Geothermal heat pumps: Everything you need to know

Ground-source heat pumps (also known as geothermal heat pumps, GHPs, or GSHPs) use heat from the earth to warm or cool air for your property. Like air-source heat pumps, ground-source heat pumps take advantage of naturally occurring temperature differences to provide warm or cold air in an energy-efficient manner. GSHPs differ from traditional heating technologies that run off gas and oil because they concentrate and move heat instead of producing it through combustion.

Simply put, a ground source heat pump heats your home or business by transferring heat from underground to inside your building. This happens through a four-step process:

1. A mixture of water and antifreeze circulates through an underground pipe

This piping is known as a ground loop. It can be installed horizontally or vertically underground, depending on your available space and the ease of installation.

2. The ground loop absorbs heat from the ground and transfers it into the piping system 

As the ground loop circulates the antifreeze mixture through the earth, the fluid captures heat in the ground. This occurs because the earth's constant temperature is warmer than the fluid, and heat naturally flows from warmer to colder areas.

3. The system's heat exchanger concentrates the collected heat

Once the fluid collects heat from the ground, that heat needs to be concentrated to provide a comfortable climate in your home. In a typical GSHP system, fluid cycled through a ground loop passes through a heat exchanger, where the captured heat is transferred to a refrigerant fluid cycling through a separate looped system. This fluid is pumped through a compressor that concentrates the absorbed heat to much higher temperatures in vapor form.

4. The heat moves into your home

The concentrated hot vapor passes through a second heat exchanger, which supplies the heat to your house when air is blown across this second heat exchanger, absorbing heat and moving throughout your home. As your home warms up, the compressed vapor cools and is pumped back through the first heat exchanger to collect more heat energy from the outside ground. This happens continually to maintain your indoor air temperature.

This process runs in the opposite direction in warmer months to cool your home. Instead of transferring the heat from the ground to your home, the refrigerant is pumped through a heat exchanger inside your home first, where it absorbs heat energy and moves it down through the ground loop to disperse heat into the earth.

Four main types of GSHP systems differ based on their ground loop setup. Three types are classified as "closed-loop" systems, while the fourth is an "open-loop" system. Closed-loop geothermal heat pumps are much more common for residential installations than open-loop systems.

Closed-loop geothermal heat pump systems

Closed-loop ground source heat pump systems are characterized by the fact that they all circulate an antifreeze solution through a closed-loop of piping underground, usually made of plastic tubing. The heat exchanger in closed-loop systems transfers heat between the antifreeze solution in the closed-loop and the refrigerant in the actual heat pump. Closed-loop heat pumps make up the majority of GSHP installations in the United States.

There are three types of closed-loop systems: horizontal, vertical, and pond/lake. A horizontal closed-loop system, typically best for residential installations, involves laying a ground loop horizontally underground, usually between four and six feet. A vertical closed-loop system is better for large commercial buildings using a geothermal heat pump setup or for those looking to minimize alteration to the existing landscape above ground. In these systems, the ground loop runs straight down for between 100 and 400 feet (depending on the local soil and geology) and then returns to the surface. Vertical ground loop systems are also suitable for areas where the soil is too shallow and/or difficult to trench.

Lastly, a pond/lake closed-loop system involves laying a ground loop under a nearby water source at least eight feet below the surface. This is often the lowest-cost option, but it's only possible if a suitable body of water is nearby.

One variation on closed-loop systems is called direct exchange. This approach has no heat exchanger and instead involves pumping refrigerant directly through a ground loop (usually made of copper). Direct exchange systems use a larger compressor and work best in moist soils. Still, you may encounter environmental regulation issues because a direct exchange system cycles the refrigerant through the ground instead of a water mixture.

Open-loop geothermal heat pump systems

A less-common option for a geothermal heat pump system is an open-loop setup. Open-loop systems circulate well- or surface water through the heat pump instead of using a closed ground loop with an antifreeze fluid inside. Open-loop systems must comply with all local codes for groundwater discharge, as the water is released back into the environment once it circulates through the heat exchanger.