Water is strange stuff when its hot it expands and when its cold it
expands. However there is always air in it, just more air in some bits and
less in others. So I think the answer is that being lighter than water it
takes some time to get to a high point in the system where it gets trapped.
Brian

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The Sofa of Brian Gaff...

Blind user, so no pictures please!
"Adam Funk" wrote in message
...
I keep finding air in the tops of the same two radiators. This made
sense in the old vented system, because air could dissolve into the
water at the cold surface in the cistern, then come out of solution
later in the hot parts. But we have a sealed system (with a
combination boiler) now.

I'd also expect to get a bit of dissolved air (to come out later) in
the mains water when I add more to the system. But usually I bleed
the two radiators, check the pressure gauge on the boiler, & find I
don't need to add more water, but still find some air a week or two
later. Where is it coming from in that case? Just more air from the
previous top-up coming out a while later?