Use a pump to circulate white vinegar through the heat exchanger for about to dissolve mineral deposits. Safety First:
Crossed plumbing pipes or dip tube failure. Manual solution: Check if a single-handle faucet is allowing cold water to mix into the hot line. If not, the internal dip tube (which sends cold water to the bottom) may have broken. Replace via the cold inlet. xyfb water heater manual
As a service feature, here is the real advice for anyone trapped in the XYFB void: Use a pump to circulate white vinegar through
Water is too hot / too cold? → Adjust temperature in small increments (wait 30 sec) → In winter, incoming water is colder – reduce flow rate at faucet xyfb water heater manual