how can i fix my allient 628/e?

when i turn on the hot water the water stays hot for about 2 mins max . and then the water will go hot and then cold ect back in january of this year, i had a plate to plate heat exchanger fitted. but about 2 months ago, i looked on my water gauge and it read nearly empty i then turned the gate key open to fill my gauge to about 1 -1.5 but i dont know if i by doing this has put air in to my system