Fridge to cool your food
This is what fridges do. They get cold inside because the refrigerant coursing around the pipes in the fridge (the blue and red lines in the diagram to the left) suddenly loses pressure as it passes through the expansion valve (2). It evaporates and cools the inside of the fridge. Heat from inside the fridge is transferred to the refrigerant (3). Then, the refrigerant gas is compressed by the intuitively-named compressor (4). High pressure forces the refrigerant gas molecules close together making them extremely hot.
The pressurised refrigerant must be cooled down before it once again travels through the expansion valve. This happens through the coiled heat-exchangers at the back of the fridge, condensing the hot gas back into a cooler liquid (1). The heat of condensation is radiated away from the fridge and out into the kitchen (A).
Heat pump to warm your home
Look at that Refrigerator Cycle diagram again. This time (A) is the inside of your house that we are trying to heat, and (B) is the air outside at temperatures around -10°C. Instead of the compressor belonging to your fridge, it is part of your Eco Cute CO2 air source heat pump. The CO2 doesn’t refer to the CO2 in the air—it is the refrigerant coursing through the red- and blue-coloured pipes.
Since the goal now is to heat the inside of the house, the size and operating conditions of the compressor are a modified, as are the design of the evaporator and condensers—but the principle is exactly the same. The system is just tweaked so that enough heat flows into the house to sufficiently warm it during winter.
Refrigeration diagram licensed under GFDL v1.2.