I recently changed all the rads on my heating and fitted TRVs to all
but one. I had a CORGI installer fit a WB Greenstar 30 CDi system
boiler. When most of the TRVs close, the system now makes really loud
noises in the lounge rads (2x4kw) due to high water flow rate. I
assumed that the modulating pump in the boiler would cope with this but
it just isn't happening.

I can't actually detect any slowing down of the pump when the TRVs
closed, so called out a WB service engineer, who tried to be be helpful
but didn't have a lot of information about this boiler. He changed the
pump head and the system board but no improvement.

The pump seems to be an OEM version of the Grundfos Alpha and doesn't
seem to have any manual control. I don't know whether there is a
problem with pump not adjusting to the system requirements, some other
problem, or it's something I have to live with. Can anyone suggest
where to go from here?