Sev wrote:

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.

You got it right. If you look at this page about halfway down they
show these valves installed after the zone pumps.
http://www.bellgossett.com/homeowner...ntrolvalve.asp
I think the reason his are allowing flow all of the time is 'main' pump
is on the return side of the bolier and forcing all three flo-control
valves open even though the zone pump isn't on.

I'm thinking of disconnecting the main circulator and then running each
zone to see if that cures the problem. Although having the pumps on
the outlet side of the boiler _could_ lead to a boiler overheat more
easily than if the pump was on the inlet side.

If it were my house I would disconnect the zone pumps and install zone
valves and run it for a season just using the main pump. If all went
well I would pull the zone pumps and have three spares.