Kitchen area tank uses hot water from a/c water recovery. When breaker for tank is turned on, hot water comes thru cold line. Never had this happen before until last year. I changed thermostat on tank, but it's still a problem.
One thing that can cause this problem is the old single handle Moen faucets that used the old 1225 cartridge. If you have a Moen faucet in the vicinity that either pulls straight up or straight out, you may have one of these faucets. Replace the brass cartridge with the new plastic one. Problem solved.
If you have a utility sink with a hose shutoff adapter on it or some other type of similar setup, if you turn on hot or cold and then block the flow out of the valve, the water pressure can force itself through the opposite temperature pipe.
There is an adapter for utility sinks which prevents this. It's called a vacuum breaker and will not allow the water to flow back up into the opposite pipe. Instead, it will exit out of this adapter.