nobody wrote:
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"

I don't understand where the pump comes into the picture if the cold is
mains pressure and the hot is gravity fed, and why isn't the hot water also
mains pressure. Most mixers require hot and cold pressures to be close to
equal. If it is a pressure control mixer that adjusts the water pressure to
keep the temperature reasonably level, it will have problems with unequal
supply pressure. A temperature control mixer which a Grohe probably is,
would be better but probably age, wear and with possibly some particular
matter in the works is now hampering performance. Have you tried a new
cartridge in it?