The RMS voltage is halved - the voltage profile stays the same for one half-cycle (hence same RMS for this portion) and is zero for the next half-cycle, so the average over a full cycle is halved (ignoring diode voltage drop).. Nonetheless, the confusion on this point accidentally gets the correct answer that the half-cycle version of 240v is effectively equivalent to 170v in terms of power, but thats because for a resistive loud the power consumption goes as v^2 (ignoring any temperature dependence of heater resistance)