Worcester 8000 life 35kw combi losing pressure when zone valves shut

[Mark Wilson]

Hi can anybody offer any ideas as to why a newly installed Worcester 8000 life combi would drop pressure to nearly 0 when both zone valves are in off position (one for ufh other for rads) causing boiler to go into fault mode when hot tap is open. Pressure returns to normal as soon as any of the zone valves are open? auto bypass has been fitted and no drips or discharge from prv