How to not get air in my boiler-changing rad

[jameswillo]

I changed a couple of rads and managed to get air in my boiler.
I got the air out but I just wondered, for next time, how do I change rads without getting air trapped in the combi boiler?

 

[terryplumb]

if you drain out the system water ,its inevitable that pipework and boiler will fill with air. when refilling a combi system the mains water pressure in conjunction with aav ,and manual venting of rads ,should push air out easily.