All About Geothermal Heating Systems

Learn how geothermal systems work to deliver earth’s ground-heat indoors and how they can serve as alternative cooling systems.

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by AdobeStock/Alis Photo

If you dig down about five feet or so in the ground to below the frost level, you will find the ground temperature to be amazingly constant, 40 degrees to 70 degrees F (4-21 degrees C), depending on the location.

It is cooler than the air in the summer and warmer in the winter. The earth’s subsurface is an enormous heat sink — a solar battery — and it takes a large amount of energy to keep it in equilibrium. This heat energy comes in great part from the sun, a renewable and inexhaustible source of energy. In lesser amounts, it also comes from the center of the earth that we now know is a heat generator. The inner core of the earth is primarily made of a solid sphere of iron within a larger sphere of molten iron. Calculations show that the earth, originating from a molten state many billions of years ago, would have cooled and become completely solid without an energy input. It is now believed that the ultimate source of this energy is radioactive decay within the earth that continues to this day; the decay produces gradually diminishing temperatures from the earth’s center to the surface. This does not mean that dangerous radioactivity is a hazard to us. We can tap into all of this heat energy, transfer it into our home for heating and return that energy back to the earth during cooling: thus we are really borrowing heat from the earth.

black and white diagram of a refrigerator's cooling system

Geothermal units use the same 100-year-old technology found in your refrigerator. They are both devices that move heat energy. It is worth noting that the refrigerator is the most reliable, longest-life appliance in your home. As the diagram above explains, a refrigerator removes heat energy from food and moves it into your kitchen. A geothermal system removes heat energy from the earth to heat your home and in the summer removes heat energy from inside your home back to the earth.

Heat naturally flows “downhill” from the warmest medium to the coolest medium. A heat pump is a machine that causes heat energy to flow in the direction opposite from its natural tendency, or “uphill” in terms of temperature. Because work must be done (energy must be applied) to accomplish this, the name heat “pump” is used to describe the device.

A refrigerator and a heat pump are about the same physical size, are quiet appliances usually contained within a single enclosure, have similar components (compressor, evaporator, etc.), and both transfer heat energy. And they each require a refrigerant, a material used in a refrigeration cycle which undergoes a phase change from a gas to a liquid, and back again.

But you will not find a gleaming white exterior or an icemaker on a heat pump. I view a geothermal heat pump as a refrigerator on steroids because it is designed to transfer a much higher level of heat energy. This requires a larger and more sophisticated compressor. In addition, a GHP is instantly reversible to provide heating or cooling. Of course, while a refrigerator is “plug and play,” a GHP must be connected to the home’s air ductwork or a hot water system, and to thermostats, water pipes, pressure gauges and valves. Oil and gas boilers also require electricity to run, but they need a constant supply of fuel in storage, a huge amount of oxygen to be burned, a chamber to hold the combustion, a heat exchanger to extract the heat energy and an exhaust system to get rid of the combustion products.

The most important difference, however, is in the efficiency. No refrigerator or oil burner is 100% efficient because like any machine, they have losses. Efficiency is basically output divided by input. Achieving greater than 100% efficiency means that you are getting out more energy than you are putting in. But surprisingly, a GHP can and does exceed that limit to provide 400% or even greater efficiency.

How It Works: Heating

The major advantage of a geothermal heat pump is that it uses a small amount of electrical energy to transfer a much larger amount of ground-heat energy. In the process, most of that electrical energy is also converted to heat energy.

diagram of a three loop geothermal system in a house

To make this happen, a geothermal system uses three-loop transfer to capture and transfer the earth’s heat energy. Each loop carries heat energy to the next loop. The first loop is a series of closed-loop pipes buried in the ground below the frost line in either a horizontal or vertical configuration, or placed in a lake or pond. Cool water from the unit in the house circulates through the ground pipes and the warmer earth releases heat energy into the water as it travels back to the GHP unit in the house. Non-toxic antifreeze is added to the ground loop water since it can sometimes be cooled below freezing by the GHP before the water circulates to the ground loop.

Or, as in my case, it is also possible to use an open-loop system by piping some of the well water directly through the GHP unit and returning it back to the well. There is no contamination of the well water; it merely circulates through plastic pipes before emptying into the well.

The warmed water from the earth passes through a heat exchanger (coaxial copper pipes) to the second loop containing an even cooler liquid refrigerant. Heat transfer takes place which cools the incoming water and sends it back to the ground loop to pick up more heat energy. The refrigerant accepts the heat energy and becomes a gas as it heats up. The now gaseous refrigerant is sucked into a compressor where it is compressed and superheated to about 165 degrees F (74 degrees C).

In a water-to-air system, the heated refrigerant then passes through a radiator-like heat exchanger (air coil) over which air is passed. Or in a water-to-water system, 120 degrees F (49 degrees C) refrigerant is passed on to a hot water heating system by means of a heat exchanger (hydronic coil) connected to baseboard registers or in-floor heating tubes. Air delivery ductwork of course, permits air conditioning.

After the refrigerant transfers the heat to the air coil, it goes through a thermostatic expansion valve (TXV) and the pressure is released. The refrigerant becomes very cold (sometimes below freezing) as it circulates back to pick up more heat from the ground loop.

The third “home” loop: In a water-to-air system, the fan-driven cool air is heated as it passes over the air-coil heat exchanger and is then ducted into the home at about 105 degrees F (41 degrees C), transferring its heat energy to the walls, atmosphere, you, etc. As the room air cools, it is returned to the GHP to pick up more heat energy. In a water-to-water system, heat is transferred throughout the home through tubes in the floor or walls or baseboard registers.

diagram of a geothermal system heating a house in winter

How It Works: Cooling

Cooling is a similar process except that with just one tap of a button on the thermostat, a clever reversing valve sends the hot compressor output to the returning ground loop instead of to the indoor air loop.

While a geothermal heat pump operating in cooling mode uses essentially the same theory of operation as a residential AC system, there are two major differences.

First, a standard AC system requires a large outdoor heat exchanger in your backyard to dump heat energy into an already hot and, in some locales, humid atmosphere. This works, but is very inefficient because it takes a lot of electrical energy. The GHP needs no noisy outdoor box; all components are underground and inside the home. In my house, I send cold well water to flow over the heat exchanger pipes, heating the water which then flows back into the well, which in turn puts heat back into the ground.

Second, a standard AC system and a fuel burner/boiler are two separate systems connecting only at the ductwork. The GHP is a single system that can be reversed with a simple switch to provide cooling or heating. This reduces costs and increases efficiency. Alternately, we can set the thermostat for a given temperature and let the system determine if heating or cooling is needed.

That is it — a very simple concept, just three loops to bring in comfortable, quiet heat and cooling using well-established technology.

diagram of a geothermal system cooling a house in summer

Don Lloyd, author of two geothermal books, lives in the Hudson River Valley region of New York with his wife, artist Martha Lloyd, in a home powered by the sun and heated/cooled by the earth. Reprinted with permission from Geo Power, by Donal Blaise Lloyd and published by PixyJack Press, 2015.

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