G
Gary71
As you seem to have movement thru the plate heat exchanger then as a guess it's either because the diverter valve assem is defective(less likely as you have replaced it) or as water will take the path of least resistance it's easier to force it's way round the diverter valve than do a circuit of your heating system, pointing at your system design/defects(as posted by others). While you have heat bypassing the heating circuit the boiler is going to achieve temperature before it would do under normal operation and the heat ends up pulsing out.
