All TRV on full each radiator should be set between 10Â°C and 15Â°C drop between input and output but no need to be exact as once rooms heat up TRV take over. It only matters on first switch on and stops the boiler reducing output before all radiators are hot. On old on/off type boilers it does even less and before TRV's was used to adjust room temperatures.
Once hot water starts to return to boiler it starts to reduce output and if all were full open then it would first heat easy room and switch back boiler output as the water would all go through just one radiator and once that room heated up the TRV would work and it would start on the next room once whole house is hot then TRV's take over anyway so it only matters on warm up from cold.
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