Towel rad in bathroom needs bleeding every week

[John D v2.0]

That's what got me stumped. If it's not a chemical reaction there most be some part of the system getting below atmospheric pressure in order to pull air in.
Leaving the inspiration to the more experienced members of the forum now...

 

[pilsbury]

Bled it yesterday. But I will check for flammable gas as safely as possible!

But just to reiterate, inhibitor was added.

 

[pilsbury]

Still getting air in my towel rad. I’d say I bled it 3 weeks ago and today noticed the top rung of the towel rad was cooler. Used a lighter on the escaping gas and am certain not hydrogen. It’s most likely air. Where it coming from I do not know. Clearly not ingress into the system as it’s always minimum of 1.5bar.

I could easily keep up this routine but it just seems so wrong in a closed system. Chances of me having a brain wave are slim. What’s the chance of pressure vessel problem as someone mentioned earlier?