PEX barrier pipe – can anybody answer this?

I have a question that I’m hoping those of you who are experienced central heating engineers may be abler to throw some light on. But first I need to tell you about my system, and the problem I’ve been having. Just over 7years ago I had a new CH system installed in my small semi. I say new – in fact some existing upstairs copper piping was retained, and also a stainless steel towel rail radiator (which had only been installed a year or so earlier as part of a new bathroom. The combi boiler is a Vaillant Ecotec Plus. 18 months after installation I had a pressure sensor replaced under warranty. The Vaillant engineer said the system water was dirty (surprising given that the system was new, dosed with a bottle of Sentinel X100 and equipped with a magnetic filter). On the advice of a local plumber I emptied & flushed the system, refilled it, added 1.5 bottles of Sentinel, and have kept an eye onn things ever since. The boiler pressure is steady and there are no discernible leaks anywhere. To cut a long story short, I discovered over the next few years that the level of inhibitor (which I monitor regularly using Sentinel quick test kits) fell quite rapidly if the system ran several hours per day at a temperature of 68 degrees (as in the recent cold weather). For example I added a full bottle 3 months ago, and the other day the test showed there was insufficient inhibitor. Over the years I have tried to find possible reasons. Two of the downstairs radiators were connected using PEX pipe (total length of pipe about 20 metres). I wondered if the installer had failed to use barrier pipe, thus allowing oxygen to get into the system. However when I took up the floorboards and checked the pipe, I found they had used the right stuff (QUAL-PEX Barrier Pipe BS7291 Class S). I then wondered if even barrier pipe becomes ineffective at temperatures from 65 to 70 degrees, but I could find nothing about this when I Googled it. So my question is: does anybody have any information...

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