While agreeing that if you actually need the space taken up by the
header tank a sealed system may make sense, header tanks worked
perfectly well for many years before them. Sometimes the simple way
is actually the best.

I see absolutely no advantages to open systems than these marginal ones:

1. Your boiler might require it.
2. You already have one and don't want to spend 50 quid upgrading.
3. Certain heat bank applications with common circulating water.

The advantages list goes on and on. Header tank systems rarely work well or
predictably, which is why sealed operation is now the norm.

Christian.