BUT HOW GREEN ARE THEY?

All geothermal heatersrequire electricity. Heat pumps require even more. The "greenness" is determined by the efficiency of the overall design and the type of electricity going in. There is no shortage of strong opinions here.

Before designers even consider adding a geothermal system, it is critical to reduce building energy loads by upgrading the envelope and, if appropriate, HVAC ductwork. "The technology is fundamentally sound," explains Joe Lstiburek, principal of Building Science Corp. "But a lot of people are using them for really, really poor buildings." For every dollar spent on conservation-better windows, insulation, ductwork-two dollars can be saved on the size of the geothermal system. But as many buildings are designed inefficiently, geothermal has become the "greenie-weenie technology du jour," Lstiburek says.

Carbon is another factor. In areas where electricity is derived from coal, heat pumps will slow energy use but never stop emissions. "In the United States, we don't typically use electricity for heating," says Cornell's Joyce. "We haven't had as much of a need to conserve electricity, but now as the society tries to do heating without fossil fuels, that's pushing us toward heat pumps because you can get electricity from [cleaner] sources like hydro, wind, solar, and nuclear."

Dwyer adds that solar panels can be used to power the heat pump, lessening the home's dependence on the grid. He also says that the entire building team has to be "on board" to ensure that the system is integrated properly across various trades.

This brings us back to the question of whether or not geothermal is renewable. Again, no shortage of opinions. Lstiburek points out that the "geothermal" moniker is nothing more than smart marketing. Others think they "should be considered the same as solar energy," says Bererton. "It's exactly like solar power for heating. If you look at a COP of four for a heat pump in heating mode, one unit of electricity gives you four units of heat, so where did those three units of heat come from? They came from the sun, so you really are 75 percent solar."

But if we take into account that on average, grid electricity in the U.S. delivers a negative COP of three, this argument loses traction: "I'm 100 percent convinced that it should not be included as a renewable energy," Gauthier responds. "You are still using electricity to pump heat. You wouldn't call a window unit [ASHP] a renewable-energy technology, would you? All that makes GSHPs unique is that they are doing it more efficiently."

As we showed earlier in the Cornell case study, geothermal systems that use no refrigerant have much higher COPs because there is no need to run a compressor. The LEED rating system does not recognize geothermal heating as renewable, but many projects that earn the Energy and Atmosphere Optimize Energy Performance credit get a boost from their geothermal systems.

Like it or not, the debate over geothermal heating as a source of renewable energy remains highly political. After all, the Internal Revenue Service now provides special tax credits for renewables, including GHPs. The debate rages on, but experts unanimously agree: These systems are green so long as they are intelligently planned, executed, and maintained.