Persistant need to bleed. Why?

[rebelbuttmunch]

Hi,

I notice that I'm quite frequently over in my dads house bleeding the same radiator.

I open the valves to feed the closed system from the header tank, then I bleed the rad, then close the valves again. A few weeks later the rad is cold again. I cant imagine that air is getting back into the system from somewhere but it seems like it.

Any ideas?
Thanks!

 

[tincup]

Is there the required concentrate of inhibitor in your system.
Adding fresh water does include oxygen.

 

[rebelbuttmunch]

Id bet money there is Zero inhibitor in his system. I'll add some in, thanks!