When Tom Rust says it took him two years to get the bugs out of this house, he isn't referring to cockroaches. He is talking about adjusting to living in a solar home.

Rust, president of Rust Construction Co. in Alexandria, lives in Old Town in an 18th century flounder house. His home was one of six buildings included on a recent tour of solar power systems sponsored by the Northern Virginia Chapter of the American Institute of Architects. b

Rust's family had to get used to an average temperature of 60 degrees. It meant taking fewer showers and becoming more aware of raising and lowering blinds, using lights, and maintaining the balance between the "active" and "passive" features of the solar power system.

But, Rust is quick to point out, "It doesn't have to be this way. I'm just a fanatic about it. Once you make the commitment to solar, you become more and more of a fanatic."

The focal point of the Rust's three-story masonry home is a glass-enclosed atrium. Its southern exposure admits light and provides ventilation and window space for the six windows that open onto it.

Rust said he could have chosen a house in a better location for solar power. "But I wanted to prove that I could build a solar home in the historic district," he explained.

Rust invested $20,000 in solar equipment, which includes 24 rooftop collector panels. He says he expects the investiment to be returned over the next 20 years.

Using a fireplace, a stove with a hotwater coil and Swiss blinds that can seal off sunlight, the alternative systems provide all the household's hot water and 60 percent of its heat.

Rust designed the house so the "nonliving" areas, such as closets and the room housing the 1,000-gallon water tank, would serve as a buffer to prevent loss on the cold northern wall.

One of the more striking solar systems shown on the tour was the Lindsay Cadilac building at Osage Street and Kenwood Avenue in Alexandria.

In 1977, the owners invested approximately $150,000 in solar equipment to heat a 40,000-square-foot addition to the building.

They expected the solar investment to pay for itself in 20 years. "But that was using the cost of oil in 1977," said Jack Lindsay, a partner in the firm. "Everybody knows what has happened to the cost of crude since then."

The company now foresees a return on its investment in half the time originally expected.

Architect Robert S. Long said the Lindsay building, a curving white, three-story structure near Fairlington, is in an ideal location for a solar system because of the site's southern exposure.

The building's convex rooftop collectors, which contain copper coils full of water and antifreeze, operate similarly to a car heater. Hot air is blown over the coils to produce heat.

The most elaborate example on the solar tour was the home of Randell Vosbeck, an architect with the design firm VVKR Inc.

Vosbeck's hillside house at 820 Rapidan Court is surrounded by trees. He describes it as a "three-zone" house, because each of its three living areas can be shut off from the rest by use of a complicated control panel in the basement.

The contemporary design of the house embraces both active and passive energy concepts.

The solar collectors -- active components -- are set at the optimum angle for the latitude of the Washington area; at 38 degrees, 51 minutes north.

The collectors provide 80 percent of the household's heat and 90 percent of its hot water.

One of the passive features is an automated louvre-awning system. The house is heavily insulated and its northern side, which receives little sunlight, has a minimum of glass. The fireplace has a ventilation system that passes air over the warm masonry. Another passive feature is the main entrance, a walk-in vestibule on the north side that can be shut off from the rest of the house.

The solar features of the house accounted for about $45,000 of its total $400,000 cost.

Vosbeck says his main concern just now is mantenance of the system, along with some "fine tuning to make sure that everything comes on when it is supposed to." He says no one really knows whether a solar system can last 20 years but, based on 1979 heating oil prices, he expects to recoup his investiment in 18 to 20 years.

Four of the systems on the tour, including its own headquarters at 210 S. Payne St. in Old Town Alexandria, were designed by Rust Construction Co.

The building, using many different types of solar equipment, serves as a demonstration model, so all machinery and controls are exposed. Nine of its solar collectors are air -- rather than water -- filled for space heating.

Other buildings on the solar tour:

-- The Meyer house, 623 Pitt St., an old structure refitted for solar hot water, with heating augmented by a glass interior wall.

-- The Wysong house, under construction on Nicholson Lane near Del Ray, is designed and equipped for optional use of solar power, with massive southern window space, a sealed northern exposure and several fireplaces.