We just moved into a house with 4 zones of forced hot water (plus a 5th zone for DHW.) Lochinvar gas furnace with Bell and Gosset NWF-25 circulator pumps. We did a fair bit of renovations which required rerouting and changing some of the zones.

I am noticing that zone 4 is getting warm when other zones are active even if zone 4's thermostat is switched 'off'. I've verified that the zone 4 pump is running correctly, not when it's off. It seems that the pipe at the exit of the pump on zone 4 doesn't get hot, when it's off, even though the zone is warm.

My suspicion is either a bad circulator pump that's allowing water through when it's off (but it pumps just fine when it's on.)

Or more likely, backflow from some negative pressure.

I do hear water gurgling when the system starts so I suspect it needs purging.

I don't know if there any check valves to prevent backflow. If there were, where would I expect to find them?

Does anyone have any thoughts on what could cause heat to this non-active zone and what I might do to remedy it?

Thanks