A Case Study: Ground-Source Heat Pumps
Heating a 3,000-square-foot house for $1.25 a day
Originally published January 2008
By Fred L. Walls, Ph.D.
and Kay Turnbaugh
As his house was being built in Lafayette, Colorado, Fred Walls installed the loop field for a ground-based heat pump (GBHP) five feet under the basement. His research on GBHPs indicated they were the most energy-efficient means of heating and cooling, so he asked the developer of his neighborhood to dig a little deeper and allow him to install the necessary pipe work before pouring the foundation for his new house.
Walls, a physicist and Fellow of the American Physical Society and IEEE, documented his energy savings during the first year he and his wife lived in their new house.
During the first year his GBHP was on line, 2007, the energy usage for heating his home was only 21% of comparable homes with gas heat. His total energy usage is less than 50% of comparable homes. His average heating costs in 2007 were $38 per month, which included 100% wind adjustment for electricity.
Heating/cooling consumes roughly 45% to 70% of the energy in a typical home. GBHPs have efficiencies of 350% to 500% compared to a fuel-based furnace, which is typically 65% to 92% efficient. “Replacing an old 65% gas furnace with a new GBHP would reduce energy consumption for heating by an astounding 85%,” Walls says.
“GBHPs also provide very efficient summer cooling and can provide pre-heat for the hot water.”
Walls also points out that the gas that is saved by using a ground-source heat pump (GHP) can then be used to displace coal used in the generation of electricity. “This is important because gas generators produce 45% less CO2 and much less heavy metal pollutants than coal, and electrical output of gas generators can easily be controlled to accommodate changes in load and/or generating capacity of intermittent renewables such as wind or sunlight.”
“It’s a value-added system,” Walls says. “Investments in selected energy conservation projects show a savings in utility costs of roughly 5% of investment. The expected increase in energy costs should create an additional return of roughly 3% to 10% per year due to appreciation in the value of the energy conserving projects. Total rates of return could easily approach 8% to 15%. These factors make it feasible, in most cases, to reduce energy consumption, reduce the average monthly cost of ownership, and at the same time increase the value of your home.” Walls expects the value of his house to increase faster than the rate of inflation because of its energy-saving GHP system.
According to the National Renewable Energy Laboratory (NREL) and Colorado’s Xcel Energy, geothermal heat pumps, also known as ground-source heat pumps or GeoExchange systems, are the most energy efficient way to heat and cool a home and provide hot water. The Environmental Protection Agency (EPA) says that GeoExchange systems are the most energy-efficient, environmentally clean, and cost-effective space conditioning systems available.
Ground-source heat pump systems typically show a savings in utility costs of roughly 5% and add value to the home of about $20 for every $1 reduction in yearly home energy costs. (See The Appraisal Journal; “More Evidence of Rational Market Valuation for home Energy Efficienty,” October 1999.)
There are federal tax rebates for some energy conservation projects, including ground-based heat pump systems, and many utility companies offer large rebates for the installation of a GHP system. More than 1 million GHP systems have been installed in the United States, including over 1,000 at colleges and universities, according to NREL.
HOW IT WORKS
A GHP system moves the heat from the earth, or a groundwater source, into the home through a heat pump/exchanger in winter and pulls the heat out of the house and discharges it into the ground in the summer. Underground piping loops serve as a heat source in the winter and a heat sink in the summer. A pump circulates temperature-sensitive fluid through the ground loop.
Walls’ system is installed under his house, which makes it more efficient than a system that is detached from the structure. Of course, if you’re retrofitting a system to an existing structure, the pipes have to be buried in the yard or a field. An alternative approach when there is not much space is to drill several wells approximately 200 feet deep in a narrow strip of land or beneath the driveway.
A few feet below the earth’s surface, the ground temperature remains at a relatively constant temperature. Depending on latitude, ground temperatures usually range from 45 degrees F to 75 degrees F (7 degrees C to 21 degrees C), even when temperatures outside can range from sub-zero in winter to scorching highs in summer. A GHP system can take advantage of this constant temperature by exchanging heat with the earth through a ground heat exchanger.
Walls has plotted the inside temperature and the outside temperature over the course of a year. As the first year of using his system progressed, it got more efficient. At left is a graph of Wall’s data for a day during the GBHP’s first month of operation.
