Measuring at the group head is the critical part here, as opposed to at the boiler, because in most machines there is a lot of thermal mass which will actually

Measuring at the group head is the critical part here, as opposed to at the boiler, because in most machines there is a lot of thermal mass which will actually equalize temperature fluctuations that happen on the scale of seconds or minutes. This means the effect of having a PID is less about smoothing out the short term fluctuations and more about the ability to set the average temperature