Thank you again for you replies. I never had this issue with my previous two boilers and I have lived in this house with the same pipework for almost 25 years. There is a possibility the boiler is the issue. I have just got my guarantee through after two years - plumber forgot to register the boiler so kicked up a fuss and finally got it a few months ago and was able to get a Vaillant engineer out but he refused to touch boiler because flue was incorrectly installed.
I agree with you BlueLoo- not sure how boiler can generate air that's why I was asking some expert engineer on here. I don't keep topping up because not much water escapes when I bleed radiator each night. I keep an eye on the pressure and it is always around 1-1.1 KBar when cold.
Corrosion is a possibility. As you said I am starting from scratch to try to figure air and banging problem (in other pipes/radiator) as the plumber not sure what to do and although he is a nice guy his performance hasn't been great. Someone on this site mentioned a device to remove air bubbles in system (Spirovent) but trying to understand how system generates air in first place. Maybe it is just corrosion in which case system needs a proper clean.
Boiler will not be creating air as internal pump has a auto air vent on it*
You have fitted new vertical radiators, turn these off and see if air issue persists
Series 6 and series 4 Vaillant’s are nit the same, your boiler is for sealed system and series 4 is open vented and both these boiler have heat exchanger with high internal resistance
A filter does not need to know how boiler works, a misstep can creates issues like you have found
Internal bypass in the vaillant is quite robust
You are running the boiler at 15Kw- that would amount to a system with 10 radiators ( approximation) and no cylinder unless cylinder is ancient non insulated
*Series 4 vaillant can either work as open vented or be a sealed system. As open vented, if pipe configuration is incorrect, air will be pulled in from the open vent.
Your boiler is a sealed system. To overcome high internal boiler resistance the pump runs at higher power. If you have old radiator valves, it is not unusual for pump to suck in micro air bubbles through the worn valve stems or poor joints.
At neutral point in the system, one Often gets air in the radiator
Some vertical radiators break down the water to create airlock you are experiencing. This is common with bathroom towel rail types.