Lon- sorry for the response delay, I was out of town on a regional single action cowboy match.
Head pressure: this is the highside pressure being discharged out of the compressor into the condenser coils. If the condenser cannot remove sufficient heat (for numerous reasons) the pressure goes up.
Temperature/pressure are relative dependent upon the type of refrigerant. So for R-22 refrigerant we generally do not like to see the pressure over 260 psi. The head pressure varies with the outdoor air temperature on an air cooled condenser, and we generally like to see refrigerant temperature/pressure of the system not significantly more than 30° F above the outdoor air temperature. (90°F +30°F = 120° F. Converts to 260 psi). Anything significantly higher than 260 psi indicates a problem at the condenser i.e. airflow, dirty coils, excessive refrigerant charge and in geothermal, water flow problems.
When the temperature becomes excessive, the liquid refrigerant that comes out of the condenser, heading to the evaporator coil has too much heat and as it passes through the metering device some of the refrigerant must flash off to cool the liquid refrigerant entering the evaporator coil (this is an efficiency loss). Head pressure/temperature also increases the evaporator pressure/temperature. So at 69 psi you may have a coil around 40°F. Raise the psi with the head pressure to 76 psi, then the evaporator coil temperature goes up about 5°. You can’t cool the house to the design temperature/moisture if those temperatures are too high. It is inefficient and takes more time to accomplish its purpose.
As for your question on compressor damage during cold weather: the opposite is happening as I described above. The condenser lowers the refrigerant temperature/pressure too much, which drops the evaporator temperature/pressure below the freezing point. (If you drop evaporator pressure 3 psi, the evaporator temperature will drop below freezing, depending on the heat load within the house, which is why I recommend running the heat before testing the operation of the air conditioner). Reciprocal Refrigeration compressors have extremely tight headspace clearance between the piston and the head of the compressor (for best efficiency) and any oil or liquid refrigerant which does not have any space within that clarence becomes a hydraulic force that generally breaks the valves or crankshaft. This does not apply to scroll compressors which can handle this slugging of refrigerant, however it’s extremely noisy and significantly inefficient. Refrigerant slugging also washes the oil out of the compressor and you’re running the compressor dry.
Yes, they are very expensive initially and they can be much more expensive to maintain, but they are very efficient because of what we discussed above. The system runs under much lower pressure which uses less horsepower, thus less electricity, and the heat of compression which accelerates metal fatigue is hardly there. You may also want to note that the refrigerant in the system has a primary job of cooling the compressor after it cools the house. That is why we want the section line coming back to the condenser to be cold and sweaty. The house may be cooling down perfectly fine but if that section line is hot, it’s like running your vehicle without antifreeze.
So next, people are going to say, we don’t have all these gauges to do this. If you understand the temperature/pressure correlation above you can determine these factors just by the temperature of the components. You can use a thermometer (if you know what you’re going to do with those temperature readings), or you can simply use your hand. If the section line feels like a cold beer, the superheat is probably fine. You can even just look at it; if it’s dry you probably want to touch it and see. If there’s ice on it you have an overcharged system or air restriction (dirty filter). As for the small refrigerant line coming out of the condenser, you should not feel any significant heat. The body (your hand) is 98°. The outdoor air span falls within this temperature range and you should not feel significant heat. If it’s cold you have a problem. If it burns your hand you have a problem.
As for equipment not being equal; there are different types of geothermal and yes they are not all equal in efficiency. System design will determine what this is (not your purview) but under any working design, geothermal operates under much lower temperatures than is experienced from the outdoor air. If you have a heat pump, there’s more heat in the ground/water in the winter than in the air, and the ground/water is always cooler than the air in the summer. Its ability to remove heat mathematically determines your savings.
Longevity of the equipment is always longer than air cooled equipment if properly designed and installed.