Radiator Gaining Air

[BigBuilder]

I think this is a very strange one.
I have a combi-boiler running radiators on two loops but the loop I'm having trouble with is the upstairs one with 5 radiators on it.
The middle radiator of the 5 just keeps gaining air. However, the central heating system doesn't lose pressure.
It does, of course, when I bleed the third radiator but doesn't otherwise.
None of the other radiators either on that loop or the other loop gain air or lose pressure.
When I bleed the third radiator, it lasts about two weeks before I have to bleed it again.
If it was losing water then, I think I'd have a chance of isolating it but these symptoms are very strange.
I've power flushed the system and introduced leak sealer but it doesn't appaer to have made any difference.
I've checked the connections and they seem tight.
I'm starting to suspect the radiator itself but would appreciate any other thoughts/ideas.

Thanks.

 

[Simon F]

air can be drawn into the system through fittings typically loose rad valves or plastic pipe without the barrier
the level you describe would likely suggest corrosion taking place and causing a build up of hydrogen.
next time you bleed the rad try to ignite the gas.
how old are the radiators? was it badly sludged when you power flushed?

 

[BigBuilder]

Hi, Simon,
Thanks for this response - and an interesting suggestion.

The system is 22mm plastic pipe under the floor, converting to 15mm copper where it exits into the room and into the radiator - with brass compression fittings for the conversion.
Yes, the plastic pipe is a combination of HEP20 and SpeedFit barrier pipe.
No, the system is slightly over 2 years old and the radiators are Acova new, (2 years ago), radiators. (The boiler's a Worcester HE, also brand new, (2 years ago), by the way).
No, the system appeared to be fine when power flushed. The water was slightly straw coloured but that's to be expected I guess.
I also have a Magnatec 2 on the return and the water in there's looking a bit dirty so that's probably due for taking off and cleaning.

Hope this helps and thanks.

 

[Blackcatgas]

As simon F has sugested, try and ignite the "air" that comes out of the air pip.