November 1, 2012 – March 31, 2013
This case study summarizes the operating costs for five months of geothermal heat pump operation for a 3,200 square foot home in central Connecticut during the winter of 2012–2013. The home was built in 1995 and is well insulated, with R29 walls, R48 ceilings, and careful sealing around windows and doors.
Figure 1. 3,200 sq. ft. home in Central Connecticut with geothermal heating and cooling system.
The geothermal heat pump system consists of two Florida Heat Pump model AP035 water-to-air heat pumps (three tons nominal capacity), each serving one floor of living space. The AP035 heat pumps were new in December 2011, replacing two older units of similar capacity. A third, homemade three-ton nominal water-to-water heat pump makes warm water that provides some radiant floor heating in one room and provides some preheating of domestic hot water.
Figure 2. Geothermal heat pumps located in the basement.
Two closed loop vertical geoexchange wells, each 500 feet deep, serve the heat pump system. The loop pipes are joined together by a header inside the basement, so that each heat pump has full use of both wells. A 20% methanol in water solution is used as the geoexchange fluid. A pair of Taco model 0011 circulator pumps, piped in series, serves each of the two-stage heat pumps. When a heat pump operates in part load, only one circulator pump operates.
Figure 3. Geothermal lines entering the basement, coming together into a header.
Electricity consumed by the heat pumps has been metered for the past 5 months by an E-Mon D-Mon model 208100 kilowatt hour meter. Its accuracy is reported by the manufacturer to be 1% of the measured load. Heat extracted from the earth is metered using a GxTracker system, provided and monitored on-line by Ground Energy Support, of Dover New Hampshire. That metering system consists of two temperature sensors that measure the temperatures of geoexchange fluid entering and leaving the loop field, and a flow sensor. Accuracy of the GxTracker system is estimated at 5% for the temperature sensors, and 5% for the flow sensor, for a total of 10%. Data collected by this system may be viewed in real time at the Ground Energy Support website.
The heat purchased was measured by the E-Mon D-Mon meter and compiled to determine the kilowatt-hours consumed by the geothermal systems. Then, using the electric rate calculated from each monthly electric bill, and the kilowatt-hours measured, the monthly operating cost was calculated as shown in the chart below. The total geothermal operating costs for the 2012-2013 heating season was $770.00.
Chart 1. Monthly geothermal heating bill – Winter 2012-2013
With the data generated by the metering systems, it was possible to calculate the amount of heat (BTUs) extracted from the ground and delivered to the home for heating. Over the 5 month metering time, 17.2 million BTUs were purchased and 56.4 million BTUs were delivered. Using that data, it is possible to compare operating costs for other types of heating systems. The table below summarizes the operating costs for four different heating systems delivering 56.4 million BTUs of heat. It should be noted that the values shown do not account for standby losses, so they are conservative in favor of the fossil-fueled systems.
|Heat Source||Efficiency||Unit price||Operating Cost|
|No. 2 Fuel Oil||82% (rated)||$4.00/gallon||$2,055|
|Natural Gas||95% (rated)||$1.40/ccf||$942|
Resources used for calculating operating cost:
- Homeowner provided electricity and natural gas bills
This case study shows that the geothermal system heated the 3,200 square foot home for substantially less than fossil fuel heating systems – even when compared to the most efficient natural gas-fired system currently available. The operating cost for fuel oil and propane would have been more than double the cost for geothermal.
To learn more about this geothermal system, please contact:
HydroDynamic Engineering, LLC
547 Shuttle Meadow Road
Southington, CT 06489
Phone: (860) 628-4622