More information would help.Originally Posted by fitsjohn
What kind of radiators? The big standup type, or copper fin baseboard?
Is the system using water circulation or steam?
If water, is it pressurized? What is the pressure and how is it maintained?
If steam, what is the pressure?
What's an "emmersion"?
Why a circulation pump at "the highest point in the system"? That is not the place to put it.
It's an old house. What was changed between the time it worked and now?
Are there air vents at the high points in the system?
Is there an air scoop and air vent near the boiler?
Is there an expansion tank in the heating loop?
How many zones and how are they controlled?
How do you get makeup water into the system?
Tell us a bit more and maybe someone can give you some suggestions.