Are 120- and 240-volt heaters compatible with both 120- and 240-volt circuits?

120-volt heater
Do not wire a 120-volt heater to a 240-volt circuit. The heater will try to deliver four times its rated wattage, causing it to overheat. This will permanently damage the heating element and also void the product warranty.

240-volt heater
We do not recommend wiring a 240-volt heater to a 120-volt circuit. The heater will be underpowered and will only receive 25% of the wattage it was designed to deliver.