5000W heater temperature
The wiring should be:

Black: one side of the double breaker to heater terminal A

White: Other side of the double breaker to heater terminal B (this wire should be tagged black or red. Use a marker or some tape.

Single-wire 220v means there is no 120v in the circuit, like a stove.

There is no fixed temperature for a given power consumption. How hot does the manual say it should get?