Power shower pressure fall (not electric)


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"
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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?
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wrote:

The pump is a dual circuit rotary one which pumps hot and cold separately. I have not tried a new cartridge as with all Grohe items this is expensive. It has a strange pin in its center which can be easily lifted out vertically but there is no mention of this in its installation instructions. Perhaps its to insert lubricant for the mechanism? I agree that the pressures should ideally be equal but they are both within the mixer's spec.
regards
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nobody wrote:

Have youy discussed this question with Grohe?
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Grohe is a high end product, most high end products have a lifetime guarantee on their cartriges, call them they may ship one out at no cost with instructions on replacing it. I have done it with other better brands.
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nobody wrote:

Just out of curiousity, why is the loft tank not pressurized? It seems to me kike this complicates the tank system immensely.
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