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A massive drill rig sitting in The French Laundry's now obliterated courtyard is boring holes in the earth 260 feet deep — not to find truffles for some newfangled dish — but to install a geothermal air conditioning and heating system.

Now a month into what is expected to be a four-month renovation project, the once pristine courtyard is hardly recognizable — muddied with the mast of the giant rig jutting high in the sky.

When construction is finished, the geothermal system — technically known as a "Ground-Source Heat Pump System" — will be practically invisible, less noisy and far more energy efficient than most heating and air conditioning systems.

"All of the reasons to do it were compelling," said Thomas Keller, executive chef and owner. "You'd have to be blind not to see all the positive impacts of it."

Up until now, the kitchen has been without air conditioning, making it very uncomfortable for staff in the summertime, Keller said.

"What we're doing is putting a modern heating and cooling system in a historic building," said Paul Kelley, the architect directing all renovations.

The building that now houses The French Laundry was built sometime around 1900 out of basaltic fieldstone taken from surrounding hillsides.

It is significant not only for its craftsmanship, but also for "its reflection of the vernacular architecture carried from Europe by the ethnic groups which inhabited this part of the Napa Valley in the late 1800's and early 1900's: the French and the Italians" according to a Napa Landmarks Inc. report provided by Kelley.

One of the greatest benefits of the geothermal system is that it will never be seen or heard by anyone, and will just quietly perform its job, Kelley said.

Like the architecture of the building, which uses materials from the surrounding landscape, the geothermal system uses the temperature of the deep earth to heat and cool.

Essentially, the restaurant is installing a closed system of pipes filled with water that loops throughout the restaurant, its side buildings and 260 feet into the earth.

At that depth, the earth maintains a constant temperature of about 55 degrees year around, according to engineer Mike Keani. He explained that the water in the pipes will be 65 degrees when pumped up to surface level. It will be circulated in the pipe-loop to maintain comfortable temperatures inside the restaurant.

In the summer, when the air is hot, the cool temperatures of the deep earth will act like a heat sink, absorbing heat from inside the restaurant. In winter, the earth will act as a heat source providing a baseline heat of 65 degrees which radiators can use, according to Kelley.

"The thermal mass of the earth is considered to be so extreme," said Frank Borges Jr., the general contractor, "it's used for heat and as a cold sink. Whatever you have, it'll absorb away."

People living in the houses on every side of French Laundry will not be disturbed by the noise of an outside condenser, Keani said.

There also won't be any awkward-looking heating or cooling structures attached to an otherwise historic looking building, he added.

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"The California Preservation Society and the State Office of Historic Preservation have both expressed interest in this project as a historic project with a new energy efficient 'green' system that works in harmony with the historic nature of the site," said Kelley.

"The only reason why it's not used more often is it's so expensive initially — $3,000 a hole," Borges said.

But Keani said that the cost-benefit analysis makes geothermal systems worth it within several years. Although, it has been rarely used in Napa County, it is much more frequently found on the East Coast and middle America.

"It's about 50 percent more efficient than the average air-conditioning unit," Keani said.

Keani explained that the earth's temperature is actually the same at 260 feet below the surface as it is at just several feet. But in this case, the pipes, which have a diameter of about an inch, needed to go down 260 feet to put more surface area in contact with the temperatures of the deep earth.

One main purpose of the geothermal system will be to draw heat and air out of the kitchen, which needs constant circulation to avoid fire and other hazards.

As the renovation work continues, not only on the geothermal heating and cooling system, but also on new buildings — an office, wine cellar, employee changing room and restroom and expanded kitchen — The French Laundry looks less like a five-star restaurant and more like the site of a natural disaster.

"This is kind of the ugly duckling phase of the project," said Borges.

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