Geothermal HVAC systems are among the most energy efficient systems available. Depending on the situation, typical energy savings are from 25% to 70%. These units have two efficiency ratings. The unit's cooling efficiency is given as an EER (energy efficiency rating). This is the cooling effect of the unit divided by the electrical input required to the unit. The heating efficiency is given as COP (coefficient of performance). This is the heating effect of the system divided by the electrical input required by the unit. In both cases, a higher number with the rating means the unit is more efficient.
According to the U.S. EPA, these systems have the lowest life cycle cost of any hvac system. Although they usually cost more to install, they generally require less maintenance and repairs.
Depending on the local conditions, the time required to recover the installation expense can be pretty short. There are now do it yourself kits available. This can lower the installation costs considerably and make the systems more affordable.
These units can be central heating only, hot water only, or heating and cooling systems. Heating only systems are common in Europe, especially in colder climates. Some of them have the ability to provide free passive air conditioning. In that case the water is pumped through the loop system and into a cooling coil. The unit's compressor does not operate. In the U.S.A., combined heating and cooling units are the most common.
The basic geothermal system consists of three parts. The first part is called the loop system. There are two main types, open loop and closed loop systems. These systems carry either water or a water/antifreeze mixture.
Open loop systems are commonly called "pump and dump" systems. They pump water from a well to the system then return it to another well or discharge it on the surface. They are not very common and many local codes do not allow this type.
Closed loop systems are the industry standard. These loop systems use a sealed water system where the water is used over and over instead of being dumped. There are three basic ways these systems are installed. The first way is a series of plastic pipes buried in trenches about four feet deep. This is called a horizontal loop system. It is an economical solution where sufficient land area is available.
In the vertical loop system, the pipes are buried in a series of holes drilled down around 200 feet. The holes are filled with a material called grout. This ensures good heat transfer between the pipe and the ground. This method is usually more expensive to install due to the specialized drilling required. But, much less land area is required, so it is a more common solution in smaller cities and towns.
The last method involves submerging the loop system in a pond or lake. Usually the loops are placed on a rack and submerged. The latest innovation for lakes, ponds, etc. is the Lima-1 system from Limnion. This system includes the loops in a compact case that is anchored to the bottom of the lake or pond. It has several advantages including easier installation.
The earth's temperature at the depths of these loops stays relatively constant all year. In the U.S., it is about 45-55 degrees all year. In the warmer parts of the world, the temperature can be as high as 70 degrees. But even at that temperature, they can still provide superior cooling capacity.
The next part of the system is the actual heat pump. They work like a typical air source heat pump operating in 50-60 degree outdoor air (which is where an air source heat pump is usually most efficient). Unlike the air source heat pump, there is usually no equipment located outside the home. The main difference is that their heating and/or cooling capacity and efficiency do not change as much because the ground temperature is relatively constant.
The unit uses a reversing valve to change the direction of refrigerant flow. This determines whether heat is added or removed from the air in the home.
Most of the units can provide a good portion of the homes hot water requirements. This is accomplished by the addition of a heat exchanger that is referred to as a desuperheater. This is usually combined with a regular electric water heater. In these systems, hot water is only produced when the home requires heating or cooling. Other units can provide for all of the home's hot water needs. These systems use integrated demand hot water heating. These systems cause the geothermal unit to start and operate as necessary to provide for the homes dhw needs.
The final part of the forced air geothermal system is the duct system. This is similar to a conventional duct system, although the ducts are typically bigger.
The traditional system puts the compressor section and a blower compartment into a single appliance. They are typically used in new construction or when replacing an entire existing hvac system. Water lines are ran outside the home to the loop system. Combination systems also provide hot water for radiant floor heating systems, ice melting, and even pool heating.
There are many manufacturers of these units, which makes it hard to compare brands. We have performed the research and put the results in a simple format.
A split system contains the compressor and associated components but does not have a blower and is not directly connected to the home's duct system. This is usually combined with an existing furnace or air handler instead of replacing the entire hvac system. They are sometimes combined with a gas furnace and referred to as a hybrid heating system although that is much more expensive than a traditional packaged system.
The latest innovation is a unit that replaces the old existing air source heat pump with a geothermal unit. It is designed to be placed outside where the old heat pump was located. The geothermal loop system is completely outside. Refrigerant lines run from the outdoor unit to a coil on the furnace/air handler. It uses many of the existing hvac system components to minimize installation costs. One of the main advantages is that normally no changes are required to the existing electrical wiring. You can compare the brands on our ratings page.
The most advanced geothermal systems are called direct expansion (DX) units. They eliminate the water/antifreeze mixture and the associated pumps. Instead, the compressor pumps the refrigerant through copper lines that are buried instead of the traditional water filled plastic lines. This results in better heat transfer and more efficient operation. You do not need purging/charging carts for startup of the system and they are easier to maintain and repair. They also are a lot easier to install and usually require less ground area for the loops (captors). The refrigerant lines are buried like the traditional polyethylene pipe loops. One of the main concerns with this type of system is corrosion of the buried copper refrigerant lines. This is usually only a concern in areas where the soil is acidic. In some cases, a cathodic protection system is installed to minimize the possibility of corrosion. Before shopping for a unit, you can compare the brands on our ratings page.