Two things. One the water maybe circulating too fast and have to be throttled back on the return pipe close to the boiler. Works like a thermostat in your car to slow the flow to allow the heat to transfer to the metal instead of flowing inside a insulating boundry layer of water inside the pipe. I still think there should be bleeds at the high points.
OK three things. The open supply to the boiler puzzles me. There may not be a traditional valve but there should at least be a thingy that works like a check valve and adds water as needed. It's an automatic feed and they always screw up in hard water.
If it is truly open, then when the heating system gets hot the water expands back into your domestic supply. Yuck. I'll try to find out more on that tomorrow. Look for bleeders.
