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Heat naturally moves from warmer places to cooler places. Heat pumps use electricity to move heat from cooler places to warmer places, making the cool space cooler and the warm space warmer. Your refrigerator is a heat pump, moving heat from inside the refrigerated cabinet into your kitchen. An air conditioner is a heat pump, moving heat from inside your home to the hot outside summer air.
Heat pumps are also used to heat homes and businesses. While air-source heat pumps extract heat from the outdoor air, a ground-source heat pump (GSHP) takes advantage of the fact that a few feet below the earth's surface the ground remains at a relatively constant temperature. Depending on latitude, ground temperatures range from 45°F to 75°F. Like a cave, this ground temperature is warmer than the air above it during the winter and cooler than the air in the summer. This leads to the exceptionally high efficiency of GSHPs.
GSHP systems consist of three parts: the ground-loop heat exchanger, the heat pump unit, and the air or water (hydronic) delivery system (ductwork or piping throughout a home or building). The ground-loop heat exchanger is a system of tubes called a loop, which is buried in the ground near the building. A fluid (usually a mixture of water and environmentally friendly antifreeze) circulates through the tubing to absorb or relinquish heat within the ground.
In the winter, the heat pump unit extracts heat from the heat exchanger and pumps it into the indoor air or hot water delivery system, moving heat from the ground to the building's interior. In the summer, the process is reversed, and the heat pump moves heat from the indoor air into the heat exchanger, effectively moving the heat from indoors into the ground.
There are different types of GSHPs, based on variations in the ground loop and indoor unit configuration.
Ground-loop heat exchanger. The ground loop is the heat exchange surface for your heat pump. A home will typically need several hundred to thousand feet of piping in the ground loop to provide enough heat in the coldest part of the year. Ground loops are divided into “closed” and “open” loops:
Some closed-loop systems are installed in horizontal configurations, in which the ground loop is run through multiple trenches approximately 5 ft. deep and around 300-400 feet long (see below).
2. Open-loop systems can be used where there is a source of groundwater available on the property from a well. Instead of circulating an antifreeze mixture, an open-loop system pumps groundwater into the indoor unit for heat extraction before returning the water to the ground. Open-loop systems are typically cheaper to install and more efficient (due to the more consistent year-round temperature of groundwater), though they require a readily-available source of clean groundwater and may require additional permitting or environmental review.
The configuration of the indoor unit will also vary depending on whether you use ductwork (forced air) or a water (hydronic) distribution system:
There are numerous benefits to using GSHPs:
While GSHPs can be a great fit for many homes and businesses, there are some potential drawbacks:
GSHP retrofits can work in most homes. If you answer “Yes” to any of the questions below, a ground-source heat pump may be a good fit for you:
Ground-source heat pumps are considered to be clean heating and cooling systems because they do not create heat, but rather they move existing heat from the ground into your building. This process is powered by electricity, which one can purchase or generate from carbon-free sources like solar, wind, or hydro. Electricity supplied by the Concord Municipal Light Plant was 54% carbon-free in 2018. That percentage will be even higher in 2019!
Yes. Ground-source systems can be installed with desuperheaters that can provide you with about half of a typical home’s annual hot-water needs. A ground-source heat pump operating in cooling mode will typically store unwanted heat in the ground. A desuperheater will use that waste heat to pre-heat your hot water before it enters your hot water tank. A desuperheater add-on will cost around $1,800 but will cost virtually nothing extra to operate.
Ground-source systems require relatively little maintenance. The ground loop is designed to last for up to 50 years or more, and no other components are exposed to the elements. Periodic checkups and filter changes are the most common maintenance requirements. Some adjustments to the system’s performance can be done remotely if a monitoring system is installed, without your GSHP contractor needing to come out to your home or business.
The ground loop piping is designed to last for up to 50 years or more. The indoor heat-pump unit has a life expectancy of around 20 years, similar to conventional heating and cooling systems. Some pumps, controls, or other components may require replacement sooner than the indoor unit.
A ground-source installation will typically take 2-3 months to complete from when you sign a contract. This includes:
While horizontal ground-loop installations require more space, a vertical ground-loop system may require as little space as your driveway. A qualified installer will generally offer to assess your lot at no charge.
The cost of a GSHP system will range due to the customization needed for your home. For Concord homes, most ground-source systems installed have ranged from $31,000 to $36,000 after all rebates, state incentives and federal tax credits.
Depending on these factors—and the value of the AECs at quarterly auctions, you could receive a total of $3,500 to $11,000+ over the lifetime of a GSHP system. Owners of GSHP systems may choose to receive 10 years-worth of AECs upfront (“pre-minting”) rather than on a quarterly basis, effectively turning the APS into an upfront rebate program. If this option is pursued, no additional AECs will be issued after the upfront “pre-minting.”
An Excel calculator developed by the Dept. of Energy Resources is available here. Please note that a default AEC value of $20/MWh is assumed in this calculator. We would recommend using a more conservative estimate of $15/MWh or lower to represent a lower auction value of these AECs and an aggregator fee of about 7% or more.
Speak with your installer to learn more about the AEC’s and the incentive you could receive for your system.
The Concord Municipal Light Plant (CMLP) offers rebates of $625 per heating ton, up to $3,125, to its residential and small business customers in Concord when they install a GSHP that meets the program criteria. Residential GSHP installations in Concord are averaging 4 tons of heating capacity. Learn more here.
Note: CMLP customers are not eligible for Massachusetts Clean Energy Center or Mass Save rebates.
The Massachusetts Clean Energy Center maintains a list of ground-source heat pump installers that participate in their ground-source heat pump rebate program, which is available to electricity customers in communities served by Eversource and National Grid. CMLP customers are free to use installers that have participated in the MassCEC program.
The International Ground-Source Heat Pump Association and the New England Geothermal Professional Association maintain directories of GSHP professionals, including installers.
We suggest soliciting proposals from at least three ground-source heat-pump installers.
Keep in mind that for any type of home heating or cooling system to work well, it must be selected, sized, and installed properly. The low-cost installer may not be your best option.
Consider asking the following questions:
Will you determine my building’s heating and cooling design loads using ACCA Manual J for residential or Manual N for commercial, and size the equipment accordingly?
Qualifications and Experience
Can you provide documentation that you hold one of the following credentials?
Can you provide references from previous customers with similar systems in my area?
Are you familiar with the application process for CMLP’s ground-source heat pump rebate and are you willing to complete the process for me?
Contact Jan Aceti, the Concord Municipal Light Plant’s Energy Conservation Coordinator, at firstname.lastname@example.org or 978-318-3151.