Tankless Hot Water Heater
check the outside shower water valves, if the showerhead is closed, (the little button on the side of the showerhead) and the water valves (hot and cold) are open water will circulate and cause a pressure issue, I had this problem and could not get hot water in the rear bath, it cost me a few hundred bucks because the factory ensured me it was the main circuit board was bad.
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