Plumbing Questions? Ask a Plumbing Expert.
I'm going to open this backup to my colleagues so one of them can help. Please don't respond (even to acknowledge this post)until you get a response from us. If you respond first it will only delay your answer.
Sounds like you have a cross over, hot water crossing over to the cold water. This can be caused by a malfunctioning mixing valve, usually in the tub/shower or possibly due to a failed check valve on a hot water return line f your have a recirculation ine on your hot water