I work for a heating company and we are having a problem with a 1 year old rinnai boiler install. The initial install was 3 zones, a 1" zone for radiant and 2 other 3/4 zones and all has been fine. We installed another 3/4 zone for baseboard heat in the basement and now when the new zone calls we get backflow thru 1 of the 3/4 first floor zones and thru the radiant,first floor zone, on the return side. There are conbraco 125s flowchecks on all the returns and all the circulators on the supply side have internal flowchecks. The wiring is fine as only the zone calling energizes and I confirmed with testing the taco zone control and it is fine, so there is a pressure or over pressure issue, the boiler is at 16 psi+-
IMO, Internal Flow Checks SUCK.
It doesn't take much imbalance to make them leak by.
When you pipe the three zones in place in the original install, and you go back and add another zone, it is very easy to pipe in an imbalance.
Zone valves rule. If the valve is closed, no flow. Valve open, flow commences.