What is necessary to maintain a space temperature of 70 °F when a heat pump can only heat to 65 °F?

Maintaining a space temperature of 70 °F when a heat pump is only capable of heating to 65 °F necessitates the addition of supplemental heat. This solution is essential because the heat pump on its own cannot reach the desired temperature, meaning that an external source of heat is required to compensate for the shortfall. Supplemental heating can come from electric resistance heaters, gas furnaces, or any other heating system that can effectively increase the temperature of the space.

Increasing insulation could improve the overall energy efficiency of the space but would not directly address the immediate need to achieve the 70 °F temperature. Lowering the thermostat would actually lower the target temperature, which does not solve the problem at hand. Changing the refrigerant is a more complex process that would typically be related to system efficiency or cooling capabilities, not directly to achieving a higher temperature when the current heat output is inadequate. Thus, the best approach in this scenario is to add supplemental heat to reach the desired temperature effectively.