Running a 230V heating element using 120V
I would guess not. The power output will only be 25% of the design power when operated from 120VAC (power is proportional to the voltage squared, so cutting the RMS voltage in half, cuts the power to \$1\over4\$. 

The actual answer will depend on the heat loss (at operating temperature) from the boiler assembly, which is difficult to guesstimate. 

The heater would not be damaged by low voltage, but if there's an electric pump in there, it could be (if it stalls it could draw excessive current and overheat, even at reduced voltage).