How They Work

Basic Concept:

Ground temperature is less volatile than air temperatures. At depths below 6ft, the ground is a constant temperature, about 50ºF in Connecticut. This is why some animals hibernate underground for the winter or why cellars feel cool in the hot summer months. A geothermal system uses this constant temperature as a heat source in the winter and a heat sink in the summer.

Heat Pumps:

It may seem counterintuitive that a ground temperature of 50ºF can be used to heat a home to a reasonable comfort level. However, this low-grade thermal resource is converted into a higher-grade thermal resource (hot air or water) by an electronically operated device called a heat pump. A heat pump is essentially an air conditioner or refrigeration unit running in reverse, and concentrates the ground heat into some medium, either air or water, at a much higher temperature.

System Components:

Each geothermal system, whether it is for heating, cooling, or both, consists of a ground loop, a heat pump, and a thermal distribution system. The ground loop is used to absorb or reject heat, the heat pump is used to add or remove heat from the thermal distribution system, and the thermal distribution system is the means of heating or cooling the building.

Ground Loop:

The ground loop is the heart of the geothermal system. It may be horizontal, at a constant depth below the ground's surface, or vertical, like a standard water-well. Vertical loops may be open systems, circulating groundwater, or closed systems circulating any number of fluids such as water or a water-antifreeze mix. Loops are typically made of polyethylene tubing and often come with a lifetime warranty. Fluid in the ground loop is circulated by a pump, which can be variable speed, adjusting the flow to the heating or cooling demands of the building.

Heat Pump:

A heat pump uses the vapor-compression refrigeration cycle, in reverse, to move heat against its natural gradient. A heat pump consists of a compressor, condenser, evaporator, expansion valve and a closed loop of refrigerant, usually R-22 or R-410a.
The heat in warm ground water is exchanged with the cool refrigerant vapor in the evaporator, increasing the temperature of the refrigerant. A compressor is then used to pressurize this vapor into a hot vapor, which passes through a condenser, giving up heat to the thermal distribution system. Finally, the refrigerant flows through an expansion valve where it returns to a cold vapor to start the cycle again.

Refrigerators and air conditioners work in this same manner, only in reverse and they use the surrounding air as a heat sink instead of the ground. Therefore, heat pumps can conveniently be reversed and used to provide central air conditioning

Thermal Distribution System:

Every house needs a thermal distribution system to get the heat from the furnace into the various living spaces. This can be done using vents which output forced hot air, radiant flooring which circulates warm water to give off heat, or hydronic baseboards which circulate hot water and distribute heat using natural convection. There are a number of other ways to distribute heat, but these are the most common methods. Geothermal systems work with all of the above thermal distribution systems.

If a geothermal system is being used for air conditioning, however, the only compatible system is ductwork for forced-air. Circulating cold water in a radiant floor or a hydronic baseboard system is not an effective method for cooling because no humidity is removed from the air. Thermal comfort is not only impacted by temperature, but by several other factors, including humidity.