Electric resistance heaters are found in pools and hot tubs, and they use electricity to warm the water. These heaters use an electrical current to heat a metal resistor that runs through the inside of the unit.
Water from the pool or spa feeds into the heater housing, where the resistor heats it and feeds back out into the pool.
These heaters are popular because they’re relatively streamlined and take up little space inside the pool or spa compared to other forms of heating.
The disadvantage is they require significant amounts of power, and installation is pretty intensive.