Air-purger problems

I have a thermostatic air-purger. I am not shure it works properly. Therefor I have a question:
If I start the purger normally, and open for NH3 inlet from the recievers during operation, and close the air outlet, will the air gather on top of the purger and make the top ice-free or will it only go back into the system?
When I run it for some time and then try to open air-outlet, it comes only NH3.
And how long must I wait before there is some air in the purger?
I have about 8000 kg of ammonia inside my system