Did you try turning off one of the washing machine valves like JohnJh20 had suggested? Actually i am a little confused my self and curious how this would even be the problem. Considering the hot and cold water lines are usually the same pressure, why or how would the hot or cold go back up the other line? 60psi of hot water for instance trying to backfeed a cold water pipe which is also 60psi pushing toward eachother seems a little far fetched. Idk.
Does the hot water ever get fully hot, and is the cold water ever truly cold? If they both just get lukewarm, not fully one way or the other, than there is a cross connection between hot and cold somewhere. If they both get fully hot or cold, just on the wrong side, than the piping to that area is crossed.
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