A typical air-to-air heat pump has a CoP of 4 but the CoP goes down with the outside air temperature until the CoP becomes 1 at around -20C. Below that temp, yo

A typical air-to-air heat pump has a CoP of 4 but the CoP goes down with the outside air temperature until the CoP becomes 1 at around -20C. Below that temp, you're better off using a backup heat source. A ground-source heat pump typically has a CoP of 5 but the CoP doesn't go down with the outside air temperature. Because the ground water temperature is stable, it will maintain its efficiency when it is cold outside. Of course, depending on the R value of your house, it may need to work harder to make up for the heat loss through the walls, roof and windows. You really need to find a heat pump installer who has the year round temperature data for your area and can do the math to calculate whether it is worthwhile to spend the extra on a ground-source heat pump.