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I've only heard of ground loops being used for when heat pumps have to operate in absolutely frigid temps (like < -20F). What's the efficiency improvement over using ambient air? NYC doesn't get too far below freezing, even in the dead of winter, and I hear modern refrigerants are pretty dang good.

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Well:

1. You get passive cooling in the summer 2. The greater the temperature difference is the more efficient the pump becomes so they are more efficient even in not so cold weather 3. The maintenance costs of the system are significantly less. You need to clean and replace blower fans. Cleaning has to be done yearly or twice yearly depending on environment. Ground source, you only need to maintain the pump and coolant which needs replacing cca once every 20-30 years.

How would a ground loop be passive?

It's not entirely passive, you have to have a pump that moves the water through the loop, but it can cool the house without running a heatpump, with just passive heat exchange. Or with the heatpump running at a minimal level because when it is really hot outside, the ground is relatively comfortably cold.

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