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Want to Warm Up or Cool Down? Go Underground!

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Have you ever been so cold sitting in your house in the winter that you feel like icicles could grow off your nose, and your feet could turn into blocks of ice? Or have you ever been so hot in your house in the summer that sweat just drips off you? Making yourself comfortable can mean turning on the heater or the air conditioner, but that costs money. Being cozy or cool does not have to cost a lot of money if you use a geothermal heat pump. In this project, you will build a model geothermal heat pump and use it to gradually increase the temperature of water. So read on, learn about geothermal energy, and get the earth to work for you!


Areas of Science
Time Required
Average (6-10 days)
Material Availability
To make a geothermal heat pump, you will need a pond pump. See the Materials list for details.
High ($100 - $150)
Exercise caution when working with electrical equipment and water. Adult supervision is required.
Michelle Maranowski, PhD, Science Buddies
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To build a geothermal heat pump and use it to increase the temperature of water.


Autumn and winter are always tough on the heating system in your house, because it has to work around the clock to keep you comfortable at around 68°F when it is cold or freezing outside. In the summer, when the outdoor temperature is in the 80s or 90s, the air-conditioning system is working to keep you cool at around 75°F. It takes a lot of energy to keep your house at a comfortable temperature when it is very cold or hot outside, and energy costs money.

Did you know that you could use the earth to do some of that heating and cooling and help your family save money? How? By using a geothermal heat pump. The word geo means "earth" in Greek and the word thermal comes from the Greek word therme, which means "heat." Geothermal heat pumps take advantage of the fact that the ground from 5 feet (ft.)–10 ft. below the surface stays a constant temperature, between 45°F and 75°F, depending on latitude. The geothermal, or ground-source heat pump, system is made up of a heat pump, loops of pipe (sometimes called the ground exchanger), and a liquid that flows through the heat pump and the loops of pipe. The liquid can be water or a mixture of water and antifreeze. The heat pump is located indoors and the loops of pipe are buried outdoors underground. Figure 1 shows the operation of a geothermal heat pump.

Diagram of how a geothermal heat pump works next to an image of a geothermal heat pump

The diagram shows hot liquid colored red flowing through long pipes buried underground. The long pipes act as heat exchangers and the hot liquid cools down before being pumped back into the building. Pictured next to the diagram are two pumps used to move a subtance through a geothermal heat exchanger.

Figure 1. The diagram on the left shows how a geothermal heat pump works. On the right is a photograph of an installed geothermal heat pump.

How does the system work? In winter, when it is cold outside, the temperature underground is warmer than the air. The liquid, or solution, circulating through the pipes in the ground absorbs heat from the ground. When the heated solution comes back to the pump, the heat from the solution is transferred to air by the heat pump. The regular heating system then heats the air more and distributes it to the rest of the building. The pump then circulates the cooled solution back into the ground and the process repeats until the desired indoor temperature is reached.

In summer, when the air temperature is warmer than the temperature underground, the system works in reverse. The pump absorbs heat from the building and transfers it to the circulating solution. The solution transfers the heat to the ground and cools as it circulates through the pipes on its way back to the pump.

Using the earth to heat and cool homes and offices is a green and renewable source of energy because it is natural and easily replenished. Watch this video to learn more about how geothermal heat pumps work.

Energy 101: Geothermal Heat Pumps
Watch this geothermal video. Making your your house comfortable using geothermal heating and cooling is a good choice if you would like to save money and energy.

In this energy and power science project, you will build a geothermal heat pump and demonstrate how the the earth can heat and cool the air inside a building.

Terms and Concepts



If you need some guidance on how to dig holes, check out this webpage:

  • WikiHow Contributors. (2011, February 9). How to dig a hole. Retrieved March 24, 2011.

For help creating graphs, try this website:

  • National Center for Education Statistics. (n.d.). Create a Graph. Retrieved June 2, 2009.

Materials and Equipment

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Experimental Procedure

Testing the Pump

  1. With your trenching shovel, dig a hole in the ground that is 4 ft. deep and 2 ft. across. Find a location that is free from foot traffic where you will not be digging near any utility lines. Have your adult volunteer help you. (Not sure how to dig a hole? Read this webpage: How to dig a hole). Once you have finished digging the hole, place the wood board on top of it so that no one falls in and gets hurt.
  2. Mark a line around the inside of the cooler, 4 inches from the top.
  3. Cut and fit the insulation around the outside of the cooler on all sides, including the lid. Use tape to secure the insulation to the sides of the cooler.
  4. Position the cooler near the hole. Place 3 1/2 lbs. of ice at the bottom of the cooler and then fill the cooler with water to the mark that you made in step 2. The cold water will magnify the effect of the pump and of pumping water through the earth.
  5. Place the pond pump in the cooler. Make sure that the pump is completely submersed in the water. Cover the cooler with its lid and use the extra insulation to cover and surround the cooler. Figure 2 shows the pump inside the insulated cooler.
Photo of insulation used to wrap a cooler next to a photo of a cooler filled with water and a pump
Figure 2. This shows the insulation (left) and a picture of the covered cooler with the pump inside (right).
  1. Wait 20 minutes for the temperature inside the cooler to stabilize.
  2. Take a temperature reading. Move the cover aside a little bit and insert the thermometer into the water. Wait for 10 seconds and then read the thermometer. Record the temperature and the time you took it in your lab notebook. This temperature is the starting temperature.
  3. Plug the pump into the nearest electrical outlet. Use the extension cord if necessary. Record the time that you plugged in the pump in your lab notebook.
  4. With the water circulating within the cooler, take a temperature reading every 30 minutes for 4 hours. Record every reading and the time you took it in your lab notebook in a table like Table 1.
Trial #TemperatureTime
Table 1. Record the time and temperature of your readings here.
  1. After 4 hours have passed, disconnect the pump from the outlet and pour the water out of the cooler.
  2. Repeat steps 4–10 for two more trials (you can do this on different days). Repeating the experiment (for a total of three trials) will ensure that your results are accurate. Record all of the temperature and time data in your lab notebook, along with the trial number.

