I don't want to hijack my own thread (though I've never hesitated to put my own foot in my own mouth) so I'll pose this related question in a new one. The Euro style open 'pressurised' systems must let in some oxygen, more than closed pressurized systems, at least. So, do they required chemical treatment of the water? Maybe similar to open OWBs on this side of the pond? I guess this question is really aimed at Hansson. He's the 'star witness' on this topic, but I would be glad to hear from anyone else who has lived with and maintained this sort of system long enough to know what works and what doesn't. My guess is something like Garn boilers use would do the trick.