Maybe you have a leek somewhere in the heating system that's why you need keep airing it.
Could be a radiator valve you can check them when the heating has been off and the radiators are cold as joints expand when hot and contract when cold it's worth checking every radiator valve with a toilet tissue .
Never air a radiator when the system is on air it when it's off then wait 5 mins before switching back on.
If it's a pressurised system you will need to top it up between 1 bar and 1 1/2 bar.
You will need to pressurise at the boiler . As I say check all valves even around the boiler look for damp patches around the valves on the floor or carpet when it's cool .