One radiator needs regularly bleeding

Hi,

One of my radiators makes "bubbling" noises a lot of the time, especially when the heating comes on in the morning. After a period of time, maybe a week or two, the whole flat becomes cold as none of the radiators are heating up. This I generally resolve by bleeding the offending radiator at which point they all start warming up again.

I believe I don't have a sealed system, though not entirely sure. There is an open header tank in the airing cupboard to which a plumber added some rust inhibitor a couple of years ago. Also, when I bleed the radiator, I can hear water flowing into the tank, presumably filling up the system to replace the air that comes out of the radiator.

I recently had a new 3-way valve and CH pump fitted, but the problem predates this work. A plumber advised that I turn the pump setting (I-III) down from II to I, but this didn't appear to have an effect.

Any ideas what could be causing this air to be introduced into the system?

Many thanks in advance.

Perhaps air is being drawn in from your expansion/vent pipe, if the tank is in the airing cupboard; and you are in a flat; it might not be high enough for your set up of pipework. Hence the plumber suggesting you turn the pump speed down.

It could be just rust causing gases, but that would probably be in all radiators.

system probably full of carp