Installing a shower valve upside down causes the hot and cold water temperature controls to operate in reverse, meaning turning the handle toward the hot side delivers cold water and vice versa. In pressure-balancing valves like the Moen Posi-Temp, an upside-down installation can also disrupt proper internal spool movement and safety shut-off functions. Fixing this usually requires removing the trim and cartridge to check if the internal components can be rotated or if the rough-in body must be flipped.
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