This is a long shot but there may be someone who knows about these heaters. I am staying at a house in Austria that has a 3 phase, 21 kW electric water heater, Vaillant model VED 21 H/6. I think the problem is with the flow sensing mechanism. The appliance feeds the shower mixer, kitchen sink and bathroom wash basin. With the filter fitted to the wash basin mixer the flow is not enough to activate the appliance so it always runs cold. With the filter removed it works OK. The shower is very sensitive to lever position, it tends to go from too hot straight to stone cold. Even at a constant lever position the heater sometimes shuts off for a few seconds and the water goes cold. I can find some technical info. online for this heater but only on PDF and only in German. I mainly wanted to know if there is any adjustment for the sensitivity of the flow switch?