My issue is i have a natural gas boiler + hydronic heating system (two zones). It seems that after I remove air from each zones, the heating system is quiet until 12 hours later. In the early morning , we hear knocking in the basement/ first floor (near the return side of the boiler - the noise is not coming from the boiler though!) as the heating is turning on. If I flush each zone for air, the noise goes away and then it seems to come back the next day.
Is it possible that either the relief valve's spring is getting old and allowing air to escape dropping the pressure (I noticed that there is no water in the copper pipe coming from the relief valve). I have also tested and verified that the relief valve works if the PSI is around 25-30 (which it is rated).
What could be other sources of air getting into the system (could there be a pressure drop and the pressure regulator pulling in new water). I also checked that the expansion tank works (knocked both ends of the tank and also confirmed that there is 20 PSI with a tire guage.)
I also recall reading about that maybe the circulation pump might be running too fast - thoughts on this?