 In very mild weather, the COP of an air source heat pump can be up to 4.However, on a cold winter day, it takes more work to move the same amount of heat indoors than on a mild day.The heat pump's performance is limited by the Carnot cycle and will approach 1.0 as the outdoor-to-indoor temperature difference increases, which for most air source heat pumps happens as outdoor temperatures approach −18 °C / 0 °F.