Which of the following is NOT a common supplementary heat method used for removing frost from an evaporator coil?

The option indicating refrigerant expansion is correct because this method is not typically employed as a supplementary heat source for defrosting evaporator coils. Refrigerant expansion involves the process of the refrigerant transitioning from a high-pressure liquid to a low-pressure vapor, a process that actually cools rather than heats the system.

In contrast, the other methods mentioned—warm water, electric heating elements, and hot gas—are commonly used to provide the necessary heat to melt the frost that accumulates on the evaporator coils. Warm water can be circulated to raise the temperature around the coils, electric heaters can be strategically placed to supply heat directly, and hot gas defrosting involves redirecting hot refrigerant from the compressor to raise the temperature of the coils. These methods effectively address frost build-up, thus maintaining the efficiency of the refrigerant system.