On 24/02/17 11:32, Adam Funk wrote:
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?

usually its not air, is hydrogen or hydrogen sulphide I think, Corrosion
products

--
"In our post-modern world, climate science is not powerful because it is
true: it is true because it is powerful."

Lucas Bergkamp