COST AND SAVINGS
A geothermal heat pump system averages about $5,000 per ton of capacity. A three-ton unit, at a cost of roughly $15,000, works for a typical residence. Xcel Energy estimates that other systems would cost about $4,000 with air conditioning. “When the cost is included in the mortgage, the homeowner has a positive cash flow from the beginning. For example, say that the extra $11,000 will add roughly $95 per month to each mortgage payment. But the energy cost savings will easily exceed that added mortgage amount over the course of a year. The savings of roughly $800 to $1,000 a year in utility costs adds about $16,000 to $20,000 to the value of the house. Therefore, the homeowner can typically reduce total monthly costs, increase the value of the home, and reduce total energy consumption with its associated CO2 production.”
The graph below shows details on Walls’ utility bills and energy usage.
- Use of the ground-based heat pump reduced energy usage for heating by approximately 79% per square foot versus a conventional 80% efficient furnace.
- Total energy usage was reduced approximately 55% per square foot when compared to a conventional home.
- Cost savings were approximately $800 to $1,175 ($67/mo to $100/mo depending on rate structure). This is roughly $19/mo to $29/mo per ton of heating/cooling.
- Cost savings typically are enough to finance loan and still save on total monthly costs.
- The cost of the system added to the value of the house.
- The cost to install a system is very roughly $5000/ton.
- The size needed is roughly 1 ton to 1.5 ton per 1,000 square feet.
- Ground loop lifetime is 50 years guaranteed; expected lifetime is 200 years.
Above is a graph for Walls’ home in Lafayette, Colorado, for the 12 months from December 19, 2006, to December 18, 2007. Heating costs were about $1.25/day when all service charges, including 100% offset with wind energy, are included. Note that the heating energy consumption is only 21% of the comparison house when adjusted to the same square footage. Note that the comparison house energy usage was measured from January 1, 2007, to January 1, 2008. The Degree days are similar for December 6 (1028) and December 7 (1181) so the comparison should be valid to within 5% of December usage or a few dollars.
One of the reasons Walls’ system is so cost-effective is that he installed it under his house. He can’t allow the ground under the house to freeze, because then the house would heave, so he installed enough pipes in the ground to keep it from freezing. The pipes are buried five feet below the basement, which is 13 feet below ground level, and, on the advice of a friend in Alaska, Walls took the extra precaution of adding two inches of Styrofoam insulation between the pipes and the house.
The house itself is insulated with R38 blown-in cellulous insulation in the attic and R19 bats in the walls. The foundation walls are insulated to R11, and under the slab is R10. The attached two-car garage has R11 insulation in the walls, an insulated steel garage door, and an insulated ceiling. Windows are Amsco double-pane vinyl VLS low-E, U factor 0.24, solar gain 0.31.
Walls hired an EPA-certified inspector to evaluate his home. He gave it a 5 Star Plus rating, which certifies that expected energy usage was less than 50% of comparable homes. Based on this rating, above, Walls qualified for a $2,000 tax refund.
Xcel Energy says that geothermal heat pump systems have fewer maintenance requirements than most other systems. The underground components are virtually worry free. Walls’ says the only maintenance he has to do for his system is to clean the filter every couple of months. Unlike regular furnace filters, his GBHP filter can be washed.
Walls and his wife have not had to change the way they live to accommodate the geothermal heating system. “We can save 70% to 80% on gas consumption and not change our lifestyle,” Walls says. They maintain their house at 73 degrees F in the winter and 76 degrees F in the summer.
When they leave the house for a few days or for a vacation, they enable the back-up electric strip heater within the geothermal system in the remote chance there is a failure in the loop field or compressor.
Some geothermal systems use radiant floor heating rather than a forced air system. The Walls’ master bathroom floor is heated in the winter by electrical radiant heat.
Walls’ data shows that the payback for a geothermal heat pump system is approximately twice as fast as a photovoltaic system, even with the rebates usually associated with a PV system. In 2007, the cost for heating Walls’ house was $456.
View a pdf (Walls-GBHP-casestudy) (49k) of the case study.
Geothermal Heat Pump Consortium (includes information about incentives by state)
DOE web site with residential energy consumption data