This system in hot water only seems to create lots air which I cannot get rid of. I have put air vents on both the flow and return connections on the coil but they don't help. The air seems to come round every minute or so and you will hear it really loudly. But what might be the cause is the pump will just stop for 1 second and start again straight away, Causing the water to boil and thus create air. Why is the pump doing this I wonder. I got called out as the boiler was locking out and coil was air locked. Drained system down and discovered vent and feed were blocked. So thought that was why the air was in there as it couldn't escape. But still, it has issues.