Exploring Energy Efficiency Using Geothermal Heat Pumps

By: Andrew Lake

A geothermal heat pump, like an air source heat pump, moves heat from one location to another. But unlike an air source unit, it exchanges heat with the ground instead of the outdoor air. This enables it to operate more efficiently, as the temperature of the earth more than 5 ft. below the surface is nearly constant all year round, being warmer in the winter when the heat pump is absorbing heat from it, and cooler than the air in the summer when heat must be released into it.

Geothermal heat pumps have a heating efficiency rating called the coefficient of performance (COP). The COP is the number of watts of heat energy the unit can move into the house for every watt of electricity consumed. New systems have COP's between 3 and 4.5. The unit can operate at this high efficiency level every day of the year.

A COP in the 4-4.5 rang equates into heating cost savings of 30-75%. The savings vary depending on your current systems efficiency (you can save more if you're upgrading from an older natural gas system or even more so if you currently use heating oil) and the electricity rates in your area (electricity is cheapest in the Northwest and most expensive in the Northeast.) Unlike with an air source heat pump, savings do not vary much depending on the winter climate.

Add air conditioning and save on water heating as well

• If you have a forced air heating system (furnace) a geothermal heat pump will provide you with air conditioning without the installation of any additional equipment. It will use only about half as much energy as a newer air conditioner.
• If you have radiant heating, the lines in your floor are unsuitable to circulate cool water, so special indoor fan units will have to be installed. If you insist on having these units installed in several rooms in your home, they can hugely increase the cost.
• If you have baseboard hot water heating, a geothermal unit is unsuitable for your home, as it cannot supply the higher water temperatures required by these systems.

A special unit called a desuperheater can be installed with a geothermal heat pump. It will use the heat pump to preheat the water up to 120F before it enters the hot water tank. This can cut your hot water costs by about half as well.

Types of geothermal heat pump installations

There are four ways to run the underground piping that is required for a geothermal heat pump: in a horizontal closed loop, a vertical closed loop, a pond loop, or an open loop setup.

Through one of these loop systems, the heat pump will acquire water which has been heated up (or, in the summer, cooled down) to ground temperature. In a closed loop setup, about 1000 feet of pipe must be buried to heat a typical 2000 square foot home. In a horizontal closed loop setup, the pipes will be buried about 5 feet below the ground. This will require a very large yard. For most homes, only a vertical loop system will fit in the yard. This involves drilling a series of wells about 200 feet deep to place the pipes in. Installation of a vertical loop system can cost up to twice as much as a horizontal loop system.

If you have a large pond on your property, you're in luck. Loops of pipe can be dropped into the pond, and this will be less expensive than a horizontal loop setup.

Open loop setups are now growing in popularity. These systems pump water from an aquifer (underground rocks that have water flowing through them) from one well and return it to the same aquifer using another well. If you have an aquifer with a high enough water flow rate not too deep beneath your property, then you may be able to drill only two wells to depths as low as 100 feet to meet your geothermal heating needs. This can be less expensive than other loop options, but even in areas with ideal aquifers, many contractors are just beginning to look into installing them.

Cost of geothermal

Geothermal heat pumps sound great when the energy savings and the benefit of adding air conditioning are described, but most homeowners will lose interest when the cost of the installation is brought up. These unit are priced similarly to new cars, so for a typical home it will take 20 years or more to earn back the initial investment. However, some homes may have payback periods of less then 10 years. These are usually larger homes in areas with lower electricity costs which are currently using oil or older natural gas systems. If the homeowners require a new air conditioner or boiler (a geothermal heat pump will still require a furnace for back-up heat in forced air systems), then this cost will also be saved.

Environmentally friendly

Next to automobiles, home heating is the top source of personal greenhouse gas emissions. The average furnace or boiler actually produces more ghg's than the average car. With a geothermal heat pump, you can cut these emissions down to a fraction of their former levels. Heat pump systems do not produce any emissions on their own, but emissions are generated by electricity production. If the electricity you use is being created by burning coal, then you might decrease your ghg emissions by about 50%. If the electricity is being produced cleanly by a method such as hydro, then your heating will be responsible for a very small amount of greenhouse gas emissions.

For this reason, your local government or utilities may be providing rebates for homeowners who go geothermal.

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