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Heat Pumps – Electric – Water/Ground Source

Water source is just one name for heat pumps that are also known as geothermal or ground source heat pumps. The multiple names reflect the heat pump’s ability to extract heat from the constant temperature of the ground or a body of water just a few feet below the earth’s surface. Depending on latitude, ground temperatures range from 45°F to 75°F, warmer than cold outdoor winter air and cooler than hot summer air.

How It Works
Ground or water source heat pump systems operate similarly to air source heat pumps and fit into one of two categories: closed loop systems or open loop systems. Closed loop systems, in a horizontal, vertical or pond/lake configuration, use water or an antifreeze solution circulating through plastic pipes below the earth’s surface to collect and carry heat into the building. In cooling mode, the system reverses itself by pulling and carrying heat from the building and depositing it into the ground or water. Open loop systems operate similarly, but the water that circulates through the system comes from a well or surface body of water.

Horizontal Closed Loop System
This system is generally most cost-effective for residential installation, particularly new construction with sufficient available land. Two pipes are buried in trenches—either one at a depth of six feet and the second at four feet or both pipes placed side-by-side at a depth of five feet in a two-foot wide trench. Looping the pipe allows for a shorter trench.

Vertical Closed Loop System
Used in large commercial buildings with less available land for the system, where the soil is too shallow for trenching, or to minimize disruption to landscaping. Four-inch diameter holes are drilled at 20-foot intervals to a depth of 100-400 feet. Two pipes, connected at the bottom with a U-bend are inserted into the holes and connected at the top with horizontal pipe that connects to the heat pump.

Pond/Lake Closed Loop System
The pipe is run underground to the pond or lake and coiled into circles at least eight feet under the water’s surface. Water in the lake or pond must meet minimum volume, depth and quality criteria.

Open Loop System
This system uses water from a well or body of water as the heat exchange fluid. The water circulates through the system and is then discharged to the ground via the well, a recharge well or to the surface. Water used in the system must be relatively clean and local codes and regulations regarding groundwater discharge apply.

Efficiency
The chart below shows the DOE recommended efficiencies for commercial water source heat pumps.

Product Type and Size

Recommended Levela

Best Availableb

Water Sourcec
65,000 – 135,000 Btu/h

12.8 EER or more
4.5 COP or more

14.5 EER
5.0 COP

aEfficiency levels for air-source units sized between 65 and 240 MBtu/h meet ASHRAE 90.1 minimum efficiency requirements.
bThe best available EER and best available COP apply to different models.
cWater source heat pumps covered here use cooling towers and boilers as the heat transfer sink or source in a closed loop piping system. This may increase boiler energy use by lowering the return water temperature. Auxiliary pumping energy is not included in the water source heat pump efficiency rating.

Source: U.S. Department of Energy, Federal Energy Management Program, How to Buy an Energy-Efficient Commercial Heat Pump