RayV wrote:

I'm guessing the _main_ circulator pump is left-over from before the
system was zoned. My previous house had only one pump and three zone
valves and always worked fine. The pump was on the cold side of the
boiler. I think whoever did the zone work did not set up the system
properly. From what I read about these flo-control valves they will
allow flow with a minimal amount of pressure just not gravity flow from
rising hot water.

Unless I'm misunderstanding, aren't they designed to prevent backflow,
but allow flow from pumps, which appears to be the problem? When
circulator(s) are running, they are indeed allowing forward flow of
water through entire system. I think you need zone valves. As you say,
appears to be retrofitted system by someone who didn't quite know what
they were doing. Don't know how large the loops are, but main and zone
circulators seem redundant, though don't see why that should be a
problem(doesn't appear to be excessive electrical load at present),
once zone valves are put in.