On 2/24/2017 11:47 AM, Tim Streater wrote:
In article , 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?

Is it air? Or is it hydrogen? Try setting light to it as it hisses out.

In which case it comes from corrosion of the iron. I have never felt
confident enough to do this test; if there was air present as well
(entering system as dissolved air during a pressure top-up) there is a
theoretical risk of a hydrogen explosion.