Geothermal heat pumps are the most energy-efficient and environmentally friendly method for heating and cooling that exists — so it's no wonder that each year sees 50,000 new geothermal heat pumps installed in the United States, according to the Department of Energy. While geothermal heat pumps (also known as ground-source heat pumps) can dramatically lower heating and cooling costs, they are expensive to install — from $10,000 to $30,000 — which is probably the main reason there aren't more of them. Geothermal heat pumps cost so much because they require a buried piping system to exchange heat with the ground, which is at a constant temperature of 57 degrees Fahrenheit a few feet below the surface no matter where you are and what the air temperature happens to be.
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Homeowners who live near a large body of water have an advantage because a pond or lake can serve as a heat source in the winter and a heat sink in the summer, and water-source heat pumps are cheaper to install than ground-source ones. Whether you install a ground-source or water-source system, you're using geothermal energy — which is renewable energy — and consuming only enough fossil fuels to provide electricity for a refrigerant compressor, a circulation pump and a blower. Once installation is complete, geothermal heat pumps give homeowners the most cost-effective heating and cooling possible while significantly reducing their carbon footprints (compared to using conventional heating and cooling sources).
How Geothermal Heat Pumps Work
In general, a heat pump works like a refrigerator or an air conditioner. A refrigerant circulates through a tubing system formed into two distinct coils. In one set — the condenser coils — the refrigerant is pressurized and condenses into a liquid. In the other set — the evaporator coils — it's allowed to vaporize. Condensation is a process that releases heat, while evaporation is one that absorbs heat. A heat pump takes advantage of this by switching the airflow between the two sets of coils as needed and exhausting air from the other set of coils. In this way, a heat pump exchanges heat energy with the environment.
Air-source heat pumps, which are the most common type, exchange heat with outside air and don't work well in cold climates because the air doesn't have much heat to exchange. About 5 feet below the earth's surface, however, the temperature is constant, so a ground-source heat pump can work almost anywhere. The problem for engineers is how to tap into this reservoir of heat energy, and the solution is to run a system of tubing underground, circulate a liquid through it and extract the ground heat from the liquid.
The tubing is made from PEX or HDPE, which are both types of polyethylene plastic, and it's either filled with a mixture of water and antifreeze ( a 25 percent solution is typical), with water from a nearby body of water or with groundwater. A circulation pump keeps it moving, and it is able to transfer heat to the refrigeration coils through a heat exchanger inside the heat pump's air handler. The heat is then circulated through a ductwork system by a blower.
Types of Loop Systems
Loop systems for geothermal heat pumps can be open or closed, and closed-loop systems are the most common. System installers fill the tubing with a water/antifreeze mixture when the geothermal system is installed and, barring any leaks, it never has to be refilled. There are three types of closed-loop systems:
- Horizontal loops, which are the most cost effective for most applications, are laid out like septic drain field pipes. Two pipes are buried 5 feet deep in a side-by-side configuration, or one pipe is buried 6 feet deep, and the other is buried above it at a depth of 4 feet.
- Vertical loops are an alternative when the ground is too rocky for horizontal loops, or there isn't enough space. One or two holes are bored to a depth of several hundred feet to accommodate a pair of pipes connected at the bottom.
- Water-source loops are placed underneath several feet of water in a nearby lake or pond and draw heat from the water, which at a depth of 5 to 10 feet is about the same temperature as the surrounding earth. Underground pipes can also extend through aquifers and exchange heat with ground water.
An open loop system can be employed when an appropriate body of water is available. Instead of a water/antifreeze mixture, it takes in water directly from the water source, circulates the water through the heat exchanger and dumps it back into the body of water or into a drainage ditch depending on local regulations. Any of these systems can be employed in conjunction with a conventional heating system, such as a gas or electric furnace, to make a hybrid system that uses the heat pump for supplemental heat to keep energy costs down.
Pros and Cons of Geothermal Heat Pumps
Installation of a horizontal-loop geothermal heat pump alters the landscape and isn't feasible on some types of terrain or if the ground is heavily wooded because, just like a septic drain field, a geothermal loop is vulnerable to tree root incursion. Installing a vertical loop can be a remedy when a horizontal loop won't work, but it's more expensive because it involves drilling holes hundreds of feet deep, often through bedrock. A water-source system is the least expensive to install, but it's an option only for properties close enough to a suitable body of water.
If you decide to install a geothermal heat pump system, you earn the following perks:
- Low operating costs that can offset the high installation costs in just a few years. Once up and running, geothermal heat pumps cost 65 percent less than conventional heating systems. Even though a heat pump consumes more electricity than a conventional gas heating system, it uses far less fuel overall.
- Longevity. Heat pumps themselves last about as long as furnaces, or from 15 to 20 years, and they are easy to replace. Geothermal loops are more difficult to replace, but you shouldn't ever have to do that because they last from 50 to 100 years.
- Clean, emission-free heat. Geothermal heat pumps are safe for the environment and present no danger of carbon monoxide poisoning inside the home. Emissions are essentially nil (apart from emissions related to the electricity production).
- Extra energy savings. You can connect a heat pump to your water heater and use it to heat water. Besides being an energy-saving strategy, this is cleaner than heating water with gas or electricity.
The two most important drawbacks are upfront costs for installation and location dependence.
Three Questions to Ask Yourself
If you're wondering whether you should be considering a geothermal heat pump for your house, ask yourself questions like:
1. Is my house in a good location?
If it's in a congested urban setting with municipal water and waste systems, you probably don't have room to do much digging, and you're unlikely to get the necessary permits. If it's in a severely sloping or heavily wooded setting, installation may also be impractical.
2. Do I love my landscaping just as it is?
The required digging is disruptive, so you have to be ready for change. That's one reason geothermal heat pumps are best installed as part of new construction when the landscaping has yet to be developed.
3. Is my house tight enough?
Heat pumps don't provide as much heat as furnaces, and you'll lose a lot of it through unsealed windows and gaps in poorly insulated floors and ceilings. Sealing gaps and upgrading insulation is an option, but it adds to the already high upfront costs of installing a geothermal heat pump.