Okay, I think I’m narrowing in. To me it sounds like the flow control valve on the system is not working properly. My local plumber though says he has not seen one of these fail in 20 year. I went and rapped on it the other day in case it was stuck, and now when the baseboard zones go on it seems like I hear three distinct thumps coming from the flow control. Is this the sound of it opening? Could it be possible that it is undersized and that the actually gravity flow is opening it?
Like you said in your reply, it seems that I do not get total garvity flow through the radiant…only half the floor seems warm.