Air in central heating system

[AnonymousImport]

My central heating flue in the loft, which I though was to allow water to escape, is actually sucking air into my central heating system, any suggestions why this might be, I constantly having to bleed the radiators.

 

[AnonymousImport]

Pump speed too high, or incorrect design of pipework. First thing to do is reduce the speed of the pump if you can, some pumps have a three-speed selector switch on the side.

 

[AnonymousImport]

Thanks for the reply. The pump and three way switch are both new, the pump is set to number two of three which was fine until recently. If I set it to one the upstairs radiator don’t warm up.