So I have a vaillant combi boiler. 2 zones with a valve each controlled by Nest system. When my Nest calls for heat zone one opens up and I have hot flow to manifold, pipes thus hot radiators. When the other Nest calls for heat zone 2 valve opens up hot flow to manifold but no hot radiators. I’ve bled all radiators in affected zone no air, I’ve tried closing all them off leaving one radiator on. Still the same... can’t think of what to do next. The system worked perfectly up until Friday. I’ve been in the flat 2 years and drained the system twice, refilled and used inhibitor. The water was clear each time.