Heat pumps

I hope this is the right section for this energy conserving question.


As I understand it (and my knowledge is quite limited) there are various sorts of heat pumps.
Some exchange with outside air and their their efficiency (COP) drops to 1 when the outside temperature drops below -10C.
On the other end of the spectrum is the geo-thermal heat pump with lots of tubing in the garden.
The last option is the best but also the most expensive.
I'm looking for a way to calculate earn back period of each system. (ROI)
For example the ROI on a air heat pump instead of full electric heating may be 4 years. But the extra cost of a full geothermal system may have a ROI of 15 years extra.
I read all sorts of fantastic claims like COPs of 3, 4 and even 5. Always using the magic words "up to".
While that advertising may be 100% true, due to the climate I live in possibly COP won't get above 3 for all sorts of reasons when using a full geo-thermal solution.
Air pumps drop to COP 1 fairly soon, which obviously mean when it gets colder their COPs slowly starts dropping. So during winter the average COP may be only 2 instead of the advertised maximum.

Can an air heatpump be upgraded to a full geo-thermal solution?
Hints? Tips? Sites with useful calculators?

BTW read my signature for more info.

Move to CO2 natural Refrigerant absorption fridge fist stage. Lowest heat -30*C. Power outage all mechanics, volume per second by pressure.

Geothermal heat pumps get alot of attention for good reason but I dont think you can say they are the best for every situation by any means. At the risk of over-generalization I would call the mini-split heat pump the best to use for most of the projects that inhabit the permies realm.

https://permies.com/t/27465//Efficient-Air-Conditioning

Geothermal heat pumps come with a very big price increase and are only financially feasible thanks to tax incentives which should probably be directed to other areas. Geothermal tends to make the most sense on very big expensive homes that have a poorly built envelope. This would describe many existing homes but if you are building new it would be wise to focus more attention on air sealing and continuous insulation which puts the "payback" possibly outside of the life of the system.

If you want to make your current system more efficient, focus on air sealing your home and existing ductwork. Lots of money is wasted on efficient heat pumps that should be spent on ductwork (sealing and bringing into conditioned space).

I also like the split ductless heat pumps (air source heat pumps without duct work and that generally use variable speed compressors). Ducting, whether it's insulated or well-sealed, or not, is wasteful of energy. For example, the duct fan alone for most central systems with ducting uses a fan motor at 500 to 1000 watts (generally). By comparison, a single one ton air handling unit has a small blower fan motor that generally draws on the order of 50 watts. All else equal (same EER, etc.), a system with typical duct work will consume more energy as compared to a split ductless system. Also, the variable speed compressor feature makes for superior efficiency at part load operation. When this is considered, then the efficiency gains are even greater. Now, a geothermal system will see better efficiency as an air source heat pump when the air temperature is very low. It's possible to mitigate against this with intelligent design. For example, the evaporator (i.e. outside unit) can be placed for solar gain during the day even placing in a sort of green house enclosure. Add a thermal mass and things could get interesting. However, even without this approach, I suspect these units will win out on a $ basis in the long run.