Electric furnaces can be installed as which type of furnaces?

Electric furnaces can indeed be installed as single-phase, three-phase, or combination furnaces, making 'all of the above' the correct answer.

Single-phase electric furnaces operate on a standard single-phase power supply and are typically used in residential applications where the electrical demand and load do not require three-phase systems. They are simpler in design and often sufficient for smaller homes.

Three-phase electric furnaces, on the other hand, utilize a three-phase power supply, which provides a more balanced and efficient power consumption. This type is often used in larger commercial or industrial settings where higher power needs exist.

Combination furnaces can leverage both single and three-phase designs, offering versatility depending on the specific heating requirements and available electrical supply. This adaptability allows installers to choose the best configuration for a given situation.

The inclusion of all these options reflects the adaptability of electric furnace designs to meet varying electrical supplies and heating demands across different settings.