Using geothermal energy to heat and cool your home

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With all respect to Carole King and James Taylor, maybe their song lyrics should have been “I feel the Earth heat under my feet.” It rhymes nicely, plus there is an enormous source of energy right beneath our feet that we can tap into to both heat and cool our homes. Is it possible? Yes. Is it practical? Sometimes.

Most of us are familiar with air-source heat pumps, small fan-like boxes that sit outside the home. On a cold winter day, this appliance uses a refrigerant to extract heat from the surrounding air and pump it into your home. On a hot summer day it will do the opposite, extracting heat from the home and pumping it outdoors. One challenge for these appliances is that they have to work hardest on the days when they are needed the most. On the coldest days, when you need a lot of heat, it is hardest for the heat pump to pull that heat out of the air. And similar for the hottest days. That is when they are least efficient.

An alternative is a ground-source heat pump that exchanges heat with the ground instead of with the air. They are more efficient because the ground below the frost line, about 5 feet down, stays around 55 degrees throughout the year, whether on the coldest winter night or the hottest summer afternoon. Because they use a moderate temperature source year-round, they use less energy than an air-source heat pump.

A ground-based heat exchanger can be installed horizontally in shallow trenches. This is a relatively inexpensive installation but it requires a lot of undeveloped land. A commercial building might install a horizontal loop under a parking lot, for example, or a rural home might have room under an open grassy area.

But these configurations are not feasible on densely developed or smaller lots. That is where vertical boring comes in. These holes are more expensive to dig but require less space.

A general rule of thumb is 200 feet of tubing yields about one ton of heating or cooling. An 1800 square foot home might need 3 tons, or about 2 300-foot boreholes or 600 feet of trenches. A geothermal engineer will assess the property and the ground characteristics to determine how many trenches or boreholes are needed and of what length. The holes or trenches should also be spaced sufficiently so they are thermally isolated from one another.

Water (sometimes mixed with antifreeze) is circulated through pipes underground and back into the house, where a heat pump uses a refrigeration cycle to make that water even hotter (or colder). That fluid then conditions either air that is blown through ducts in the house, or water that is pushed through a radiant floor. Voila, a comfortable house!

Even though these systems are very efficient, vertical boring can make them very expensive to install, often a premium of $20K or more over other options. They make the most economic sense when a house is first being built or when replacing a system that burns costly fuel like heating oil. There is a 30% federal tax credit through 2032, which also helps. A company called Dandelion Energy that is working to install residential geothermal efficiently and at scale in the northeast has gotten the total price down closer to $25K. They are also providing free financing with payments of around $200-$300/month for 5-10 years. For homes that are paying $1000 or more each winter month for heating, that can start to make a lot of sense.

The Swedes have installed over 500,000 small geothermal wells. When a carbon tax pushed up fuel oil prices in the 1990s, geothermal systems became popular. (Cold-weather air-source heat pumps were not yet available.) Take a look at the map below, from an area south of Göteborg. Canada’s CBC News quotes a heat expert saying that in the early 2000’s “there was a drilling rig in the neighborhood pretty much every single week.”

Costs will be lowest where geothermal is more common, but it is certainly possible to install a system in the Bay Area. Palo Alto resident Cedric de La Beaujardiere and his wife Susan Stansbury decided to put one in at the end of 2020. They wanted to reduce their emissions and their small lot made it difficult to find a place for an air-source heat pump. Cedric explains: “We considered an air-source heat pump, but our side yards are narrow and these units are not permitted in the front yard, so the only place to put one would have been in our meditative place and I was concerned this would be disruptive.”

They were planning to redo their old driveway and garage floor anyway, so it made sense to put the boreholes under the driveway. The contractor they hired, Cold Craft, recommended two 300-foot holes about 12 feet apart for their 1400 square foot home.

Early on a December morning in 2020, a large drilling rig backed into the driveway. The company had a smaller rig but it was not available that day, so they used their large commercial vehicle.

The contractors began by drilling the first bore, about five inches wide, 300 feet down. This took a few hours as they repeatedly attached new pieces of casing as the hole got deeper. Mud that they used to facilitate the drilling combined with clay from the hole to create a muddy pool at the base of the rig. A pump siphoned this mixture out of the driveway, where it was sorted so that some of it could be reused for the drilling.

Cedric says they encountered no rock at all. “After the drilling we got a report of the layers of soil detected at various depths — all various types of clay and two water tables were encountered as they drilled down.”

Once the boring was done, the plastic tubing that forms the heat exchange loop was inserted into the hole.

A small pipe was inserted with the tubing so that mortar could be poured into the hole from the bottom up. The mortar helps to extend the life of the system, as Cedric explains: “This mortar provides thermal contact with the earth and stays flexible, so if there’s an earthquake the lines should bend and sway but not crack and leak…. I don’t recall the warranty details, but I remember the installer saying the heat exchange loops buried in the ground should outlive us.”

It took about one day to complete the first borehole and a second to complete the next.

Pipes were installed from the driveway into the house beneath what would be a poured concrete garage floor.

A heat pump inside the garage further warms or cools the water via a refrigeration cycle, then the water conditions air that is blown into the house through the duct system.

After several years with this system in place, Cedric says “We are happy with it. It provides heating, cooling, and the ability to just run the fan to filter the air. The latter two are a new benefit for us that our gas furnace did not provide.” The system cost considerably more than an air-source heat pump, but it was the best option for their needs. Cedric considers it to be a good investment in their home and in the planet. “I strongly support heat pumps for space heating and for water heating, whether air-source or ground-source. Second only to passive designs, these are the most energy efficient way to heat and cool air or water.” Cedric and Susan also have a recently updated rooftop solar system that provides ample electricity for not only their heating and cooling, but also for about 10K EV miles.

Some of you may be wondering whether this technology makes sense in Tahoe’s colder climate, where air-source heat pumps are less efficient and where the Earth’s heat could be used to de-ice driveways and sidewalks. There are both newer and older homes in the area that use geothermal, but I was not able to get in touch with an installer to learn about specifics. I think for now it is a niche option, where the practicality of geothermal will depend on heating fuel prices and property characteristics.

Perhaps more interesting is the potential of so-called “networked” geothermal heating, in which a utility installs wells under the streets that are then used to heat homes. A Truckee resident is interested in piloting that at the local high school campus. I will cover these systems in a future blog post.

You can learn more about residential geothermal in this short video from the Department of Energy, or this longer one from This Old House. YouTuber Matt Ferrell does an economic analysis of his installation here. I look forward to hearing any questions you have about this technology.

Current Climate Data

Global impacts (February 2024), US impacts (February 2024), CO2 metric, Climate dashboard

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