Energy upgrades for existing homes often focus on the demand side, and involve major modifications to the building envelope, such as improving the air-tight envelope, boosting insulation levels, and replacing windows and doors. But the supply side can be just as important — particularly in situations where upgrades to the envelope would be costly and complicated.

Coastal Connection is keeping tabs on just such a job in the town of Rockport, Massachusetts, on the Atlantic shore north of Boston. Treehouse Design, a design-build firm with decades of experience in the area, is handling the remodel of an oceanfront house that has just been purchased by new owners.

Company founder Tim Thurman says that the existing building, constructed in 1987, places difficult constraints on any effort to improve insulation or air-tightness. The footprint is complicated, with a multitude of boxy room sections, complex intersecting hip roofs, and cathedral ceilings. Adding insulation would require demolishing and replacing interior finishes that are still in good shape — and the intricate shapes involved would make that an expensive proposition. "We could do it," says Thurman, "but it would be so expensive that it would almost be cheaper to tear down the house and start over." Instead of that complex and risky endeavor, the new buyers have opted to install a more efficient heating and cooling system — in this case, a ground source heat pump (GSHP) with three 400-foot geothermal wells.

In two previous geothermal projects, says Thurman, he has figured out a way to hit a cost-effectiveness "sweet spot." Massachusetts has a heating-dominated climate, where houses need more energy for heat in the winter than for cooling in the summer. But Thurman has chosen to size his geothermal systems for the smaller cooling load, not the larger heating load. He makes up the difference with a backup oil boiler and indirect water heater — which also supplies ample domestic hot water for the home (sometimes an issue with geothermal systems). The result is an HVAC system that cools effectively all summer long, and meets the home's heating needs for all but a small number of winter days (when the standby boiler kicks in to make up the difference). The savings on the up-front cost of the geothermal ground loop (the heating system's biggest cost factor) is significant, says Thurman: "Basically, with the boiler as backup, we only have to drill two wells instead of three."

For this job, however, the new owners are adamant that the house should not rely on fossil fuels at all (ironically, they are engineers working in the coal and oil industries themselves). So Thurman has sized the geothermal ground loop to supply all the home's heat. An electric on-demand water heater will supply domestic hot water. This means forgoing the up-front cost advantage of the hybrid geothermal and propane setup Thurman has used in the past. But he says that in terms of the owners' wallets, this cost issue may be a wash. Current tax law provides a tax credit of 30 percent of the up-front cost of a geothermal heat pump system. "That tax credit is very useful to clients with high earnings," Thurman notes.

Coastal Connection will be following Thurman's job in coming weeks. For a look at the drilling of the three wells for the ground loop, see the slideshow.