whats up with my hot water??

For my family to take a hot shower we have to turn on a
cold faucet or flush the toilet. My plumber spent the afternoon replacing the old hot water heater. to his surprise, Both the cold and the return pipe has cold running water. He put a shut off on the copper pipe after the water heater to stop the flow while he tried to solve the mystery. We have seperate handles on all sinks and showers therfore no mixers. The hot water is off going to the washing machine. My plumber was perplexed. He's never seen this before. Any ideas?

Have you got any single handle Moen faucets, or a Mixet shower valve.
Maybe something where hot and cold can be turned on but the end of the faucet can be blocked?