Physics: how a heat pump heats a house

Tutoring physics, everyday ideas come up. The tutor mentions the basic concepts behind a heat pump.

A law of physics is that, all by itself, entropy will increase. Entropy means randomness or disorder. An example is that, when you put food colouring in water, it spreads out on its own.

The heat pump might use refrigerant whose boiling point, at normal pressure, is -25°C. The refrigerant is in a closed, sealed system and circulates between the outdoors and the indoors. It’s so-called because it’s from the same class of compounds found in fridges, which work the same way but in the opposite direction.

In the house, a compressor and condenser work together to concentrate the heat present in the gaseous refrigerant so it can be liberated within the house. As the refrigerant relinquishes its heat, it liquefies and is passed outside of the house.

Relieved of its heat, the liquid refrigerant might find itself, as mentioned above, at -25°C. If the outside temperature is -10°C, that’s easily warm enough to evaporate the refrigerant, which absorbs heat from the surroundings. This happens spontaneously, since gas is more disorderly than liquid.

It’s the heat pump’s ability to condense the refrigerant to liquid, bringing its temperature, for example, from -10°C down to -25°C, that enables it to function. The heat lost by the refrigerant during that transition is what heats the house.

Source:

technologyreview.com