Why is a geothermal heat pump better than an air-source heat pump?
Water holds a huge amount of heat, compared to most substances. You can take 62.42 BTUs out of 1 cubic foot of water (at 40°F), and its temperature will drop just 1 degree. In contrast, air that is dry, or almost dry (which it will be at 40°F), holds very little heat. You can only take 0.019 BTUs out of 1 cubic foot of air before its temperature drops 1 degree. So, comparing equal volumes, water holds 3274.78 times as much heat as dry air does!
What does this mean for heat pumps? An air-source heat pump must move about 3000 times the volume of air through its heat exchanger, than a geothermal heat pump does water, to get the same heat. Even though air is less viscous than water, and easier to move, the air-source heat pump still ends up working harder: to move that much air, an air-source heat pump must run a fan consuming 400 watts of power per hour. An equivalent geothermal heat pump's water circulating pump only consumes 90 watts per hour!
Air has another problem for air-source heat pumps: the drier it is, the less heat it can hold, and colder air is drier than warmer air. So, as the temperature drops linearly, the amount of heat in the air drops exponentially! At about 30°F, almost all of the water vapor left in the air will "frost" out of it, which drastically lowers the heat available to the heat pump. The only way to compensate is to speed up the heat pump's fan, and move even larger quantities of air - but this really ratchets up the power consumption. The combination of transferring less heat, and using more energy to do it, is crippling to the air-source heat pump's efficiency.
But water vapor isn't through causing trouble yet. When all that moisture starts frosting out of the air, it freezes right onto the heat exchanger coil of the air-source heat pump. Eventually the ice gets so thick that the fan can't pull any air through the heat exchanger coil, and the heat pump stops working. To solve this problem, the air-source heat pump has to defrost the outdoor coil several times per hour - and it takes quite a lot of power to do it. A geothermal heat pump, on the other hand, never requires defrosting (antifreeze keeps the loop fluid flowing).
Finally, the most important reason a geothermal heat pump is better than an air-source heat pump, is that the earth has more stable temperatures than the air. This gives a geothermal heat pump a much higher entering fluid temperature in the heating season, and a much lower entering fluid temperature in the cooling season, than an air-source heat pump, making the geothermal heat pump tremendously more efficient, with a much longer lifetime.
A 3-ton Amana ASZ air-source heat pump will have 14,700 BTUHs of heating capacity (1.23 tons), and a COP of 2.1.
A 3-ton ClimateMaster TTV geothermal heat pump (entering fluid temperature of 32°F) will have 29,000 BTUHs of heating capacity (2.42 tons), and a COP of 4.0.
The geothermal heat pump gives you twice as many BTUHs, at half the cost per BTUH!
The 3-ton Amana ASZ air-source heat pump will have 33,500 BTUHs of cooling capacity (2.79 tons), and an EER of 11.84 (it only gets two-thirds of that EER and capacity if it's covered with grass clippings and dirt, which obstruct the airflow).
The 3-ton ClimateMaster TTV geothermal heat pump (entering fluid temperature of 77°F) will have 38,200 BTUHs of cooling capacity (3.18 tons), and an EER of 18.2 (mowing the lawn won't hurt a geothermal heat pump's efficiency or capacity).
In all cases, a geothermal heat pump beats the pants off an air-source heat pump - in fact, no matter what air-source heat pump you compare it to, ANY geothermal heat pump will perform better!