specs: - Potterton Profile Boiler - Grunfos Selectine pump - Hercal h/w
tank

I have a 3 bed house, my central heating system used to be fine, bar
the banging of an old boiler.

I got the boiler and the hot water tank changed. After this a small
amount of air everynow and then used to get into the radiator in my
small bedroom. So I'd bleed it once a month and everything was fine.

This year I got 2 radiators changed (a large one in the lounge and a
small one in the bathroom). Now more air is entering the small bedroom
radiator. The frequancy of the air is un-acceptable, not only making
bubbling noises that wake people up but also requiring to be bled once
a week or every other week.

What I want to know is 'why' is this air getting in ?

I have not seen any leaks in the system, my plumber says that it's
through the vent and the pipework needs to be changed to 'fully pumped
system'. BUT the question in my mind, is why is this happening? The
pump hasn't changed, so surly the vacumn in the system hasn't changed,
so why whould the water in the vent pipe now be getting sucked lower
for air to get into the system? Also why would this be made worse once
again by 2 new radiators.... it doesn't add up in my mind, any ideas?

- also tried to knock the speed of the pump down to test, by this has
made no difference.