Henry Gifford is a man who designs mechanical systems for very energy efficient, comfortable, and affordable apartment buildings in New York City, along with his partner, architect Chris Benedict. In a recent article in Fine Homebuilding, Henry explained how geothermal heat pumps work in a way that I will always remember. I paraphrase:

Dig a hole in the ground. Put some buckets of water in the hole. If you are deep enough below ground, the temperature of the water in the buckets, after a while, will be about 55⁰F. Take the bucket into your house and put it in your refrigerator. The fridge will cool the water down to say 50⁰F, and the heat produced in the coils behind the refrigerator will add some heat to your house. Voila! You’ve created a geothermal heat pump.

Notice that the heat produced is not free. It takes electricity to run the refrigerator. And if you don’t want to spend your days hauling water in buckets from the hole in the ground to your refrigerator, you’ll want to install a water pump, which uses more electricity.

The very best residential geothermal heat pump system, according to Henry, has a coefficient of performance (COP) of about 3. This means that for every 2 watts of energy the system pulls from the ground, you have to provide only 1 watt of electricity. You get 3 watts out for 1 watt in. But a typical system has a COP of about 2.

Given that electricity is produced at power plants that use fossil fuels, and depending on the mix of fuels your utility uses to produce electricity, you will probably burn more fossil fuels using a geothermal heat pump with a COP of 2 than you would using an efficient gas- or oil-fired furnace. And geothermal heat pumps are much more expensive to install than traditional furnaces.

At Home Energy Magazine, where I work, we always tell people that if you have your house air sealed, insulated, and provided with the right amount of ventilation to keep you healthy, you can do better with a medium-efficiency furnace than you would with a high-end system—ike a high efficiency gas furnace—and a leaky house. For most of us, that’s the best choice of all, for heating and for cooling.