My Grohe mixer is fed from a hot/cold pump. The cold is mains pressure and
the hot is fed from a loft tank so the head is around 4 feet. The pressure
at 38C temp was very acceptable when new 8 years ago but now the pressure is
too low at this temperature. If I reduce the temeperature by rotating the
mixer the pressure returns to normal but the temperature is too low.

I have changed the shower head and flexible tube but this does not improve
the pressure. I also tried lifting the hot tank temperature but this did
not improve the pressure.

There is a maximum temperature limit adjustment on the mixer accessed by
removing the knob and rotating a inner knurled knob but this just sets the
temperature higher before the knob stop position is reached.

There is no air in the pump so can anyone advise why this is happening and
how I can fix it?

regards,

"nobody"