Right, Vege, at 0.75V drop time times the current through the diode being dissipated as heat, these are going to be big through bolt type diodes, (several in parallel) mounted on a big aluminum heat sink when you get to higher currents. At high currents, the diode noise (EMI) will also be impressive without RC snubber suppression. Forget AM radio reception near the heater.
Diodes are sometimes used in portable, 1500 watt or less resistance heaters. You're not likely to see them being used in high wattage devices because the cost and heat sinking required. Most higher wattage devices, like electric boilers, will have multiple elements and sequencing on start up and/or selection of elements for variable ouput levels.