On 2017-02-24, Roger Mills wrote:

On 24/02/2017 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?

As others have said, it could be hydrogen - caused by corrosion. Is
there any corrosion inhibitor in your system?

The plumbers (who I think are very good) drained & power-flushed the
whole system in spring 2014 when they installed the new boiler, &
added corrosion inhibitor. They may have added inhibitor in spring
2015 when they serviced the boiler, & definitely added it when they
moved & replaced a radiator & serviced the boiler in spring 2016.