: We have a single handle flow control for the shower. It also has another handle to divert water to two alternate shower heads. One handle turns on the water, another will turn on the alternate heads and the third will regulate the water temperature. If this third handle is not completely closed then hot water gets into the toilet and into the cold water faucet of the sink. I read somewhere that there could be a diverter problem at the hot water heater. My plumber is stumped. Thank YouWhat is the brand of the Mixing valve? If it is a MIXIT brand you need to replace the control cartridge. These are notorious for cross feeding hot to cold. I have never seen a diverter at the water heater. Maybe a dip tube problem, but that would show up at every fixture and not in the manner you describe. I would rebuild the valve first and see what happens.