I am fitting a new thermostatic mixer shower in my bathroom to replace an old Trevi Therm 310 which worked well but has finally given up the guost! I have a low pressure system with 22m pipes directly from the cold water tank and the hot water cylinder feeding the shower. The height from the shower head to the cold water tank is about 2 metres so I guess I have no more than 0.2 bar pressure. I want a setup that will provide the most flow from the shower head but I do not want to fit a pump. Anybody have any advice, preferably naming models and quoting flow rates that will at least equal the old Trevi performance, as it seems difficult to get this info from brochures and salespeople! I am also aware that there are some models out there (like the old Trevi) that have 3/4" inlets rather than 1/2" inlets and I believe that they would provide better flow rates - is this correct?
Don't know too much about thermostatic mixer showers. However I would imagine that, if the cold water supply to your current shower comes from the same cold water storage tank that feeds your hot water cylinder, you should have balanced flows of hot and cold at the shower head. If that is the case and you can acquire a shower that is suitable for a low pressure system you ought to get adequate flow from it. In this case I'd keep the 22mm pipes and reduce them to suit the new shower inlets as close as possible to the shower mixer.
If, however your current shower is fed directly with mains cold water you may need to incorporate a pump into the hot water side of the shower to achieve a balanced supply.
Bear in mind I'm only a DIYer myself and have no expertise in such matters. I'm simply applying logic to the situation you report.