Tom and Susan Rust, who live in the only solar-heated new house in Old Town Alexandria, are young enough not to be disillusioned by the problems of building a unique dwelling. In their early 30s, they are old enough and now experienced enough to know they will be able to do it more effectively next time.

Tom Rust heads a small construction firm started by his grandfather. He and Susan have been married nine years and they have two young children. The Rusts said they had been talking for more than five years about building a solar-heated house.

"Before we started, I talked with people who had built solar homes, attended a bunch of seminars and read a lot," Rust recalled the other day.

But doing a "special" house was not easy, even for a builder who has a brother who is an architect. (Brother John designed the house.)

"In the face of feul shortages and rising fuel and utility costs, our solar house was built to demonstrate the practicality of using solar energy in a Northern Virginia house," Tom Rust says.

"This house integrates a unique floor plan and many modern features with an architectural design compatible (on the exterior) with the colonial theme prevalent in Old Town Alexandria. In outward appearance, the house is similar to a flounder house (with one sharp, angled side), a design popular in 18th century Alexandria. In addition, we incorporated advanced concepts of using active and passive solar energy into an energy-conserving house."

One of the truly unusual features of the house is the three-story atrium on the south side. The interior of that atrium is finished in regular outdoor plywood siding. The atrium area, which has an insulated glassed area facing south, is literally walled off from the rest of the house.

"Our heating system works by thermal converstion," said Rust. "The sun's infrared light is absorbed by blackened metal plates (on the roof) which conduct the heat energy to copper tubes which are in physical contact with the plates. The metal plates, or conductors, are located on the south-facing roof and are slanted to get the maximum energy from the sun. A flow of water absorbs the heat from the plates and is pumped to a storage facilitiy - in this case, a 1,000-gallon, insulated tank on the second floor.

"The heated water in the tank is then pumped through air flow coils which have attached warm air ducts to disseminate heat, when needed, throughout the house. Also, liquid flow coils also provide heat for domestic hot water use. Typically, the liquid from the collectors ranges from 140 to 200 degrees Fahrenheit. Enough energy can be stored to provide space heating for three days. The efficiency in this system, as in any such system, depends on the quality of design, workmanship and materials used."

In addition, the atrium is designed to provide light and heat for the house. When the winter sun heats the air in the atrium to 80 degrees, Rush said, automatic dampers open, converting the area into a return air duct for the main heating system. In effect, that circulates warm air throughout the House. During the summer, a blower at the top of the atrium will draw warm air out of that central three-story are and from other parts of the house and move it outside - on th principle of an attic fan.

The Rusts moved into their house last November but did not realy complete it until February of this year. They figure that the house itself cost about $108,000, of which about $18,000 was for the solar system. In addition, they paid $22,000 for the 24-by-128-foot lot of 615 S. Royal St.

The site had been previously undeveloped due to a high level of underground water. As a result, the house has no basement. Rust and his crew emplaced 24 30-foot pilings on which 85 yards of concrete and 30 tons of steel rods were used for the foundation. Special supports also had to be built into the structure of the house to support the big hot water tank of the second floor.

"The house was designed to fit around the heating system," said Rust, who with his wife supplied the general plan to his brother. John Rust's drawings had to be approved by the architectural review committee of Old Town Alexandria.

Visitors to the Rust home get a slight view of the flounder roof on the south wide and aslo see an attractive brick-exterior in the colonial idiom. An attractive brick storage shed is part of the bricked court in front of the entrance.

An entry foyer to the right acts as an air lock to prevent heat and cooling loss. Another inside door leads to the open, no-cupboard, no-cabinet kitchen, where pans hang from one unfinished wall. The kitchen also has a fireplace with a coil to heat hot water. "We use it a lot," Susan Rust said.

Food items are stored in an open pantry area, where the refrigerator is located. The kitchen also has a dining area. In the rear part of the first floor, separated from the kitchen by the big atrium, is the comfortable living room with a fireplace at chest level height in the black wall.A patio and garden area are in back of the house. There's a rear entry with air lock, similar to the front entrance.

On the second floor, reached by a winding star stairway with a well-placed grab post, are two bedrooms and the mechanical-tank room. On the third level are two bedrooms for the children, with skylights on the ceilings.

Besides the air-lock entry compartments on the first level, the Rust house has other energy-saving features.

The roof has 9-inch fiberglass insulation with an R-30 rating and the exterior walls have 2-inch urethane and urea-formaldehyde insulation with R-14 and R-22.5 ratings. The exterior walls are seamless and windows and doors have interior window shutters.

The Rusts went through a laborious process to get the design of their solar system but wound up with the Barber-Coleman firm's Maryland shop doing the control system design. Sunworks of Connecticut supplied the roof collectors and the Rusts plan to add more collectors. The water tank was bought from the J.E. Hurley firm in Virginia.

"The monitoring system for this installation is highly important," Tom Rust said, "and we had a friend, James Slinkman, design our system to learn how wfficent the operation is and to learn how we can make it more so."

Before they moved into the new solar-atrium house, the Rusts lived in an older, redone house in Old Town. Rust Construction also has other houses under way. "We'll probably live her for a few years and then build another," said Tom Rust, after talking for an hour about the home just completed.

Although their house has 3,000 square feet of living area, they agree that the smaller the house the more practical it is for solar heating. They recognize that a completely solar-heated house, with only electric service and an electrical heat backup for sunless periods, is still a fairly expensive exercise. But they insist thay installation of solar heating for domestichot water is now practical for any new house.