We only get a hot shower if we turn on a cold water faucet, or flush the toilet. I had a plummer over for the day replacing the hot water heater. to his surprise we had cold water running running from the supply line and return line both. we had a mess when cold water sprayed out of the hot pipe. He said he never saw this before. We have individual hot and cold handles in all showers and sinks with no mixers. The hot water supply to the washer is off. This amazed him. Any ideas??