Connecting a separate 110v source without physically disconnecting
the 220 circuit is an accident just waiting to happen. You may not be
the person who flips the circuit breakers back on.

As for boiling water, if 1KW is consumed long enough it can
increase tank temp to boiling point, but it will become way too
dangerous long before that point. As hot water over 125 degrees F
represents a scalding hazard.