The reason the rad is not circulating is probably that CP1 is limited to a 1.8M head which realistically is a bit low, CP2 at 3M should be a good compromise to get circulation, theoretically, at least, the flow rate should increase from 0.4 m3.hr to 0.52 M3/hr and you may be able to nurse another few kw from the boiler without cut out. CP3 at 4.5M is probably the best choice, hydraulically, you may loose a few % in boiler efficiency but maybe a small price to pay for comfortable heating until Vaillant respond.
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