Sorry if anyone has already said this but there seems to be no bypass after the pump? Pump shouldnt be set on 3. Boiler stat shutting on and off correctly?? If you shut heating off can you hear motorized valve shutting down...no air heard by pump and vent/coldfeed??
At present there is no official bypass, but the boiler can only fire when the UFH is running - the radiators come on at the same time (no separate zone as such: 'wrong', I know). Before there was UFH, the room served by the UFH has rads on manual valves and the air situation was no better. Y plan, so always somewhere for pump over-run to go.
Pump is on proportional pressure curve 3, not speed 3. The 3 pp curves are not that different each from the other.
Boiler cycles correctly, albeit with quite a high hysterisis. I think the gas rate is too high (but not by much), but last RGI to work here refused to set it down for me on the grounds that it would most likely cause the diaphragm in the gas valve to fail for not much gain.
Y plan, so motorised valve stays in CH mode until call for DHW.
After I removed the pump, air could be heard travelling up the vent. In normal use, with boiler kettling, you do hear the occasional bubble running up the vent, and sometimes a bubble manages to get pulled into the pump.