How can i stop getting air in my heating system?

Are you 100% certain it is air, not hydrogen that you are continually venting?

Hydrogen is given off due to reaction between you systems metal components and corrosive water.......
If you are venting, you also must be adding top up water too then, worsening the problem.

Has your system water been properly treated, you may need to get your system flushed, nutarlized and treated to solve this problem.

T
March 2008