First, a solar collector; then a cozy nest inside

In this area of New England it is common for wood-heated homes to burn 6, 7, or even 8 cords of wood a year. But Jonathan and Marsha Nourse get shirt-sleeve comfort and do all their cooking on a mere 2 cords. Heating requirements alone probably would run to less than a cord.

If theirs were an oil-heated home, a single tankful of the expensive stuff would be more than adequate.

Such efficiency -- it makes some folk shake their heads in disbelief -- stems from the fact that the Nourses live in a passive solar collector. In other words, the collector doubles as a home. It's called a Trisol, because of its triangular shape and the fact that 75 percent of the heating comes from the sun. It is so efficient that even when the couple left on vacation for a week in the winter of 1979, the sun's rays alone never let the home drop below 59 degrees F.

That's the sort of economical performance that leads the young Massachusetts farmer to believe he lives in a home destined to be the wave of the future.

Fact: It was no more expensive to build than a conventional home. Indeed, at the area. The design is such that a more economical version can be built for $ 33 a square foot -- half that if the owner does the construction himself.

Fact: the passive solar structure requires no motors or other machinery to be effective -- machinery that needs no servicing or runs no risk of breaking down.

Fact: The massive reinforced walls (60 cubic yards of poured concrete) needed for adequate heat storage, along with the surrounding earth berm, make the basic structure almost indestructible.

"Given a direct hit in a nuclear attack," says the designer, Leandre Poisson of Harrisville, N. H., "all that would go is the [wooden] second floor." He's jesting, of course, but it makes the point. The banks love the concept; their mortgage money couldn't be safer.

Fact: The same conviction currents that heat the home in winter also cool it in the summer. While winter heating costs are minimal, summer air-conditioning costs are nonexistant. The Nourses say temperatures stay consistently 10 degrees cooler than the hottest outdoor temperatures in summer.

The Trisol is essentially an isoceles triangle with a 90-degree angle at the back and two 45-degree angles in front. The rear of the home points as close to due north as possible, so that the largely glaass-filled front wall faces the sun all day. There are no windows in the two side walls.

The roof rises slowly from the front to the rear point. This means that rising hot air streams to this rear high point in the winter, then is forced by a duct back down into the home, setting up a constant circulation of warm air. In the summer an open hatch allows the warm air to be vented out via a wind turbine. At the same time this venting draws outside air into the house through two underground pipes, which are buried in the always cool soil of the earthen berm.

Having the floor with the tile or slate overlay be thick concrete, as are the groundfloor walls (10 inches) and a kitchen wall immediately back of the greenhouse, provides the considerable thermal mass needed if a solar home is to prove effective. It does this by radiating back the stored heat from the winter sun and by soaking up unneeded heat from the air in summer.

In the Trisol, the concrete walls are insulated with polystyrene sheathing on the outside and then are backed up with a berm of soil. This does two things: The insulation prevents the loss of heat from the walls into the surrounding berm; while the berm surrounds the home with a cool but stable temperature (about 48 degrees F.) all year round.

Around Jan. 15 (when average temperatures reach their seasonal lows), the angled sun streams in to strike all thermal walls as it travels across the southern sky. This is possible because of the triangular shape. The rising sun in the east hits the west wall; the setting sun, the west wall. At midday all the walls receive the impact of the sun.

In fact, at this time of year nearly all of the sun's rays striking the home, except fo r the roof, do so where the radiant heat can be absorbed and stored.In summer, when too much warmth could be a problem, the sun is so high that it penetrates only a few feet into the home.

Heavy insulated curtains or shutters drawn across the front glass wall at night would improve the performance of the home still further. But, says Mr. Nourse, "the home performs so efficiently as it is that I don't feel the need to invest several thousand dollars in shutters."

On the face of it a triangular home seems odd. But this is less obvious inside than out. In fact, the house is a square that has been cut diagonally in two. Thus, many of the rooms and living spaces leading out from the conventionally angled rear walls are square or rectangular in shape.

In the Nourses' home, the foyer on the one front corner and a bathroom on the other are the only two obviously triangular rooms in the house.

The Nourses had begun to think of building a home about seven years ago, "when the first oil crunch hit home." This made them rethink the sort of house they wanted. They were ignorant of solar at the time but, in their own words, "We wanted a custom-designed home that would function in this region; that would work with little energy input."

Their search along these lines led them to architect Poisson of the Harrisville-based Solar Survival Inc. His response was to design a solar collector, which he then made habitable. The result was the Trisol, a design that had begun taking form in his though some years earlier.

Now, after two years in the house, the Nourses are convinced they have a winner.

"The whole technique is so basic it's beautiful," says Mr. Nourse, who believes that active solar systems will soon be obsolete. "Why go complicated and expensive, when passive does it all?," he asks with a logic that is hard to refute.

As Mr. Poisson sees it, ultimately "the only ally we have is the sun. Our future survival depends on being able to live with the sun." To this end the architect, a graduate of the Rhode Island School of Design, the royal College of Art -- London, and the London School of Economics, has been inventing solar devices of all types -- from simple cones and cold frames to promote out-of-season vegetable growth, to solar dryers and heaters, to houses. His interest in home food production prompted him to add a greenhouse to the front of the house, which also adds significant heat to the home in winter.

The Nourse home was his first Trisol. Since then four others have been erected or are nearing completion. Other custom designs are on the drawing boards. Mr. Poisson also encourages people to build their own homes. Fully half the cost is in labor, he says.

If you can build it yourself, why not?" People living in rocky terrain should build the walls of stone, he says. "Stone provides better thermal capacity than concrete." Concrete blocks are suitable, provided they are solid. "If you use hollow blocks, fill them with sand," he adds.

Plans are available for 1,200-square-foot units with 2 bedrooms, one bath, solar greenhouse, and root cellar.Write to Box 275, Harrisville, NH 03450.