Dual fuel geothermal heat pump systems are not as efficient as other types of geothermal heat pumps, because dual fuel systems must turn off the geothermal heat pump's compressor when auxiliary heating is needed. Our experience with these systems is that when it is very cold and the thermostat calls for auxiliary heat, the geothermal heat pump's compressor will shut down, the fossil fuel furnace will fire to warm the home up, and then the system cycles off. Because it is very cold outside, in only a few minutes the thermostat will call for heat again. The geothermal heat pump will start and begin to heat, but by the time it is making heat the home has already dropped in temperature enough that auxiliary heat is called for and the geothermal heat pump's compressor is shut down again.
Using a number of systems to adjust the controls and stop this kind of cycling has resulted in no success. When you use this kind of system, and it gets cold enough outside that you need auxiliary heat, 90 percent of your heating will be from fossil fuel. Heating with fossil fuel as the majority heating source defeats the purpose of the geothermal heat pump.
If you have the space, you can leave your existing fossil fuel furnace and connect a geothermal heat pump next to it. This is different than using a dual fuel split geothermal system because your geothermal heat pump's compressor doesn't have to shut down when auxiliary heating comes on. This means auxiliary heating will come in much smaller doses, and more importantly, that the geothermal heat pump will still do the majority of the heating. There are some controls that need to be installed for the air flow, but typically it is not more complicated than that.
Please note: Dual fuel split geothermal heat pump systems must be installed by a certified HVAC technician. Since they have 2 sections, the refrigerant lines must be connected and the system charged with refrigerant, and this MUST be done by a certified HVAC technician.