my shower mixer runs off of a combi boiler. but it never runs hot. if you turn the mixer on a little bit the water comes out at like a normal electric shower, at a decent pace, but there is no temperature control, the temp dial does not have any effect. sometimes the water will come hot for a moment but then go icy cold again. if you open the mixer fully, suddenly it runs at a realy high pressure. and stays cold. again the temp dial does not work no matter where it is. is there anything i can do to make this system wwork?