I have Keston Gas boiler with a pressurized central heating currently set to 1.2 bar. The system is 5 years old and I recently have drained to install 5 Drayton TRV. After running it with Fernox restorer for a week I add Fernox Protector and bled all the rads, pump multiple times (with the pump off and TRV set to Max) and I also have de-airiator (or whatever is called) next to the pump but 2 radiators upstairs are still making some bubbling noises when the CH starts. I've turned the pump to min (from III to I) and this has improved the situation but I can still hear the bubbling when the CH starts. Also I have realised that I've fitted some of the TRVs on the return part of the flow but this should not matter, based on Drayton instruction TRV being bi-directional. It seems to me that I'm getting air into my CH (because always one if the radiator downstairs close tot he boiler have air in it) but there are no leaks from anywhere OR there is an air that I can't get rid off. Any ideas of suggestions will be highly appreciated. Thanks