Building and Testing the Geothermal Heat Pump

  1. Perform this second test at the same time of day as the first test. Move the cooler close to where you dug the hole.
  2. With the box cutter knife, cut out two notches, 6 inches apart, in the rim of the cooler. The notches should be big enough to fit the bottom half of the PVC tubing, but no bigger. The cover should fit snugly over the cooler and the tubing.
  3. Shape the tubing into a "U" and then place the curved part of the tubing in the bottom of the hole you dug in step 1 of Testing the Pump. Try to space the two arms of the bent tubing about 6 inches apart from each other (see Figure 3). Shovel dirt into the hole and bury the tubing. Tamp down the soil around the tubing.
Plastic tubes connect to a pump inside a cooler and are buried underground
Figure 3. Geothermal heat pump. In this example, the pond pump is connected to the tubing going through the ground. Notice that the tubing is sitting in the notches in the side of the insulated cooler.
  1. Place the ends of the tubing in the notches and then into the cooler. Put 3 1/2 lbs. of ice at the bottom of the cooler. Fill the cooler with water until the surface of the water reaches the 4 inch mark on the cooler.
  2. Surround the tubing that is still above ground (but not inside the cooler) with the pipe insulation. Cut the pipe insulation to the correct size.
  3. Put the pump back into the cooler. Connect one end of the tubing to the outlet of the pond pump. The other end of the tubing should hang free in the cooler. Cover the cooler with its lid and use the extra insulation to cover and surround the cooler. Now let the temperature stabilize for 20 minutes, then take a temperature reading. Record the time and the temperature in your lab notebook. This is the starting temperature and should be identical to the starting temperature from Testing the Pump.
  4. Plug the pump in. The water should move freely through the tubing buried underground and back into the cooler. Record the time that you plugged in the pump in your lab notebook.
  5. Create a new table, like Table 1 from Testing the Pump, and label it accordingly. Take a temperature reading every 30 minutes for 4 hours. Record every reading and the time you took it in the new table in your lab notebook.
  6. After testing is completed, unplug the pump and pour out the water from the cooler.
  7. Repeat steps 4, 6, 7, 8, and 9 two more times (for a total of three trials). Repeating your experiments ensures that your results are accurate. Record all temperature and time data in your lab notebook, in a new table, along with the trial number.

Analyzing Your Data

  1. Plot your data on a scatter plot. If you would like more information on scatter plots, or if you would like to make your plots online, try this webpage: Create a Graph.
  2. Make a scatter plot for each trial. Label the x-axis Time and the y-axis Temperature and plot the data for each time carefully.
  3. What do your scatter plots tell you? What is the difference in the final temperature between just running the pump with the water inside the cooler and pumping that water through the tubing in the ground? Is using a geothermal heat pump a viable option for heating buildings?
  4. More-advanced students should:
    1. Calculate the change (final minus starting) in temperature for each trial.
    2. Average the change in temperature for each condition (just running the pump versus pumping water through tubes in the ground).
    3. What is the difference in the average temperature change between the two conditions? Hint: Use subtraction.
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Ask an Expert

Do you have specific questions about your science project? Our team of volunteer scientists can help. Our Experts won't do the work for you, but they will make suggestions, offer guidance, and help you troubleshoot.

Global Connections

The United Nations Sustainable Development Goals (UNSDGs) are a blueprint to achieve a better and more sustainable future for all.
This project explores topics key to Affordable and Clean Energy: Ensure access to affordable, reliable, sustainable and modern energy.



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General citation information is provided here. Be sure to check the formatting, including capitalization, for the method you are using and update your citation, as needed.

MLA Style

Maranowski, Michelle. "Want to Warm Up or Cool Down? Go Underground!" Science Buddies, 22 June 2021, https://www.sciencebuddies.org/science-fair-projects/project-ideas/Energy_p040/energy-power/want-to-warm-up-or-cool-down-go-underground. Accessed 30 Nov. 2023.

APA Style

Maranowski, M. (2021, June 22). Want to Warm Up or Cool Down? Go Underground! Retrieved from https://www.sciencebuddies.org/science-fair-projects/project-ideas/Energy_p040/energy-power/want-to-warm-up-or-cool-down-go-underground

Last edit date: 2021-06-22
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