Cold water going back flowing to hot side.

[Jay gary]

So, I have a odd one here. I have low hot water pressure to the bathtub but great pressure from the bathroom sink directly beside the tub. If I turn on the hot water to my bathroom sink then turn on the hot water to my tub, the hot water stops coming out the sink but keeps coming out of the tub at a low rate. Okay, so I leave both faucets open, then I turn on the cold water faucet on the sink and now I get crazy water pressure out of the hot side of the bathtub, but it is cold water. I can not for the life of me see how this could happen. Could this also be related as to why my hot water pressure is soo low on the tub?

 

[Reach4]

Hot water goes to kitchen OK even with the hot to the tub on?

Look for a valve in line with the hot water for that bathroom. Maybe that valve is partially closed.

I am thinking there is some restriction in a path shared by hot for lavatory and tub.