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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.
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.

 

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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.

My post was in context to the setting on Bob's unit - 32 F. Below that is a mute point.

And the chart proves the point. "The efficiency goes down in a fairly straight line". On your chart, the Coefficient of Production (COP) is ~2.6 at 0 C (32 F). At 10 C (50 F) the COP is ~3.2. That's an 24% gain in COP. At 40 F, somewhere around 11.5% COP gain.
 
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