Pleas help me with the following. If an electrical device is used for heating purposes with a rating voltage of 240 volts, why does it take longer to heat up when connected at a 208 volt source. It seems to me that the current will be higher than a 240 volts and consequently the heating device will heat up faster.

Ohms Law is involved.

When suppying a 240 volt heater with 208 volt, the heat output is lower as the wattage consumed is lower proportionally.

The resistance of the unit heater remained the same.