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Geo-Thermal Heating & Cooling PDF Print

Heating and Cooling are two of the highest energy use processes in a home.  53% of the energy use in the home is for heating and cooling (Energy Kids).  There are many ways to reduce that energy cost.  These can include better insulation, more efficient windows, thermostat temperature settings, home design and higher efficiency equipment.   The discussion for this article is going to focus on one of the most efficient ways to heat and cool your home.  This involves Geo-Thermal Ground Source heat pumps.

 There is one large advantage with Geo-Thermal Ground Source Heating Pumps.  The source for heating and cooling is derived from the ground temperature rather than the air.  The temperature of the ground varies by location but in the Midwest, it stays around 50 to 55 degrees (GreenCast).  Air source Heat Pumps must use Air temperature for their heating and cooling.  As you are well aware, air temperature varies widely during the year.  In the summer it may reach above 100 degree temperatures and the winter it may dip below 0 degrees.

The main question is why is the ground temperature more efficient for the cooling and heating of your home?  The best place to start with this answer is to explain how a heat pump works. 

A heat pump has the ability to switch directions by opening and closing valves.  This allows it to cool in the summer and heat in the winter.  A heat pump actually removes heat from the air.  In the heating cycle, it is doing the reverse of air conditioning.  It extracts heat from the outside air and sends it to the inside.  In the cooling cycle, it takes the heat from the air inside the home and sends it to the outside.  To perform this function, it uses a refrigerant that is constantly expanding and contracting. 

In the heat cycle, the cycle starts with the refrigerant in the heat pump passing through the heat exchanger and absorbs heat from the outside air.  The refrigerant evaporates as it is heated into a gas.  The gas refrigerant is put through a compressor and is pressurized until its temperature is over 180 degrees.  The hot gas passes through a refrigerant to air heat exchanger where the heat is extracted and sent through the heat ducts.  The refrigerant changes to a liquid as it loses heat and then cools as it passes through an expansion valve to start over. (Ground Loop). 

In the cooling cycle, the process is reversed.  The refrigerant is leaving the house as a warm gas.  It is compressed and gives off more heat outside the house.  The fan in the pump cools the heat coming off of the compressor as it compresses the refrigerant into a cool liquid and sends it into the house.  It passes through an expansion device and cools rapidly before entering the condenser.  The hot air from the house is blown across it.  The air cools as it passes over the condenser.  The heat is drawn from the inside air and begins to heat the refrigerant as it exits the house as a gas.  The cycle begins again.

Now that we understand how the heat pump works, let's discuss the in-efficiencies of the Air Source system.  In the winter, the heat pump is extracting heat from the outside air to aid in heating the refrigerant and changing it into a gas.  The colder that the outside air is, the less heat is available to aid with this process.  In the summer, the heat pump is trying to release heat from the compressor and cool the refrigerant to condense it into a liquid.  In the summer, however, the outside air temperature can be extremely high which forces the heat pump to work much harder to extract the heat from the system.  Even though a heat pump is a more efficient system than a standard split system, it still has several efficiency hurdles to overcome as it does its job.

This brings us to the Ground Source Heat Pump solution to the problem.  The ground source system circulates a liquid through the ground so that it takes on the temperature of the soil. This temperature is a fairly constant 50 to 55 degrees for the Midwest.  This liquid is used to perform the same work as the air in the Air Source system.  In the winter the heat pump is extracting heat from 55 degree liquid instead of battling with much colder air.  This warmer liquid will be much more effective in changing the refrigerant into a gas in the winter and producing the heat cycle.  In the summer the liquid from the Ground Source is much more effective in cooling the refrigerant in the system than the hot outside air. 

Now that we understand the efficiency of the Ground Source liquid cooling, let's discuss how it gets to the heat pump.

There are 3 basic types of closed loop systems.  They are as follows:

  • Horizontal Loop systems are used in areas where land is plentiful and soil may be shallow.  The system of pipes is placed in trenches that are 4 to 6 feet deep.  1 to 6 pipes may be placed in each trench.  Trenches can be 100 to 400 feet system ton.  The trenches are placed 6 to 10 feet apart.  The overall land area required can range from 750 to 1500 square feet per system ton.
  • Vertical Loop system is used when land area is scarce.  Drilling equipment is used to bore small diameter vertical wells.  The pipes are lowered into each well to produce a U-shape at the bottom and form a continuous flow down and up.  Hole depth can vary from 100 to 300 feet per system ton.  The holes are separated 10 to 15 feet.  The land area required per ton is only 100 to 200 square feet.
  • Lake Loop systems are very economical when bodies of water are readily available.   The coils of pipe are laid on the bottom of the lake and in most cases, ¼ to ½ acre of water surface at a depth of 8 to 10 feet will accommodate a typical residence.

Installation of the loop system is one of the most expensive parts of the Ground Source system. In either the Horizontal or Vertical loop system, excavation or drilling equipment is needed. Each well in the vertical system can cost as much as $2500.  One well is installed for each ton of cooling.  This expense plus the increased cost of the Ground Source equipment can cause the cost of a Geo-Thermal system to be 30% to 100% times the cost of a high efficiency system.  A great source for cost comparisons is a website called (HVAC- Op Cost).  This site allows you to input the efficiency of the system that you are contemplating and to see the cost return on the system.  You may need someone familiar with the mechanical systems to help you with all of the numbers to input.

If the cost of a Geo-Thermal system is that expensive, will it pay to install?  The payback is largely dependent on the climate and the demands of heating and cooling.  The payback can be as quick as 5 years and can extend to 7 or 8 years (EcoHome). 

So Geo-Thermal is not for the faint of heart.  It takes a commitment for the long haul.  If you are willing to make the investment, you will have a system that runs quietly and efficiently for many years and will certainly increase its value as energy costs continue to rise.

Much of the information for this article was supplied by my local Heating and Cooling subcontractor.  (Vogel Heating & Cooling) has been installing quality mechanical systems in the St. Louis area for over 60 years.  They are a (Carrier) dealer and have always provided me with quality equipment at a fair price.

 

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