redundant to me.

Derek, I am curious of your remark about the 'better understanding of
pumping'. Could you clarify this or point me to a URL please?

Best Tim