So the Mitsubishi heat pump we replaced our gas furnace with comes with electric coils that kick in when the outside temperature is too low for the heat pump to heat effectively by itself.
The catch is, Mitsubishi doesn't determine that the heat pump isn't keeping up by comparing the temperature you set the thermostat for with with the temperature in the house. It goes by the *outside* temperature: above 10⁰F, the coils stay off.
57⁰ in the house? Just put on another sweater, peasant!