A shower temperature that swings erratically between scalding hot and freezing cold without middle ground is usually caused by a faulty, mineral-scaled pressure-balancing spool inside the shower mixing valve. When mineral scale or wear prevents the internal spool from sliding freely to balance incoming water pressures, a sudden pressure drop on the cold supply line will cause an immediate surge of scalding water. Verification: Shut off the main water supply, pull out the mixing cartridge, clean or replace it, and test handle movement.
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