How Does Geothermal Heating and Cooling Work?

Geothermal HVAC systems are known for being an energy-efficient and environmentally-friendly way to regulate indoor temperatures using energy from the ground. While this innovative technology is becoming more common, many people are still unsure about how it actually operates.

If you’ve ever wondered “How does geothermal heating and cooling work?” this article will help you understand how these systems control temperatures in residential and commercial buildings. Once you have a grasp of how these systems function, you can make an informed decision as to whether ground-source HVAC is right for your climate control needs.

The 3-Part System

The short answer to the question, “How does geothermal heating and cooling work?” is that a 3-part system–consisting of a heat pump, ground loop and distribution system–circulates water underground to regulate indoor temperatures. For a deeper understanding of how the system functions, we first need to take a closer look at each part individually:

Ground-Source Heat Pump

Although traditional HVAC systems use a furnace for heating and an air conditioner for cooling, some use heat pumps, a two-in-one solution that can both warm and cool indoor spaces by transferring heat as needed. While most heat pumps exchange heat between inside and outside air, ground-source heat pumps first transfer heat between the ground and the indoor space, then use a heat exchanger and compressor to provide warmth or cooling as needed.

Ground Loop

As explained by geothermal HVAC provider Dandelion Energy, ground loops are U-shaped pipes buried in the ground that circulate a mix of water and antifreeze from the heat pump, through the soil and then back to the heat pump. This thermally-conductive mix releases heat into or absorbs heat from the ground as it moves through the underground loop, facilitating the heat transfer that the ground-source heat pump needs to lower indoor temperatures in the summer or raise them in the winter.

Distribution System

Once the geothermal heat pump conditions air, the distribution system delivers it throughout the building. Although the most common method is a forced-air system in which an air handler pushes air through ductwork and out of vents, water-to-water systems use water-filled pipes that radiate heat through the walls and floors. While water systems use closed-loop pipes that circulate the water back to the heat pump, air systems use return ducts to bring hot or cold air from throughout the building to the heat pump, allowing the unit to exchange heat through the ground loop and begin the underground heat exchange cycle.

How the System Works

As previously explained, geothermal HVAC systems can dissipate excess heat into the ground in the summer and extract stored heat from the ground in the winter. To better understand this process, let’s take a closer look at the step-by-step process, depending on whether the system is in warming or cooling mode:

Heating Mode

• Step 1. Cool liquid starts flowing through the ground loop - A cool mixture of water and antifreeze begins flowing from the heat pump through the ground loop.
• Step 2. The liquid draws heat from the ground as it moves through the loop - As the liquid moves inside the loop, it passes through relatively warm ground and absorbs the heat.
• Step 3. Warmed liquid flows back to the heat pump - Once the liquid is heated, it moves back to the heat pump.
• Step 4. Heat is extracted from the liquid - As the lukewarm mixture of water and antifreeze passes through the heat pump unit, the heat exchangers, refrigerant and compressor work together to extract heat and transfer it to air or water, cooling the liquid in the process.
• Step 5. Heat is distributed throughout the building - The warmed air or water then moves through the building’s distribution system. Forced-air systems rely on air handlers and air ducts to distribute the air while other systems use water-filled pipes to radiate heat into indoor spaces.
• Step 6. The cycle repeats - Now that the cold liquid is back at the beginning of the ground loop, the cycle begins again.

Cooling Mode

• Step 1. Warmth is brought to the heat pump - Forced-air systems use an air handler and return ducts to draw warm indoor air from the building’s rooms to the heat pump. In water-based systems, pipes absorb heat from the indoor environment and carry it to the heat pump.
• Step 2. Heat pump transfers warmth to the liquid in the ground loop - The evaporator coil, refrigerant and heat exchanger work together inside the heat pump to move heat from the warm air or water to the thermally-conductive mix of water and antifreeze inside the ground loop.
• Step 3. Liquid moves through the ground loop and releases heat into the soil - As the warmed mixture of water and antifreeze flows within the loop, it passes through cool ground, allowing the heat taken from the indoor space to dissipate into the soil.
• Step 4. Cooled liquid flows back to the heat pump - The cooled water and antifreeze mixture flows back to the heat pump where the cycle began.
• Step 5. The cycle repeats - Now inside the heat pump once again, the cool liquid absorbs heat transferred from the indoor air or water system. The warmed water and antifreeze mixture then begins moving through the ground loop to continue the cycle.

Geothermal Heating and Cooling FAQs

Now that you have a good understanding of how ground-source HVAC works, here are some frequently asked questions and answers that people have about these systems:

Do these systems use electricity?

Yes. While geothermal HVAC systems don’t use electricity to heat or cool a building directly, the heat pump, air handler, ground loop circulation pumps and other pieces of hardware need an external power source.

Do these systems generate electricity?

No. Even though there are geothermal energy systems that harness the heat from volcanoes and geysers, geothermal heating and cooling systems don’t make their own electricity; instead, they use it to power hardware that leverages subsurface temperatures to condition indoor air efficiently.

What types of ground loops do geothermal HVAC systems use?

There are 4 main types of underground loops, depending on their location and position:

• Horizontal Loop - Pipes are buried in the ground horizontally.
• Vertical Loop - The coils are positioned vertically.
• Pond Loop - The loop is submerged in a nearby body of water, allowing the system to transfer heat with water instead of soil.
• Open Loop - Instead of traveling through a sealed pipe, open loops take water from a well or pond and then discharge it at the end of the cycle.

Is the antifreeze toxic to the environment?

Possibly. A GreenMatch.co.uk article explains that some types of antifreeze used in geothermal heat pumps, such as methanol and ethylene glycol, can be very hazardous while others like propylene glycol are safe. If you’re considering a ground-source heating and cooling system, it's important to check local laws regarding antifreeze usage to ensure environmental safety and regulatory compliance.