What law of thermodynamics dictates that the range of 40 - 32 is least efficient? The efficiency goes down in a fairly straight line until it reaches unity (or below). This is a typical curve, all the ones I've seen have a similar profile.Most advice says 40+ degrees for heat pumps. That bottom 8 degrees, from 40 down to 32, is where the pump is least efficient. That's dictated by the physics of thermodynamics.