Simply put, geothermal works by connecting the space you wish to heat or cool (e.g. your house) to the earth and moving heat between the two. At the heart of any geothermal heating and cooling system is a heat pump, also known as a ground sourced heat pump or GSHP.

As the name implies, the heat pump “pumps” or moves heat from one place to another but, unlike a furnace or boiler, it does not create any heat. The heat pump is connected to the space to be conditioned – the building – by either an air distribution system (typically a blower with duct work) or a series of distributed pipes recirculating water or other liquid, commonly called a hydronics system. On the opposite side of the heat pump, it is most often connected to the earth by a series of buried pipes (known as the earth or ground loop) through which a water solution is recirculated. In heating mode, heat is moved from the earth to the building and in cooling mode from the building to the earth.

As the laws of thermodynamics tell us, heat will only move from an area of higher temperature to one of lower temperature, so the heat pump must concentrate the heat extracted from the earth which might only be 30 to 50F at a depth of 6 feet or so (where the ground loop is) in order to raise its temperature so that its heat can be moved to a house maintained at approximately 70F. To accomplish this, the heat pump utilizes a refrigeration cycle, just like your refrigerator or freezer. The important difference is that the cycle is reversible, i.e. it can move heat to or from your house.

How the Refrigeration Cycle Works

Key to a refrigeration cycle is the use a special fluid (called refrigerant) that has a low boiling point which can be significantly altered by changing its pressure. The refrigerant is circulated in a closed two-chamber system: the low-pressure chamber is commonly called the “evaporator”; the high-pressure chamber, the “condenser”. The heat pump extracts heat from a source (e.g. the earth in heating mode) and concentrates it to raise its temperature high enough so that it can be moved to the targeted area (e.g. your house). The schematic below shows a heat pump (in heating mode) between the earth and your house.


After circulating through the ground loop, water enters the heat pump at approximately 40F and comes in close thermal (but not physical) contact with the refrigerant in the evaporator where its boiling point is approximately 20F. The higher temperature water causes the refrigerant to boil or vaporize.

HP Schematic (evap)

Once vaporized, the refrigerant can be compressed or pressurized which raises it temperature significantly.

HP Schematic (comp)

Now the heat from the refrigerant can be transferred to your house by blowing air across (or bring water in close thermal contact with) the coiled metal tubing containing the hot refrigerant gas in the condenser. This causes the refrigerant to condense back to a liquid.

HP Schematic (cond)

Finally, the warm refrigerant liquid is passed through an expansion valve as it returns to the evaporator causing its temperature to drop so that it can absorb more heat from the ground loop.

HP Schematic (expan)

And the cycle repeats.

In cooling mode, a reversing valve switches sides of the heat pump: the cold evaporator is in contact with the building’s distribution system and the hot condenser with the ground loop.

Click here to learn more about geothermal heat pumps and ground loops.