Hello Lads, got a central heating problem

[lattin]

Got a 10 year old heating system and one rad needs venting every month or so. The primary F & R come straight to the cylinder which is fairly central in the house. (house is a bungalow). 28mm x 15mm manifold system feeds rads radiating from cylinder. The rad is not last in the run. It is an open system with F & E tank in attic.