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A heat pump works by trying to cool outside air - thereby heating inside air. However as outside temperature drops below 40F, the heat pump has little outside heat to pump inside. Therefore (and unlike other heating systems) a heat pump must also have the 'emergency heat' function. That is typically electric heat - very expensive. Thermostat must have extra functions to control that 'emergency heat' function.
Heat pumps generally are most efficient at latitudes and temperatures of North Carolina.
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