Refill ceiling heating water - How many bars?

Hello everyone,
I happened to see today that the water pressure of our air-to-water heat pump is dropping towards 1 bar. Our heating engineer last refilled it on 03.09.2017.
I actually thought that a heating system is a closed system and therefore loses little to no water. I am now asking, since I don't know what the pressure was after the last refill, is such behavior normal?

Regards
Speer

Yes, despite a closed system, heating systems lose water. The reasons are diverse, but mainly they are leaks at the connection points.

The pressure your system requires to function optimally is stated in the installation/operating manual of the device.