Geothermal HVAC systems, also called ground source heat pumps, 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). 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 systems that can mostly be home-owner installed. This can lower the installation costs considerably and make the systems more affordable.
While water to water geothermal systems do exist, combined forced-air heating and cooling units are the most common.
There are two main types, open loop and closed loop systems. Closed loop 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 non-setting material. 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 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.
There are two types of these units. The first type contains all the mechanical components required for the system. This is referred to as a package system.
The other type only contains the compressor and associated components. This is usually combined with a furnace or air handler. They are called split systems and are sometimes combined with a gas furnace and referred to as a hybrid heating system. This type is usually a lot more expensive to install but they offer more flexibility in design.
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.
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.
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.