I have read various solutions to the above problem and (I think) I have exhausted them all. Any advice would be appreciated.
System: Baxi Barcelona open vented, with two 2 port valve circuits. The pump is situated between the air separator and the two valves in the flow of the boiler. The Boiler is situated in the garage with 28mm pipework up to the valves, air separator and pump (all in an airing cupboard on the landing. The air vent goes vertically to the small header tank in the loft. History: System plumbed in about 7 years ago and originally had this problem but though I cured it by lowering the header tank water level to 2 inches and extending the vent height. Recently replaced a large hall radiator with one about 3/4 its size. During balancing the system I found it had been pumping over again (dirty water in header tank). As the pump was getting noisy I replaced it with a Grundfoss "smart" pump. When I removed the old one I foung it had quite a lot of hard black sludge on its ports. I then realized this was probably due to the oxygen from pumping over. Had the boiler serviced and is OK! I asked the engineer for advice, and was told to clean the system as there was probably some restriction in the flow. Cleaned it with Sentinel X300 and ran boiler on high temperature with all valves open, but still the problem exists. It did clean the system slightly but I have alway had X100 in it and I did take all the old radiators off the walls and cleaned them when I plumbed in the system. Symptoms: When the boiler starts and the pump switches off at the end of priming and when the boiler stops I get about a tumbler measure pumped over. I think there is also some fluid coming into the tank from the C/H circuit feed. I tried closing a gate valve (in the feed) so it was only just open, but it obviously did not improve the pumping over. Even with the pump on its lowest speed (both auto and fixed) I still have the problem. The Baxi installation manual did not specify a need for a bypass and as I have 2 towel warmers without TRV's I assumes it would make no difference!! Well that's the sorry saga so far, I would appreciate any comments on my system and any possible ways I can cure the problem as its now exhausted my knowledge!!?
I am tending to think it is my design that may be the problem as I have always had problems with air in the system!!
Thanks & sorry for the length of this message.