Bathroom rad needs bleeding every other day

[bhm1712]

as in the title,

i am having to bleed the bathroom rad every other day, it is the second rad in the system. The pump is not sucking in via the open vent (did jam jar test) and it is not pumping over either. The pump is on speed 1 so not cavitating i dont think.

the tell tale sign of needing to bleed it, is a trickling sound in that rad.
any ideas as to what may be causing it?

C-Plan system
Grundfos 15-50 selectric pump
glow-worm Space saver mk2 40br
22mm main piping, 15mm to upstairs rads

 

[MarkRainbow]

If the radiator is in the negative pressure part of the circuit it can be possible for air to be drawn in via "micro leaks"... holes or poorly sealed compression fittings that... that are too smalll to allow water to leak out but which are big enough to let air in.

Another possibility is that there is no inhibitor in the system, and the "air" is actually the gaseous products of corrosion. This includes H2S which smells of rotten eggs. It's also inflammable and will burn if lit with a match! From the rate you are having to bleed though, I wouldn't think it would build up that fast.