This page shows the actual performance of our geothermal system since it went online on July 16, 2010, and it attempts to quantify the improvement over our previous air conditioner and furnace during cooling season and heating season.

Cooling Performance

This table shows some recent data for our previous air conditioner for comparison with the performance of the new geothermal cooling system.

• This data is based on our utility bills, which arrive in the middle of each month, so "May-Oct" means "the six-month period from mid-May to mid-October", which are the cooling months here.
• Electricity used is the total of our electric bills for the time period shown minus about 300 kWh per month (which is the average amount of electricity our home would consume without heating or cooling). Note: These data exclude one month in 2009 in which we turned off our AC while we were out of town for 2 weeks, so our usage was abnormally low. Note: I installed extra meters in Dec 2011 to allow us to measure exactly how much of our monthly usage was used for heating/cooling, instead of just estimating "total-300kWh".
• Energy used is the electricity converted to megajoules. Note: 1 kWh = 3.6 MJ.
• CDDs is the number of Cooling Degree Days for the same time period minus about 10 CDD per month (our AC doesn't run unless it's at least this hot). This is computed by my utility company by comparing the average daily temperature against 65°F, which is a little misleading, because we set our thermostat to 78°F during cooling season. So, if the average outside temperature is 70°F one day, We Energies will add 5 CDDs to the total for the month, but we won't consume any energy for cooling. Note: Our utility bills may show a non-zero number of CDDs outside the May-Oct time period, but these CDDs are unrealistic.
• Performance is the ratio of energy to CDD, which is an attempt to disregard the seasonal temperature variation and isolate the overall performance of our cooling system. A smaller number is better, because it indicates we used less energy for the same cooling.
• The average performance of our old air conditioner from 2008 to mid-2010 was 4.4 MJ/CDD.
• Savings is the change in energy consumption (based on the 4.4 performance value) for the same number of CDDs, where a negative number would indicate the new geothermal system used more energy than the previous air conditioner.
• Using the initial geothermal cooling data to compare with our previous air conditioner suggests this new system uses 35% less energy. Savings are greater during hotter summers, when a backyard heat exchanger has to work much harder to push the heat out of a house.
• Using geothermal cooling has saved us a total of 1,247 kWh of electricity (4,490 MJ of energy) in its first 3.5 cooling seasons of operation.

Heating Performance

This table shows some recent data for our previous gas furnace and compares with the first data gathered from the geothermal system. We're trying different optimization adjustments, so the results are still preliminary.

• This data is based on our utility bills, which arrive in the middle of each month, so "Oct-May" means "the seven-month period from mid-October to mid-May", which are the heating months here.
• The old furnace burned natural gas to generate heat and consumed electricity to blow it around the house.
• The geothermal system will use electricity to pull heat out of the ground and to blow it around the house. However, this is only efficient for mildly cold weather, so when the outside temperature drops below a certain point, the geothermal system shuts down and changes over to conventional heating, which means the new furnace burns natural gas to generate heat and consumes electricity to blow it around the house (same as the old furnace).
  Note: The changeover point was set to 10°F initially, but I found this was too low, because the amount of gas saved was not enough to justify the much larger amount of electricity used. So, I changed it to 20°F in Dec'10, changed it to 30°F in Feb'11, and changed it to 40°F in Jan'12. However, then I found it appeared the numbers had swung too far the other way, so I changed it to 35°F in May'13.
• Electricity used is the total of our electric bills for the time period shown minus about 300 kWh/month (which is the average amount of electricity our home would consume without heating or cooling).
  Note: I installed extra meters in Dec'11 to allow us to measure exactly how much of our monthly usage was used for heating/cooling, instead of just estimating "total-300kWh". The actual number has averaged out to about 330kWh as of Aug'13.
• Gas used is the total of our gas bills for the time period shown minus about 11 therms/month (which is the average amount of gas our home would consume without heating or cooling).
• Total Energy used is the sum of the previous two numbers, converted to megajoules.
  Note: 1 kWh = 3.6 MJ, 1 therm = 105.5 MJ.
• HDDs is the number of Heating Degree Days for the same time period minus about 280 HDD (our furnace doesn't run unless it's at least this cold). This is computed by my utility company by comparing the average daily temperature against 65°F. So, if the average temperature is 60°F one day, We Energies will add 5 HDDs to the total for the month. This is a useful number for us, because we set our thermostat to 68°F during the day and 65°F during the night in heating season.
• Performance is the ratio of energy to HDD, which is an attempt to disregard the seasonal temperature variation and isolate the overall performance of our heating system. A smaller number is better, because it indicates we used less energy for the same heating.
• In Autumn 08 we had some insulating and air sealing work done on our house. This apparently resulted in a 16% improvement in our heating performance. Ignoring the data before that improvement and using only the most recent two years of data (Oct'08-May'09 and Oct'09-May'10), our old gas furnace had an overall performance of 12.4 MJ/HDD.
• Savings is the change in energy consumption (based on the amount of energy we would have consumed with the previous furnace for the same number of HDDs, calculated in a separate spreadsheet), where a negative number indicates the new geothermal system used more energy than the previous gas furnace.
• We only have a few seasons of heating data for the geothermal system so far, but it shows the gas usage dropped, the electricity usage rose, and the total energy used for the heating season dropped. This was a net savings of energy, as we expected. Initially, the geothermal system appeared to be 35% more efficient, but it was shifting too far away from gas heating during the first winter (electricity costs were too high compared to gas savings). During the second and third winters we adjusted the changeover point (so we rely more on gas heating and less on geothermal in the coldest part of the winter), resulting in efficiency improvements of 20% the second winter but 0% the third winter. This indicates we raised the changeover point too far, so we bumped it down to 35°F for winter '13-'14. This resulted in the predicted improvement, but it was an atypically cold winter, so the data may not be fully representative. We are considering 32°F changeover next winter.
• Using geothermal heating has saved us a total of 30,251 MJ of energy in its first 4 heating seasons of operation.