Hello. Its a Vaillant ecomax 635, about 3 yrs old. Its cuts out most days with error codes that translate to overheat/air blockage. I have heard what happens prior to cut-out; a boiling (bubbling/kettling) noise emanates from the boiler. Various corgi engineers have claimed to diagnose the problem, and have attempted to bleed air through the boiler pump central screw, and subsequently by draining down and refilling the system, with no effect. The problem is most likely to happen when 2 of the 3 central heating zones are turned off. When I restart the system (by pressing the ignition button) there is always a gurgling noise as if gas bubbles are being flushed through the boiler, even if the boiler has been allowed to go cold (which makes me think they are not steam bubbles). There is rarely any gas to bleed off from the radiators though. I have tried a descaler, and the system has rust inhibitor added. Have emailed Vaillant UK with details but get no reply. Any suggestions welcome